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IOBC-WPRS Bulletin Vol. 110, 2015

 

IOBC-WPRS Bulletin Vol. 110, 2015

Working Group "Integrated Plant Protection in Fruit Crops, Subgroup Pome Fruit Diseases".
Proceedings of the Meeting at Stellenbosch (South Africa), 24th - 28th November, 2014.
Edited by Alana den Breeyen, David Gadoury, Cheryl L. Lennox, William E. MacHardy, Julia C. Meitz-Hopkins & Arne Stensvand.
ISBN 978-92-9067-293-7 [XIII + 246 pp.]

 

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Introduction: Climate change symposium
William E. MacHardy

Abstract only

1

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Projected climate change effects on apple phenological development
Elmi Lötze, Stephanie J. E. Midgley, Cheryl L. Lennox

Abstract: The Western Cape region of South Africa is identified as an area that will be influenced directly and significantly by projected climate change in future. Higher mean, maximum and minimum temperatures and more frequent, warmer hot and less frequent, cold days are predicted. High temperature extremes have increased, whereas low temperature extremes have decreased significantly. Rainfall trends are more difficult to indicate and show little consistency across the Western Cape region. Extreme rainfall events will become more frequent, especially in spring and summer, with a reduction in extremes in autumn. Thus, depending on when during the season these changes will occur, and which climatic parameters are involved, the effect of the changes will impact differently on fruit production – specifically apples. The phenological development of fruit trees is to a great extent genetically controlled; however it is influenced and even shifted when temperature changes are significant during the sensitive periods. Dormancy in apple trees has been mentioned specifically as a sensitive stage that will impact on fruit production due to climate change. Similarly, a change in rainfall patterns will impact on this sensitive stage. The objective of this study is to quantify the predicted climate changes and integrate these scenarios with present phenological phases of apples in two climatically contrasting production areas in the Western Cape. Once we have identified the main phenological stages expected to be affected, the impact of these changes on fruit production will be discussed.

3-13

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Climate, asynchronous phenology, ontogenic resistance, and the risk of disease in deciduous fruit crops
David M. Gadoury

Abstract: Minimum chilling requirements of perennial fruit crops have been extensively studied, but little is known of how the degree and depth of winter chilling affects synchronization of host regrowth upon emergence from dormancy. The European grapevine species (Vitis vinifera) is a useful model system for studying the interactions between chilling, asynchronous phenology, development of ontogenic resistance, and the consequent risk of disease. Mean temperature of the three winter months ranged from -4.1 to 11.8 °C among 15 sites on 3 continents, and was associated with duration of bloom at each site: 2 d at the coldest sites, and > 2 wks at the warmest sites. This 7-fold increase in the duration of bloom directly translated to protracted susceptibility due to delayed development of ontogenic resistance to major fungal pathogens, including Erysiphe necator and Plasmopara viticola. Downstream effects of asynchronous bloom such as asynchronous ripening and sugar accumulation were also recorded. Asynchronous regrowth following unusually warm winters has been noted in grapevine, apple, and stone fruits. This asynchrony will likely increase the risk of disease in many pathosystems typified by phenology-defined windows of susceptibility. Bloom is an easily monitored phenological event that can be used to quantify the impact of winter chilling on the degree of asynchrony in perennial fruit crops. Once the relationship between winter temperatures and the duration of bloom is determined for a perennial fruit crop, the impact of climate change on the foregoing can be projected by examining these interactions across existing climatic gradients.

15-24

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Climate trends in the Lake Constance region and challenges to pest management programs from a consultant's perspective
Peter Triloff

Abstract: While annual climatic data from Lake Constance area only show a slight increase of maximum and decrease of minimum temperatures, precipitation has decreased by about 30% in the past 23 years. Calculated on a monthly basis a remarkable increase of average temperatures up to 2 °C and maximum values up to 4 °C in April, May, June, September, October and November have been recorded, while January March, July and August show a decrease. Leaf wetness hours increased by up to 40% in August, September and November, while a 20% decrease has been observed in April. Precipitation decreased by approx. 40% in April and June and 50% in November while July and August show an increase of approx. 30% and 40%, respectively. Despite no significant changes in average, maximum and minimum temperatures on a monthly basis have been recorded in February, bud burst since the early 1990s in about 50% of the years occurred about three weeks earlier than before, caused by high heat units being calculated for February of those years. The changes in temperatures enabled the production of varieties requiring high temperatures especially in May and June, while others cannot be produced any more or exhibit sunburn and show poor coloration because of too high temperatures in September. In apple scab control a severe change from regular rainfalls to only showers, which in many cases do not occur at all, leads to very high percentages of ejectable ascospores for several weeks, resulting in an increased number of treatments. Predicted rainfalls, that – if at all – finally only occur as local showers, frequently result in a remarkable waste of fungicide sprays. The increase of leaf wetness hours in August and September resulted in improved infection conditions for apple scab but also for summer diseases. The most obvious effect on pests has been observed for codling moth, resulting in an earlier begin of the first generation. Dry spells accidentally distributed over the year and lasting up to 8 weeks that frequently occurred since 2003 are considered the most dangerous effect of a further increasing climate change, because a slight increase to approx. 10 weeks may destroy a high percentage of fruit orchards from acute water deficiency. The effects of climate change are expected also to generally be exacerbated by a decrease of agricultural research and advisory in common with structures in education and research preventing the build-up of experience and overview within individual crops.

25-32

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Calibrating a disease forecaster for Elsinoe spot in organic apples using receiver operating characteristic curve analysis
Robert Beresford

Abstract: Elsinoe spot (Elsinoë pyri), which is a minor disease of apple and pear worldwide, is not a problem in New Zealand apple orchards under conventional synthetic fungicide regimes, but can be problematic in “organic” orchards where lime sulphur and copper fungicides are used. Elsinoë pyri overwinters as ascomata in fallen leaves and as acervuli on unharvested fruit on trees. Epidemics develop from midsummer to leaf fall following rainy weather. A disease forecaster to identify conditions causing high infection risk was developed to aid decisions about lime sulphur spraying. The risk model was derived from data on infection under controlled temperature and wetness conditions. The model outputs a risk indicator variable (0-10) for individual wet periods and indicator values exceeding given thresholds allow adjustment of timing and rate of lime sulphur applications to maximise disease control and minimise phytotoxic
risk. The forecaster was field calibrated using potted ‘Royal Gala’ trap trees exposed in an unsprayed Elsinoe-infected orchard to ‘trap’ infection and identify individual infection periods. Preliminary and revised versions of the forecaster’s risk algorithm were compared using receiver operating characteristic (ROC) curve analysis. Actual infection from the trap plant data and the forecaster’s prediction of infection were used to determine conditional probabilities for true positive, false positive, true negative and false negative predictions. Both algorithms gave significantly better than random prediction of infection events and the revised algorithm, which included a rainfall criterion, was substantially better than the preliminary algorithm.

