Field evaluation of rootstocks for apple replant disease tolerance

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.