Optimizing chemical management for Colletotrichum species causing bitter rot on apple in the Mid-Atlantic USA

다국어 초록 (Multilingual Abstract)

Bitter rot of apple is a disease of apple fruits caused by fungi in the genus Colletotrichum. Apple growers in the Mid-Atlantic USA have reported increased losses to bitter rot in recent years, necessitating optimized management recommendations. Over 500 apples with bitter rot were obtained from 38 orchards across Pennsylvania, Virginia, Maryland, Delaware, and Ohio. The causal species listed in decreasing order of abundance were identified as C. fioriniae, C. chrysophilum, C. noveboracense, C. siamense, C. fructicola, C. henanense, and C. gloeosporioides sensu stricto, the latter two being first reports. We screened 220 isolates through an initial fungicide sensitivity screen, from which a smaller subsample of isolates was selected and tested to obtain EC50 and EC25 values for a total of 22 fungicide active ingredients from FRAC groups 1, 3, 7, 9, 11, 12, and 29. A few isolates were resistant to FRAC groups 1 and 11 with confirmed β-tubulin E198A and cytochrome-b G143A mutations. With little resistance found, fungicide resistance is unlikely to cause increasing bitter rot incidence. These fungicides varied widely in efficacy within and between FRAC groups, with our Colletotrichum isolates insensitive to many active ingredients. Comparisons of our in vitro results with our field trials and other field trials conducted across the Eastern USA suggested that EC<SUB>25</SUB> values (concentrations that reduce growth by 25%) are better predictors of fungicide efficacy in typical field conditions than EC<SUB>50</SUB> values. With few effective fungicides, application timing is critical for bitter rot management. Spore trapping and quantification of C. fioriniae showed that spores are dispersed in orchards from bud break to leaf drop, indicating spores are present season-long. Experiments of the relative susceptibility of apples to bitter rot throughout the season inadvertently confirmed the importance of temperature and moisture in creating high disease pressure. The most effective fungicides should be applied for optimized bitter rot management before high disease-pressure conditions.