33-42

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Using long-term research station data to assess and predict disease response to climate change
Mark Boudreau, Kari Peter, Turner Sutton

Abstract: Plant disease response to climate change ideally should be evaluated with long-term disease records for specific cultivars and locations, and predicted with models operating at seasonal and regional scales. Such records are uniquely available for apple trials at research stations. We compiled nearly 40 years disease data on unsprayed ‘Rome Beauty’ apples at Fletcher, North Carolina and Biglerville, Pennsylvania, USA, spanning a latitudinal gradient from 35.4°N to 39.9°N. Disease incidence response to broad seasonal patterns was initially evaluated by categorizing all seasons into terciles based on average temperature or total precipitation, or by El Niño Southern Oscillation (ENSO) cycle during or immediately prior to the growing season. Patterns were revealed that suggested predictive power at this scale for some pathogens. For example, sooty blotch-fly speck incidence at Fletcher was 41% and 47% less in the driest tercile of growing seasons than in the mesic or wettest terciles, respectively (p < 0.01). Powdery mildew had a similar response but differences were not significant. ENSO, however, appeared to have little influence on disease. Chronological trends in disease and phenology are also considered, all of which can inform future models which are useful at the spatial and temporal scales of climate change.

43-45

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A threshold for early termination of fungicide treatments to control scab (Venturia pirina) on Conference pears based on disease history and growth level
Marc Trapman, Vincent Philion

Abstract: The moderately pear scab susceptible pear variety ‘Conference’ is the most important cultivar grown in northwest Europe and requires frequent fungicide application if high inoculum levels are present in the orchard. From previous work with bagged fruit in 2011 and 2012, we concluded that fruit infections on ‘Conference’ mainly occurred within the first weeks after flowering. Practical experience also indicated that scab control could be more difficult in orchards with a high growth level, or a history of severe pear scab. In these situations, treatments during summer still contributed to disease control. In 2012, 2013 and 2014, trials were organised in commercial IPM and Organic orchards in The Netherlands and Belgium to find relations between the last treatment which significantly contributed to scab control in these orchards, and orchard factors as disease history, growth level and fertilisation.

47-50

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ELISA detection of Apple chlorotic leafspot virus (ACLSV) and Apple mosaic virus (ApMV) in pome fruit in comparison to RT-PCR detection and the determination of genetic variation of their virus species in South Africa
Sophia S. Malan, Lizel Mostert, Dirk U. Bellstedt

Abstract: Apple chlorotic leaf spot virus (ACLSV) and Apple mosaic virus (ApMV) are responsible for reduced yield in the South African deciduous fruit industry. These two diseases are regulated by the South African Deciduous Fruit Plant Certification Scheme whereby no trees infected with these viruses are permitted for plantings. Currently the double antibody sandwich enzyme-linked immune-sorbent assay (DAS-ELISA) is prescribed as the test method for routine detection of these viruses in plant material. In the first part of this study, detection limits of DAS-ELISA and reverse transcriptase polymerase chain reaction (RT-PCR) for ApMV and ACLSV were compared respectively. The RT-PCR was found to be more sensitive than DAS-ELISA in the detection of ACLSV and ApMV. In the second part of this study, the genetic variation of ApMV and ACLSV isolates from South Africa was investigated. Extracted RNA was used for RT-PCR of the coat protein genes which were then sequenced. Phylogenetic trees were constructed using these sequences as well as reference sequences from GenBank. Phylogenetic analysis revealed that South African isolates of ACLSV were similar to isolates from the rest of the world, and that the majority of isolates are not restricted to a particular fruit group. This indicates that cross-infection between pome- and stone fruit is a strong possibility. It is also concluded that ACLSV was imported from various regions of the world, since similarity with a number of different overseas isolates was found. Phylogenetic analysis of the coat protein gene sequences of ApMV isolates indicated that two major groups occur in South Africa. Phylogenetic analysis also revealed that South African isolates in individual clades are not restricted to a single fruit group, which indicate the risk of cross-infection.

51-55

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Wood scab in apple and pear
Arne Stensvand, Håvard Eikemo, Regina Rancane, David M. Gadoury

Extended abstract

57-58

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Epidemiological research on the etiology of scab on pear
Wendy van Hemelrijck, An Ceustermans, Dries Martens, Wannes Keulemans, Dany Bylemans

Abstract: Apple and pear scab, caused by Venturia inaequalis and V. pirina, respectively, are the key parasites in pome fruit growing. If no control measurements are undertaken, the pathogen can destroy a complete harvest when weather conditions are favourable. The scab fungi on pear and apple are two distinct species. Mostly they are considered to have a similar biological cycle. Despite the similarities, there are also clear differences and these differences are significant for the control of the pathogen. For example, pear scab does not only infect fruits and leaves as apple scab does, but also infects twigs. To better unravel the life cycle of the pear scab fungus and to develop a better control strategy a 4 year IWT (Agency for Innovation by Science and Technology)-funded research project was performed. In this project the etiology of pear scab, the diversity of V. pirina biotypes and their pathogenicity on different plant parts or cultivars and the scab management was investigated. The results of this project indicate that twig scab can be an important factor in the life cycle of pear scab and that plant part or type of cultivar does not always set up a barrier for cross infections with scab conidia. Genetic and molecular studies displayed, amongst others, that lesions are result of an infection of a single spore and that the scab fungus can be present asymptomatically on pear plant parts. Furthermore, our results indicated that infections of fruits occur already early in the season, but that the expression of the symptoms is delayed.

59-67

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Alternaria species associated with leaf blotch of apple and dead flower buds of pears in the Netherlands
Marcel Wenneker

Abstract: The genus Alternaria consists of approximately 250 species and has a worldwide distribution. As plant pathogens Alternaria spp. infect a diversity of plants. Several different cultivated crops are affected, causing severe damage and crop losses. Alternaria leaf blotch and associated premature leaf drop is considered one of the major problems on apple in many parts of the world. Premature leaf drop in the Golden Delicious apple variety is a global issue that received a great deal of attention in the Netherlands in the 1960s and 70s. While the damage caused by leaf drop is manageable for apple growers farmers, in tree nurseries that cultivate trees the problem can cause a considerable loss of quality of the produced trees. Recent studies performed by Applied Plant Research showed that (specific) sprayings against the pathogen works very well against premature leaf drop. Dead flower bud are a common phenomenon in pear culture in the Netherlands, Belgium and Mediterranean countries, and is also caused by Alternaria spp. The disease is characterized by a partial or complete necrosis of flower buds during tree dormancy. This disease progresses during winter and spring. Eventually, this results in the death of most flowers and decay of buds at flowering. In field trials it was proven that fungicide treatments can reduce disease incidence significantly. There is no information on the identity of the specific pathogens, and whether one or more Alternaria species cause both diseases in the Netherlands. DNA-sequencing and morphological studies were performed on a number of Alternaria isolates from affected apple leaves and diseased flower buds of pear, and several Alternaria groups were identified.

