Bitter Rot of Apple in the Mid-Atlantic US: Causal Species and Evaluation of the Impacts of Regional Weather Patterns and Cultivar Susceptibility

Apple growers in the Mid-Atlantic region of the United States have been reporting an increase in losses to bitter rot of apple and are requesting up-to-date management recommendations. Management is complicated by variations in apple cultivar susceptibility, temperature and rainfall, and biology of the Colletotrichum species that cause bitter rot. Over 500 apples with bitter rot were obtained from 38 orchards across the Mid-Atlantic and the causal species identified as C. fioriniae and C. nymphaeae of the C. acutatum species complex and C. chrysophilum, C. noveboracense, C. siamense, C. fructicola, C. henanense, and C. gloeosporioides sensu stricto of the C. gloeosporioides species complex, the latter two being first reports. Species with faster in vitro growth rates at higher temperatures were more abundant in warmer regions of the Mid-Atlantic, while those with slower growth rates at higher temperatures were more abundant in cooler regions. Regional bloom dates are earlier and weather data shows a gradual warming trend that likely influenced, but was not necessarily the main cause of the recent increase in bitter rot in the region. A grower survey of apple cultivar susceptibility showed high variation, with the increase in acres planted to the highly susceptible cultivar ‘Honeycrisp’ broadly corresponding to the increase in reports of bitter rot. These results form a basis for future studies on the biology and ecology of the Colletotrichum species responsible, and suggest that integrated bitter rot management must begin with selection of less-susceptible apple cultivars.