Monitoring Spore Dispersal and Early Infections of Diplocarpon coronariae Causing Apple Blotch Using Spore Traps and a New qPCR Method

Apple blotch (AB) is a major disease of apple in Asia and recently emerged in Europe and the USA. It is caused by the fungus Diplocarpon coronariae ( Dc ) (formerly: Marssonina coronaria; teleomorph: Diplocarpon mali ) and leads to severe defoliation of apple trees in late summer resulting in reduced yield and fruit quality. To develop effective disease management strategies, a sound knowledge of the pathogen’s biology is crucial. Data on the early phase of disease development is scarce: no data on spore dispersal in Europe is available. We developed a highly sensitive TaqMan qPCR method to quantify Dc conidia in spore trap samples. We monitored temporal and spatial dispersal of conidia of Dc , and progress of AB in spring and early summer in an extensively managed apple orchard in Switzerland in 2019 and 2020. Our results show that Dc overwinters in leaf litter and spore dispersal and primary infections occur in late April and early May. We provide the first results describing early-season dispersal of conidia of Dc , which, combined with the observed disease progress, helps to understand the disease dynamics and will be a basis for improved disease forecast models. Using the new qPCR method, we detected Dc in buds, on bark and fruit mummies, suggesting that several apple tissues may serve as overwintering habitats for the fungus, in addition to fallen leaves.