69-71

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Towards the integrated management of apple replant disease using knowledge on disease etiology
Adele McLeod, Mark Mazzola

Extended abstract

73-75

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Leaf fertilizer applications promote white haze and sooty mold in apple
Klaus Marschall, Werner Rizzolli, Yazmid Reyes-Domínguez

Abstract only

77

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Field evaluation of rootstocks for apple replant disease tolerance
Karien Bezuidenhout, Louise van Schoor, Nigel Cook

Abstract: Apple replant disease (ARD) is a major problem in all apple growing regions of the world where apple orchards are established on sites previously cultivated with apple. The typical symptoms are stunted growth, discoloured roots and poor yield, which are exacerbated by abiotic factors such as poor soil fertility. ARD is primarily a biological phenomenon and its etiology involves a complex of various soil organisms (plant parasitic nematodes and soil pathogenic fungi). Soil fumigation used to be the preferred management tool, but recently more sustainable, long-term solutions have been researched. It has been shown that Malus germplasm contain sources of genetic tolerance towards ARD. Therefore, the aim of this project was to evaluate various rootstocks under South African (SA) ARD conditions. A selection of rootstocks currently used in SA, as well as rootstocks from the Cornell Geneva range, was planted in the field at two ARD sites. All plots were split into fumigated and non-fumigated (sub treatments) plots in order to study the effect of ARD. Tree growth (shoot growth, plant biomass and root biomass) and nutrient status (soil and plant nutrient content) were monitored and the nematode levels were determined at the end of the trial in 2012. As is expected when dealing with soilborne diseases and a syndrome as complex as ARD, the data was characterised by high levels of variation. Some general trends could however be distinguished. Such as that MM109 at both sites and G222 at one of the sites, seemed to be the most tolerant to ARD, when evaluating percentage increase in growth and weight between fumigated and non-fumigated plots. G228 seemed to be most susceptible when evaluating percentage increase in weight. G202 generally induced the highest plant mass within the Geneva range and MM109, within the Merton range. CG3007 had very poor survival in one of the sites. In general, results for the Geneva rootstocks varied between sites and variables measured. Lesion nematode data did not show significant differences, possibly due to spatial variation. Repeat trials at different field sites are in progress to confirm the above preliminary results.

79-85

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Molecular characterisation of the South African V. inaequalis (apple scab) populations
Trevor A. Koopman, Julia C. Meitz-Hopkins, Cecilia Bester, Kenneth R. Tobutt, Cheryl L. Lennox

Abstract: Venturia inaequalis, the causal agent of apple scab, is one of the most important diseases of apple (Malus domestica), in terms of economic crop losses, world-wide and in South Africa. South Africa has four principal climatically different apple growing regions. Apple scab isolates were characterised molecularly from four populations from the different regions to determine if there are any differences between these populations. In the 2012/2013 season, single spored isolates were genotyped with six known microsatellite markers from V. inaequalis,
EmVi10, Vitc2/D, Vitg9/129, Vitg11/70, 1tc1g and 1tc1a. Species identity was confirmed by using ITS and ABC2 sequencing. Results from the genotyping with the microsatellite markers indicate that the South African V. inaequalis populations are highly variable and sexually outcrossing (Vo/Ve = 1.18; P = 0.55). Overall, SSR markers used in this study were highly variable (H = 0.62). In addition, each region has ‘private’ alleles indicating moderate differences between the populations (Fst = 0.15; P = 0.001). Little population differentiation was found between the Ceres and the two Langkloof populations although these populations are more than 500 km apart. There were moderate differences between the other populations. This may have implications on the apple scab control and indicates that different management practices may be needed to control the disease in these different areas. The disease might also have adapted to the different climatic conditions in these areas. A high number of synonymous changes was found in the ABC2 gene nucleotide binding site 2 (NB2) region (TD = 0.034; P > 0.1) and six ABC2 haplotypes and five ITS haplotypes were identified. The ABC2 NB2 region might be under balancing selection, implying possible virulence differences in the pathogen.

87-91

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Host modification of Penicillium solitum during postharvest decay of apple fruit
Kari A. Peter, Eunhee Park, Verneta L. Gaskins, Tianbao Yang, Ivana Vico, Wojciech J. Janisiewicz, Wayne M. Jurick II

Extended abstract

93-94

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Preliminary identification of a new fungal phytotoxin involved with Alternaria apple blotch and fruit spot diseases
Mattia Bosello, Peter Robatscher, Klaus Marschall, Michael Oberhuber

Abstract only

95

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Determination of apple scab races occurring in South African apple orchards
Trevor A. Koopman, Julia C. Meitz-Hopkins, Cecilia Bester, Kenneth R. Tobutt, Cheryl .L Lennox

Abstract: Apple scab, caused by the fungus Venturia inaequalis, is one of the most important apple diseases in terms of economic crop losses, in South Africa and world-wide. To date, 17 different apple scab races have previously been identified. Each of these races has overcome major host resistance genes in the apple plant. The ARC apple breeding programme in South Africa currently makes use of only a few major resistance genes for breeding against apple scab, and it is not known which apple scab races occur in the country. The aim of this study was therefore to identify the apple scab races occurring in the South African apple growing regions. Apple scab isolates were collected from two Western Cape apple growing regions namely Elgin and Ceres. Differential apple cultivars with known major resistance genes were imported from Switzerland for race identification. Hardened-off plants of the differential cultivars Gala (none), Golden Delicious (RVi1), M. floribunda (Rvi7), 9AR2T196 (RVi5), TSR34R18 (RVi2), A723 (RVi10), Hansen’s baccata#2 (RVi12), Dülmener Rosen (RVi14), GMAL 2473 (RVi15), G711 (Geneva x Braeburn) (RVi3), Priscilla (RVi6), were spray inoculated in the greenhouse with spore inoculum mixtures from different orchards per region and incubated at 18°C at 100% relative humidity. The plants were evaluated at 14 and 21 days after inoculation. Isolations were made from plants that were susceptible and showed sporulation. Results showed apple scab symptoms on the plant leaves, which ranged from chlorosis and necrosis to sporulation. Venturia inaequalis races 1 and 12 were found to already occur, and the RVi7 resistance from M. floribunda still has not been overcome, in South Africa.

