Infection conditions for Neofabraea perennans and Phacidiopycnis washingtonensis on developing apple fruit in the orchard

In Northern Germany, a major share of postharvest losses of apple fruit is due to preharvest infections by pathogenic fungi. Little is known about their infection biology. Inoculation experiments were conducted with the most important storage-rot pathogen Neofabraea perennans, as well as with the recently discovered minor rot Phacidiopycnis washingtonensis, by spraying developing fruit on apple trees with conidial suspensions to drip wetness between June and harvest time (September / October). All inoculation events in three trial seasons were chosen to coincide with natural rainfall. Phenological stages and meteorological parameters of each infection event were used for correlation analyses. Both pathogens produced increasing fruit rot levels with inoculation dates closer to harvest. In addition, for N. perennans seven environmental factors were positively correlated with disease incidence, the most significant ones being the duration of post-infection leaf wetness and the scab infection quotient incorporating wetness and temperature. With P. washingtonensis, in addition to fruit maturity three environmental factors were identified. In a second step, multifactorial models for both pathogens were created using the phenological and meteorological factors. For N. perennans, scab infection quotient until first drying-off, dry hours within the leaf wetness period and post-inoculation precipitation levels were identified as important factors, whereas for P. washingtonensis only the average temperature during the leaf wetness period had a significant influence on the rot incidence. Either model was extended by the viability of conidia used for inoculation. Possibilities to deploy these models for a more accurate a priori prediction of the likely severity of storage rot and a more targeted use of pre- and postharvest fungicides and physical postharvest treatments are discussed.