Epidemiological factors and cultivar sensitivity affecting severity of apple canker in Finland

Abstract: The economic importance of apple canker has increased in Finland in recent years. The causal organism of apple canker is Neonectria ditissima. The perfect (ascospore-forming) stage of the fungus disperses by wind or rain-splash and the imperfect (conidia-forming) stage by rain-splash. The temperature and moisture requirements for perithecia development, maturation, and finally ascospore ejection are not known in the Finnish climatic conditions. Also, very little is known about the canker susceptibility of eastern or continental types of cultivars grown in Finland. To better understand the geoclimatic potential of the disease development and the monthly temporal trends of disease conducive weather periods, the Finnish weather data was used to run a canker risk model. In the experimental part of the on-going study, the densities of airborne ascospores are studied with volumetric air samplers placed in orchards in two different locations of southern Finland in 2017-2019. Trapped Neonectria spores will be counted under a microscope and species identity confirmed from DNA. The spore trapping results will be combined with weather data, and with visual observations of perithecia amounts and maturity in the orchards. Cultivar tolerance to artificial infection by N. ditissima is tested in a greenhouse in 2017-2018. In 2017, isolated ascospores were used as the pathogen inoculum and one-year old potted trees of 66 cultivars were wound-inoculated. The trial will be repeated in 2018. Later, sensitivity of frost damages in different cultivars will be evaluated by inoculating wounds induced by controlled freezing. Based on the early results, canker risk may extend beyond the current main apple growing area. Cultivar phenology and the timing of high-risk weather conditions require further observations. Various levels of disease susceptibility or tolerance were observed in both the modern and traditional cultivars, indicating potential for improvement of resistance by breeding.