97-99

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The role of latent preharvest infection on postharvest diseases of pomegranate in South Africa
Karen Munhuweyi, Cheryl L. Lennox, Julia C. Meitz-Hopkins, Oluwafemi J. Caleb, Gunnar O. Sigge, Umezuruike Linus Opara

Abstract: South Africa is an upcoming pomegranate producer and exporter in the Southern Hemisphere. The study highlights the importance of determining the etiology and potential of pomegranate postharvest pathogens using molecular techniques to ascertain when/where infection occurs to allow integrated management strategies to be implemented. High counts of Penicillium spp. and Alternaria spp. were observed from preharvest to commercial harvest time with limited fruit damage after fungicide treatment in the pack-house. Isolates of fungal pathogens causing fruit decay were plated on potato dextrose agar (PDA) and incubated at 25 °C for further microscopic and/or molecular identification. The casual agents of latent infection were Alternaria spp., Aureobasidium spp., Pilidiella granati, Penicillium spp. and Botrytis cinerea, respectively. Fungicide treatment in the pack-house reduced disease incidence significantly.

101-103

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Apple canker caused by Neonectria ditissima in Norway
Jorunn Børve, Venche Talgø, Arne Stensvand

Extended abstract

105-106

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Fungicide sensitivity of a South African Neofabraea alba population towards fungicides pyrimethanil and fludioxonil
Jessica Rochefort, Alana Den Breeyen, Cheryl L. Lennox

Extended abstract

107-108

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A comparative study of fungicide sensitivity assays
Benjamin P. Cloete, Julia C. Meitz-Hopkins, Cheryl L. Lennox

Abstract: Fungal pathogens are able to develop resistance against most single-site fungicides such as the dicarboximides and the anilinopyrimidines when used exclusively on highly susceptible cultivars with inadequate cultural practices to support control. It was therefore imperative to assay the sensitivity of Western Cape Botrytis cinerea populations to iprodione and pyrimethanil. Traditional methods of fungicide sensitivity testing involves measuring mycelial growth and conidial germination on plates, but these methods are expensive and time consuming, leaving prospects for more rapid methods of fungicide sensitivity monitoring. This study made comparative use of two microassays, namely a microtiter test and a microbioassay. For the microtiter test, conidia from B. cinerea were challenged in 96 well microtiter plates with different concentrations of iprodione and pyrimethanil. Conidial germination was read with a microplate photometer (microtiter plate reader). For the microbioassays, conidiospores of B. cinerea were incubated on glass slides containing fungicide amended media of equal concentrations used in the microtiter tests and the germ tube lengths were measured. Data resulting from these two methods were compared to determine possible disparity between methods. This forms part of a larger study to establish a rapid fungicide sensitivity monitoring system for the South African pome fruit industry.

109-111

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Molecular variation in candidate fungicide resistance genes of Venturia inaequalis from a sanitation trial orchard
Julia C. Meitz-Hopkins, Amirhossein Bahramisharif, Trevor A. Koopman, Joseph Mafofo, Saskia G. von Diest, Cheryl L. Lennox

Extended abstract

113-115

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Diagnosis and diversity of the Fusarium spp. isolates causing mango malformation in Southern Spain
Maria Crespo, Francisco Manuel Cazorla, Stanley Freeman, Juan Antonio Torés, Antonio de Vicente

Extended abstract

117-118

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Venturia inaequalis pseudothecia development in relatively warm and cold winter regions
Saskia G. von Diest, Ankia Rabie, William E. MacHardy, Cheryl L. Lennox

Abstract: Potential ascospore dose (PAD) is an estimate of an orchard’s level of risk for scab build up. Two density components that determine PAD, pseudothecial density per fertile lesion
(PD) and ascal density per pseudothecium (AD), were compared in the warm winter region of Elgin (E) and the cold winter region of Koue Bokkeveld (KB) over two seasons (2012, 2013). From the two regions, apple leaves with scab lesions were collected during leaf-drop and overwintered in their region of origin or in the other region to investigate the effect of temperature and precipitation on PD and AD. In both seasons, the PD for leaves collected and overwintered in the cold winter region was significantly higher than leaves collected in the cold and overwintered in the warm region, as well as leaves collected and overwintered in the warm winter region. Significantly lower temperatures in the first weeks after leaf drop in the cold winter region are thought to have affected pseudothecial formation. However, in both seasons, the PD for leaves collected and overwintered in the warm winter region did not differ significantly from leaves from the warm region that overwintered in the cold region. Asci developed normally and produced mature ascospores in spring, and there were no significant differences for AD values in any treatments or seasons, despite significantly different temperature differences in the two regions. This is possibly due to different fitness of the EL pathogen population that has adapted to shorter, warmer winters causing fewer pseudothecia to be formed. Additional seasons for this experiment are needed to verify these results before PD and AD constants can be determined for application of PAD in South Africa.

119-122

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Overwintering of Venturia inaequalis conidiospores in South African apple orchards
Saskia G. von Diest, Julia C. Meitz-Hopkins, William E. MacHardy, Cheryl L. Lennox

Abstract: In milder climates in Europe and North America Venturia inaequalis, the causal agent of apple scab, has been reported to overwinter in both the asexual (conidial and mycelial) and sexual (pseudothecial and ascosporic) forms. Viable asexual inoculum is not currently controlled by standard chemical spray programmes, which focus on preventing infections by ascospores early in the season. Objectives of this study were to (1) determine the occurrence of overwintering of asexual inoculum of V. inaequalis in three apple production regions of the Western Cape in South Africa, and (2) clarify their role in inciting early spring infections. Over three seasons, thirty buds were randomly sampled in late winter (August) from six ‘Braeburn’ orchards in the Koue Bokkeveld, Witzenberg Valley and Elgin regions for detection and/or quantification and for conidial viability testing of V. inaequalis. Buds were dissected to separate inner and outer bud tissue, which were analysed separately for viable conidiospores using microscopy. Scabbed pygmy apples that remained on trees during winter were also collected and tested for conidial viability. Conidia per orchard in bud-washes ranged from 0 to 677. Viable conidia were found only in inner bud tissues of orchards with high scab levels the previous season. Viability ranged from 0 to 29% and averaged 4%. Results suggest that orchards with high scab levels in the previous season have a higher risk of developing pygmy apples, and of viable conidia overwintering on pygmies and in bud tissues. Results also suggest that scabbed pygmy apples harbour a higher percentage viability and number of conidia, and may be a more likely source of viable conidial inoculum than apple bud tissue that may cause infection before matured ascospores are available to cause infections. This is the first report of viable conidia of V. inaequalis overwintering on pygmy apples in South Africa.

