A Bitter, Complex Problem: Causal Colletotrichum Species in Virginia Orchards and Apple Fruit Susceptibility

Bitter rot, caused by Colletotrichum species, is one of the most devastating summer rot diseases affecting apple production in the Eastern United States. Given the differences in virulence and fungicide sensitivity levels between organisms belonging to the acutatum species complex (CASC) and the gloeosporioides species complex (CGSC), monitoring their diversity, geographic distribution, and frequency are essential for successful bitter rot management. In a 662-isolate collection from apple orchards in Virginia, isolates from CGSC were dominant (65.5%) in comparison to the CASC (34.5%). In a subsample of 82 representative isolates, using morphological and multilocus phylogenetic analyses, we identified C. fructicola (26.2%), C. chrysophilum (15.6%), C. siamense (0.8%), and C. theobromicola (0.8%) from CGSC and C. fioriniae (22.1%) and C. nymphaeae (1.6%) from CASC. The dominant species were C. fructicola, followed by C. fioriniae and C. chrysophilum. C. siamense followed by C. theobromicola developed the largest and deepest rot lesions on Honeycrisp fruit in our virulence tests. Detached fruit of nine apple cultivars and one wild accession (Malus sylvestris) were harvested early and late season and tested in controlled conditions for their susceptibility to C. fioriniae and C. chrysophilum. All cultivars were susceptible to both representative bitter rot species, with Honeycrisp fruit being the most susceptible and M. sylvestris, accession PI 369855, being the most resistant. We demonstrate that the frequency and prevalence of species in Colletotrichum complexes are highly variable in the Mid-Atlantic and provide region-specific data on apple cultivar susceptibility. Our findings are necessary for the successful management of bitter rot as an emerging and persistent problem in apple production both pre- and postharvest.