123-124

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Characterisation of apple blotch disease infections by Marssonina coronariae
Andreas Gallmetzer, Yazmid Reyes-Domínguez, Klaus Marschall

Abstract only

125

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Temporal progress of Glomerella leaf spot on apple at different positions of canopy, related to plant defoliation
Natasha A. Hamada, Louise May de Mio

Abstract only

127-128

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The occurrence of Apple stem grooving virus and Apple stem pitting virus in pome fruit producing regions of South Africa
Magdalena C. Gagiano, Dirk U. Bellstedt, Lizel Mostert

Abstract: Apple stem grooving virus (ASGV) and Apple stem pitting virus (ASPV) are known to infect pome fruit in all pome fruit producing regions of the world. As infection with these viruses causes yield reduction, has a negative effect on fruit quality and reduces orchard longevity, they are of significant economic importance. The South African Deciduous Fruit Plant Certification Scheme regulation states that no trees infected with these viruses are permitted for certified plantings. A survey in which ASGV and ASPV was detected was performed in South African pome fruit regions. A total of 9063 samples were tested for the presence of ASGV with DAS-ELISA over three testing seasons from September 2011 to December 2013. ASGV infection rate in 2011 and 2012 was 3.3% and 0.8%, respectively, with a significant increase to 19.4% in 2013. Seventy seven of these ASGV infected samples as well as randomly chosen additional samples were tested for ASPV with RT-PCR during two active testing seasons between September 2011 and December 2013. Samples from pome fruit trees that showed visual symptoms associated with ASPV, including apple stem pitting, apple epinasty and -decline, pear red mottle, pear stony pit, quince sooty ring spot and fruit, observed during block inspections were included. From the 77 selected samples, a total of 24.7% of ASGV infected samples were also infected with ASPV. Single infection of ASGV and ASPV occurred in 57.7% and 15.6% of the 77 selected samples respectively. Four pear samples that showed visual symptoms of stony pit and fruit deformation were found to be infected with ASPV. Coat protein genes of a large proportion of ASGV and ASPV isolates were sequenced and indicated that South African isolates have multiple worldwide origins.

129-132

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Determining the baseline sensitivity for mycelial growth in Western Cape gray mould populations
Benjamin P. Cloete, Julia C. Meitz-Hopkins, Cheryl L. Lennox

Abstract: Botrytis cinerea is an important pre- and postharvest pathogen and is the causal agent of grey mould disease on pears. It infects the fruit via the calyx end, stem end or wounds. It quickly develops fungicide resistance due to its rapid life cycle and adaptability and resistance to multiple fungicides has been documented in the past. To determine the distribution of baseline sensitivities of 50 B. cinerea isolates recovered from stored ‘Packham’s Triumph’ pears, these isolates were tested in vitro for their sensitivity to technical grade benomyl and iprodione. EC50 values for individual isolates tested against benomyl ranged from 0.038 to 0.087 mg/l and between 0.146 and 0.304 mg/l for iprodione. The mean EC50 values for the population tested for iprodione was found to be 0.212 mg/l and the mean EC50 for benomyl was lower at 0.063 mg/l. Results from these tests showed that the benzimidazole and dicarboximide fungicides are still effective in controlling grey mould disease on pears. Data from this study contribute to monitoring future shifts in sensitivity of this population or future development of resistance.

133-136

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A new spore trap technique to track maturation of Venturia inaequalis ascospores throughout the primary scab season
William E. MacHardy

Abstract: A new spore trap and procedure that identifies the biofix (first matured ascospores), accelerated phase of ascospore maturation, and end of the primary scab season (all ascospores matured) based on differences in trapped ascospore density is discussed.

137-141

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Sources of Neofabraea spp. and Cadophora spp. in Dutch apple and pear orchards
Jürgen Köhl, Marcel Wenneker, Lia de Haas, Ron Anbergen, Carin H. Lombaers, Pieter Kastelein

Abstract: Post-harvest diseases of apple and pear result in significant economic losses during long storage. Pathogens causing quiescent infections in the orchard leading to late post-harvest losses in The Netherlands are Neofabraea alba (Lenticel spot disease), N. perennans (Bull eye rot), Neonectria galligena (Nectria rot), Phytophthora spp., Alternaria spp., Fusarium spp., Cadophora spp. (Phialophora spp.) and Stemphylium vesicarium. Knowledge on the occurrence of the different post-harvest diseases and their epidemiology is very limited. The objectives of our project were to develop tools for the quantitative species-specific detection of pathogens in environmental samples; to study the population dynamics of pathogens in orchards; and to identify major inoculum sources of the different pathogens. Species-specific primers and probes have been developed for three main pathogens N. alba, N. perennans and Cadophora luteo-olivacea. The amount of DNA of each of the three pathogens can now be quantified in environmental samples using TaqMan-PCR assays. Samples of various necrotic plant residues and tree parts were collected in 10 apple and 10 pear orchards during the growing season 2012 to investigate their role as potential inoculum source. In all orchard samples were taken monthly from May until September and in December 2012 from 4 plots (replicates). Neofabraea alba and C. luteo-olivacea were found in all orchards whereas N. perennans occurred only occasionally. Neofabraea alba and C. luteo-olivacea were present on necrotic tissues of apple and pear such as mummies, cankers, and dead leaves. Interestingly, the pathogens were also found in varying amounts on necrotic tissues of other plant species present on the orchard floors. The concentration of pathogen DNA showed a differential pattern on different substrate types. Based on the new knowledge on major inoculum sources of the different pathogens, preventative measures will be developed to reduce the risk of late post-harvest losses.

143-145

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Orchard detection, disease incidence and distribution of Bull’s Eye rot on “Cripp’s Pink” apples in the Western Cape of South Africa
Jessica Rochefort, Alana Den Breeyen, Cheryl L. Lennox

Abstract: Bull’s eye rot (BER) is a latent disease complex that affects pome fruit worldwide, but information of this disease in South Africa is limited. In 2009, a high incidence of BER was reported on stored apples by producers and packhouses in Ceres and Grabouw, the major pome fruit growing regions of South Africa. A preliminary investigation revealed that, of the four species responsible for BER, only Neofabraea alba was present, with the Cripp’s Pink apple cultivar affected most. In this study, timing of fruit infection and postharvest incidence and distribution of BER for Cripp’s Pink from the Western Cape Province was determined, since infection occur in the orchard and disease symptoms only become evident after the cold storage period. Orchard detection, starting one month post full bloom, was done by sampling fruit and leaves from two commercial farms known to have a high incidence of BER, one in Ceres and one in Grabouw, for the 2011/ 2012, 2012/ 2013 and 2013/ 2014 seasons. Fruit was washed, the DNA extracted and then amplified using species specific primers. Neofabraea alba was detected as early as two months post full bloom in Ceres in 2011/ 2012 and one month post full bloom in Grabouw for the 2012/ 2013 season. Disease incidence was determined for Cripp’s Pink apples collected from five different growing regions (Witzenberg Valley, Koue Bokkeveld, Vyeboom, Elgin and Hemel-en-Aarde Valley) at packhouses in the Western Cape and placed in cold storage at -0.5 °C, regular atmosphere, for seasons 2009/2010, 2011/ 2012 and 2012/ 2013. The fruit were not subjected to any postharvest treatments. Evaluation for BER symptoms was done after a four month period. Neofabraea alba was recorded in 66% of grower-lots in 2010/ 2011 and 2012/ 2013 and in 34% in 2011/ 2012 with incidences ranging from 0 to 73% in 2010/ 2011, 0 to 6% in 2011/ 2012 and 0 to 30% in 2012/ 2013. Collection of this data will contribute to a better understanding of the disease and will also allow producers and packhouses to make more informed decisions in terms of disease forecasting, storage duration and sustainable control methods.

147-149

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Colletotrichum acutatum on apple in Norway
Jorunn Børve, Arne Stensvand

Abstract: Colletotrichum acutatum causes bitter rot on apple fruit. It is a pathogen with numerous hosts and adapted to various climatic conditions, and in Norway it has been found on 38 different plant species. There is an increasing awareness of C. acutatum as a postharvest pathogen on apple throughout Europe. It may occasionally be detected in the orchard, but it is the main cause of postharvest decay in Norway. This paper reviews the biology and control of C. acutatum on apple as we presently know it. Typical symptoms and signs on fruit are round, sunken lesions, with dark acervuli and orange to pink spore masses. However, these symptoms may vary. Generative buds may harbour C. acutatum on their scales and may be important as sources of inoculum in early spring. Fruit spurs and apple leaves may contain the fungus and be sources of inoculum for fruit throughout the growing season. Infections on bud scales, fruit spurs, and leaves are all asymptomatic, with the only disease symptoms occurring on maturing or harvested fruit. Fungicide applications against apple scab normally reduce the amount of postharvest development of bitter rot. Experiments with a range of different fungicides against C. acutatum on leaves did not show clear differences between treatments. In cold storage, bitter rot developed more easily in late rather than in early harvested fruit. Disease incidence increased over time in cold storage and more so after simulated shelf life. Storage at low temperature delayed disease development, and incidence after storage at 1 °C was lower than at 3 °C.

151-157

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In vitro effects of aqueous versus ethanol diluted garlic extracts on apple postharvest pathogens Botrytis cinerea, Penicillium expansum and Neofabraea alba
Chanel K. Daniel, Filicity Vries, Cheryl L. Lennox

Abstract: Plant extracts and their essentials have been highlighted in recent years for their antimicrobial potential. Garlic extracts have been documented to be inhibitory to an array of bacteria and fungi. This study aimed to determine the in vitro antifungal activity of aqueous and ethanol diluted garlic extracts on the postharvest pathogens Botrytis cinerea, Penicillium expansum and Neofabraea alba. Extracts were tested by using distilled water and ethanol as solvents to make up the required concentrations and subsequently incorporating the extracts into an amended media. Mycelial growth of B. cinerea and P. expansum on garlic extract amended PDA media was successfully inhibited by aqueous and ethanol dilutions. EC50 values indicated that 25.5% and 11.7% ethanol extract could be used to limit the growth of B. cinerea and P. expansum, respectively. Neofabraea alba growth was similar when treated with the aqueous and ethanolic dilutions of the garlic extracts. EC50 values indicated that 11.5% of an ethanol diluted extract could be used to reduce growth of N. alba. It was concluded that the type of solvent used play an important role in the antifungal activity of garlic extracts. Overall, the antifungal activity of garlic extracts for the control of B. cinerea and P. expansum was confirmed. The antifungal activity of garlic extracts for the control of N. alba requires further investigation

159-164

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Chemical composition and antifungal activity of citrus and lemongrass essential oils in combination with cold storage regimes against Botrytis cinerea
Nokwazi Mbili, Filicity Vries, Umezuruike Linus Opara, Cheryl L. Lennox

Abstract: Postharvest diseases cause serious losses for the apple fruit industry in South Africa. The chemical composition of essential oils of lemon (Citrus limon), lime (Citrus aurantifolia) and lemongrass (Cymbopogan citratus) were analysed using gas chromatography (GC)-mass spectrometry (MS). The analysis exhibited that active components in essential oils of C. limon, C. aurantifolia and C. citratus agree with those found in literature. The analysis exhibited 45 components in essential oil of C. limon, with limonene and gamma-terpinene as the major compounds; C. aurantifolia included 20 components with limonene and gamma-terpinene as the most abundant compounds; while C. citratus revealed 27 components with geranial and neral as the major compounds. Direct contact and vapour phase methods were used to screen the antifungal activity of these essential oils against the major postharvest pathogen of apples caused by B. cinerea. Direct contact phase of lemongrass essential oil, mixtures of lemon and lemongrass and lime and lemongrass essential oils exhibited the strongest toxicity and completely inhibited the mycelial growth of B. cinerea for all concentrations tested at 20 °C, regular atmosphere and controlled atmosphere. Similar results were observed for mixtures of lemon and lemongrass and lime and lemongrass essential oils. Lemon and lime essential oils also showed antifungal activity but they were much less effective than lemongrass essential oil. In the case of the vapour phase method, all essential oil treatments significantly (P < 0.05%) controlled B. cinerea at all concentrations tested compared to control treatments. Mycelial growth of B. cinerea was inhibited by the tested essential oils in a dose-dependent manner. This study demonstrated that C. limon, C. aurantifolia and C. citractus essential oils in combination with cold storage regimes have a potential for the control of postharvest pathogen of apples caused by B. cinerea.

165-168

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Sustainable control of postharvest apple rots – from orchard to store
Robert J. Saville, Justine Perrin, Karen Lower, Angela M. Berrie

Abstract: Fungal rots can result in significant losses in storage, particularly in fruit stored beyond January. Successful control of storage rots depends on a clear understanding of the rots to be controlled. Over the past two decades we have carried out surveys to determine the rots present in commercial apple stores in the UK. The survey involves weekly visits to pack houses from January to March. The survey was conducted on fruit that was being graded at the time of the visit. Rots were identified visually and numbers recorded. The resulting data allowed us to observe trends in the rot profile over time. Historical data is also available from rot surveys conducted in the UK during the 1930s, 1960s and 1980s. The combined dataset shows large shifts in the significance of different rot species over time. Information on rot profiles and species level identification of the causative agents add to our current understanding of post-harvest apple rots, how this can be used practically by growers and future areas of research.

169-171

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Management and control of apple powdery mildew
Angela M. Berrie, Robert J. Saville, Xiangming Xu

Abstract: After scab and canker, powdery mildew (Podosphaera leucotricha) is the most important disease on apple in the UK. Most cultivars are susceptible to the disease, and some such as Cox are very sensitive with mildew levels as low as 8% mildewed leaves being sufficient to reduce yield and fruit quality. Susceptible cultivars require season long spray programmes amounting to 10-15 sprays to protect shoots and buds and prevent high levels of overwintering mildew. In many orchards control of powdery mildew is difficult. There are several possible reasons for this, but the limited range of fungicides available for control, resulting in heavy reliance on triazole fungicides (myclobutanil and penconazole) and as a result the possibility of reduced sensitivity of mildew to these fungicides could be a contributory factor. The results of two projects are described. Firstly, investigating sensitivity of apple mildew populations to triazole, QoI and SDHI fungicides and secondly, identifying new fungicides, alternative chemicals and biofungicides for apple mildew control. Management of powdery mildew is also discussed.

173-185

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Cladosporium cladosporioides H39: A new biological control agent for apple scab control
Jürgen Köhl, Christian Scheer, Imre J. Holb, Sylvester Masny, Wilma M. L. Molhoek

Abstract: Apple scab caused by Venturia inaequalis is the most important disease in apple production. Control of apple scab currently depends on the multiple applications of fungicides. The potential of the antagonist isolate Cladosporium cladosporioides H39, originating from a sporulating colony of V. inaequalis, to control apple scab development was tested in eight trials during two years in orchards in Eperjeske (Hungary), Dabrowice (Poland) and Bavendorf (Germany) planted with different varieties. Treatments were conducted as calendar sprays or after infection periods during the primary season or the summer season. Additional trials in an orchard in Randwijk (The Netherlands) focused on the effect of timing of the antagonist application before or after infection periods. The overall results of the field trials consistently showed for the first time that stand-alone applications of the antagonist can control apple scab in leaves and fruits. This was demonstrated in organic and integrated growing systems. In both systems, control levels as for common fungicide schedules could be reached. The antagonist also was effective if applied one or even several days after infection events. This has been found in several field trials
and has been confirmed by a trial with single spray applications at different intervals before or after infection events. The better understanding the biology of the antagonist will help to further exploit its use in apple scab control.

187-189

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Hot water shower against Gloeosporium fruit rot in organic apple production – how to introduce a new system from research to practice
Karl Schloffer, Marc Trapman

Abstract: Gloeosporium rot is the main storage disease in organic apple production in Austria and in untreated orchards losses can be up to 80%. The problem in organic fruit growing is not only to find a product or method with a high efficacy against Gloeosporium rot but also against different fungal diseases including apple scab, sooty blotch and Marssonina coronaria during the growing season. Using different fungicides or plant strengtheners together or alternating them could lead to potential phytotoxicity. Applying control methods against Gloeosporium rot postharvest may provide more control than applying treatments during the growing season. In 2009 and 2010 different orchard treatments were treated with products commonly used in organic apple production, such as copper-oxychloride, potassium bicarbonate + sulphur, and acidified clay powder (Mycosin) either alone or in combination with postharvest immersion in hot water. None of the orchard treatments were effective. However, superior results were obtained with the hot water dipping. The objective of the study was to develop hot water treatments for larger-scale commercial applications. Showering fruit with hot water provided logistical advantages over the immersion method. The efficacy of the hot water shower machine developed for the prevention of Gloeosporium rot is approximately 80% comparable to the degree of disease suppression obtained through the use of chemical fungicides. In 2013, hot water showers were used to treat over 4.5 million kg of apples. Under certain conditions heat damage occurred on the apples. However, there is potential to improve the efficacy of the hot water shower system by optimizing the shower time and temperature.

191-198

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Use pattern and limits of potassium bicarbonate for apple scab control in Québec orchards
Vincent Philion, Valentin Joubert

Abstract: Scab (Venturia inaequalis) is an economically important disease of apples in many wet climate production areas including Québec, where orchards are almost half planted with a single susceptible cultivar (McIntosh), thereby increasing scab severity through selective pressure. Numerous fungicide sprays directed against the primary infections of this fungus are required every year for commercial production. However, V. inaequalis populations are becoming increasingly resistant to a number of products registered for scab control. Consequently, since 2009 alternative chemistries such as potassium bicarbonate (KBC) were tested under different use strategies. We observed that KBC can be effective in controlling scab only if precisely applied within 300 degree-hours (base 0 °C) from the onset of infection. KBC mixed with sulfur had a much higher efficacy that KBC applied as a standalone. Under an optimal use pattern, the KBC+sulphur mix showed similar efficacy to liquid lime sulfur (LLS) but less than penthiopyrad or solatenol (SDHI) or difenoconazole (DMI) applied with the same strategy. Nonetheless, because KBC + sulfur is presumably less prone to resistance than current systemic fungicides, less phytotoxic than LLS, and considerably less expensive than many other product, it is likely that growers will integrate this as an additional option when post-infection efficacy is needed.

199-212

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An orchard without fungicides: a promise or utopia of biotechnology?
Cesare Gessler, Andrea Patocchi, Henk J. Schouten, Frans Krens, Henryk Flachovsky, Magda V. Hanke, Andreas Peil, Giovanni Broggini

Abstract only

213

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Effect of treatment with fosetyl-Al on the response of apple to Venturia inaequalis infection: symptom expression, qPCR
Wannes Keulemans, Bruno Daniëls, Luk Demaeyer, Mark Davey

Abstract: Apple scab, caused by Venturia inaequalis, is the most important disease in apple orchards in most countries. The most common means to control the disease is by application of fungicides. In many cases more than 10 applications per year are used. These fungicides have an impact on environment, on the financial revenues for the grower and consumers are aware about supposed effects on human health. These effects create a negative image of apple culture and they can be avoided by the use of resistant cultivars and/or by alternative management strategies. The use of plant defense enhancers and plant fortification agents can contribute to such strategies. Fosetyl-Al is supposed to affect plant defense systems. Therefor we studied the effect of the application of this compound on scab development of a susceptible cultivar ‘Golden Delicious’ and a resistant cultivar ‘Topaz’, carrying the dominant resistance gene Rvi6. We inoculated young control plants and plants pretreated with fosetyl-Al of both cultivars with V. inaequalis race 1. Plants were evaluated at regular times post inoculation (p.i.) for visual symptoms according the classes of Chevalier (Chevalier et al., 1991). At 18 days p.i., when clear visual symptoms were present on susceptible plants, we collected leaves from both inoculated cultivars for quantification of fungal DNA in leaves by qPCR (Daniëls et al., 2012). At five days p.i., before visual symptoms, we collected leaves for differential protein accumulation studies in control and treated plants of ‘Topaz’. Proteins were separated by 2 DE gel electrophoresis and stained by CBB G-250 for spot detection. Spots were picked up and analyzed by MALDI-TOF-TOF for further protein identification. Chlorosis and sporulation was visible on ‘Golden Delicious’ from day nine p.i. onward and the effect of fosetyl-Al was dependent on the leave age. In ‘Topaz’ no sporulation was observed, but chlorosis was detected earlier compared to ‘Golden Delicious’. qPCR revealed that in leaves of ‘Topaz’ fungal DNA was present at 18 days p.i., although in much lower amount than in ‘Golden Delicious’. Both in ‘Golden Delicious’ and in ‘Topaz’ fosetyl-Al reduced fungal growth in the leaves. We found many proteins that were significantly up and down regulated in ‘Topaz’ by the mock inoculation and by the fungus inoculation. There was a significant overlap in proteins that were up/down regulated by the fungus and the fosetyl-Al treatment, suggesting that fosetyl-Al stimulates the apple response to scab infection, even in this resistant cultivar, as was also found in the qPCR analysis.

215-218

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Venturia inaequalis demethylation inhibitor fungicide sensitivity after orchard sanitation
Julia C. Meitz-Hopkins, Hannie van Zyl, Saskia G. von Diest, Trevor A. Koopman, Martin Kidd, Wolf F. S. Schwabe, Cheryl L. Lennox

Abstract: Venturia inaequalis, commonly known as apple scab, causes large crop losses world-wide and in South Africa if not controlled by fungicide applications. Integrated disease management approaches such as sanitation practices for inoculum reduction are thought to help maintain effectiveness of chemical control. Applied research in an orchard environment can give an indication of the efficiency of a combination or single application of different management strategies. In this study monitoring of fungicide sensitivity of V. inaequalis populations was applied in field trials where leaf shredding, or a standard fungicide spray program during the growing season combined with the leaf shredding treatment had been applied. It was compared to an untreated orchard section, or a standard spray program alone. The aim of this study was to evaluate the impact of sanitation treatments in the orchard on levels of fungicide sensitivity of one of the most commonly used systemic fungicide classes in South Africa, namely demethylation inhibitors. Two populations from sanitation trials were tested for their fungicide sensitivity to flusilazole using a mycelial growth test of single spored isolates. In the orchard sections, where leaves had been shredded after leaf fall in combination with fungicide application, a significant reduction in resistance levels was observed (EC50 = 0.11 μg/ml) when compared to the negative control (EC50 = 0.27 μg/ml).

219-223

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Non-chemical control of replant disease, caused by Pratylenchus penetrans, in apple orchards in the Netherlands
Marcel Wenneker, Pieter van der Steeg, Gerard Korthals, Johnny Visser

Abstract: Apple replant disease is considered a problem of increasing importance in apple growing regions in Europe. In the Netherlands chemical soil fumigation (metam sodium) is the standard method to control replant disease in apple orchards in sandy soils. This disease is mainly caused by the root-lesion nematode Pratylenchus penetrans. The negative impact of chemical control on the environment and ground water makes the development of alternative control measures necessary. In a multi-year project, alternative and biological treatments were tested. The trials were carried out in an orchard were the apple trees were removed due to severe infestations of P. penetrans. The trial consisted of the following soil treatments: (1) growing of marigold (Tagetes patula); (2) growing of marigold followed by Anaerobic Soil Disinfestation (ABSD); (3) growing of black oat (Avena strigosa) followed by a ‘late’ ABSD treatment; (4) growing of a biofumigation crop, Indian Mustard (Brassica juncea); (5) compost amended in the soil; (6) black fallow (control); and (7) Chemical Soil Disinfestation (CSD; control). After the treatments the plots were replanted with apple trees (cvs. ‘Elstar’ and ‘Boskoop’). CSD, ABSD and Tagetes controlled P. penetrans significantly. There was no effect of biofumigation in controlling P. penetrans.

225-227

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Effective post-harvest eradication of EBI tolerant Venturia inaequalis with EBI fungicides in South Africa
Wolf F. S. Schwabe

Abstract: During the 2006 and 2007 growing seasons four trials were conducted in apple orchards with severe scab infection to evaluate the eradicative potential of three ergosterol biosynthesis inhibitor (EBI) fungicides, viz. bitertanol (Baycor; DC 300 g/l, 80 ml/hl), difenoconazole (Score; EC 250 g/l, 12 ml/hl and flusilazole (Nustar; EW 100 g/l, 32 ml or 48 ml/hl). EBI tolerance was recorded in all four orchards. Experimental trees were caged. Single post-harvest applications were compared with double and triple applications. When leaf drop was completed samples were collected from each cage and discs with scab lesions were cut from the leaves, put in glass fibre bags, secured on racks between shade cloths and were allowed to spend the winter in the orchard. Ascospore production of samples was determined in the laboratory at fortnightly intervals during the full ascospore dispersal season. Ascospore survival was reduced by 62, 93 and 99% after single, double and triple fungicide applications on samples from the Grabouw orchard in 2006. At the Agter-Witzenberg orchard with similar applications the reduction of ascospores was 57, 76 and 94%, respectively. In the 2007 trials ascospore reduction in an Oak Valley orchard was 66, 86 and 92% after single, double and triple applications, while in the Koue Bokkeveld orchard ascospore reduction was 90, 97 and 99% for similar treatments. Significant differences of fungicide activity were obtained after only a single application.

229-240

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A role for polyploidisation in biotic and abiotic stress resistance in Malus domestica?
Nick Hias, Katrien De Dauw, Mark Davey, Leen Leus, Marie-Christine Van Labeke, Johan Van Huylenbroeck, Johan Keulemans

Abstract: Polyploids are naturally ubiquitous throughout the plant kingdom. Their abundance indicates that polyploidy confers an adaptive advantage. In the case of artificially mitotically-doubled polyploid plants, some studies have shown an increased resistance to biotic and abiotic stresses compared to their diploid progenitors. In a previous study we observed that under water deficit conditions tetraploid (4x) plants show a higher leaf water status and a higher photosynthetic rate compared to their diploid (2x) progenitors. These results suggest that 4x plants could be more tolerant of water deficit. Based on these observations and previous studies we expected that polyploidisation could also be advantageous in the response of apple to biotic stress caused by pathogens. Hence the main objective of the present study was to determine if polyploidisation has an effect on the tolerance of apple plants to apple scab (Venturia inaequalis). Three susceptible (Gala, G40 and G61) and one Rvi6 resistant genotype (G58) were artificially doubled from nodal segments using colchicine. Visual evaluation of infection severity, according to the five ‘Chevalier classes’, showed a higher susceptibility in 4x plants for the three susceptible genotypes. An additional assessment of the sporulation showed also a clear increase in sporulation area on 4x plants compared to the 2x plants. The 4x plants of the Rvi6 resistant genotype showed a clear decrease in infection severity compared to the 2x plants. No sporulation was observed in this genotype. Based on these observations, polyploidisation has a clear impact on the susceptibility or tolerance to Venturia inaequalis compared to the diploid state of the host. Polyploidisation of susceptible genotypes resulted in a higher scab susceptibility, while polyploidisation of an Rvi6 resistant genotype resulted in a higher scab tolerance. Further physiomorphological and molecular research is however necessary to elucidate the mechanisms behind this changed tolerance.

241-243

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Temporal population change of the apple scab fungus Venturia inaequalis in mixed cultivar orchards
Thomas A. J. Passey, Michael W. Shaw, Xiangming Xu

Extended abstract

245-246

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