Frequency of Colletotrichum species causing bitter rot of apple in the southeastern United States

The occurrence and frequency of Colletotrichum spp. were determined by sampling 980 apple fruit from orchards in Arkansas, North Carolina, and Virginia in 1992 and 1993. Colletotrichum and Botryosphaeria spp, were recovered from 78.8 and 21.2% of the sampled fruit, respectively. Conidial and colony morphology, growth rate, and perithecial production were used to characterize the Colletotrichum spp. Based on conidial morphology, 68.6% of isolates were identified as C. acutatum and 31.4% as C. gloeosporioides. The C. acutatum isolates produced hyaline, elliptic-fusiform conidia tapered at one or both ends, whereas isolates of C. gloeosporioides produced hyaline oblong conidia with obtuse or rounded ends. Among the C. gloeosporioides isolates, 42.1% of monoconidial cultures produced the Glomerella cingulata teleomorph. None of the isolates identified as C. acutatum produced an ascigerous stage. All isolates of C. acutatum had a significantly slower growth rate on potato dextrose agar (PDA) than did isolates of C. gloeosporioides and G. cingulata. Among the isolates of C. acutatum, 90% were characterized as ''chromogenic types'' producing a distinct ruby red pigmentation on PDA. However, the ruby red colony color was not diagnostic for species identification as there were some isolates of C. acutatum that produced colonies orange to dark brown in color on PDA. The chromogenic isolates were only found among the isolates identified as C. acutatum. Of the Botryosphaeria spp. recovered, 84.1 and 15.9% of the isolates were identified as B. dothidea and B. obtusa, respectively. The majority of Botryosphaeria isolates were recovered from fruit with small (<0.5 cm in diameter), nondescript lesions. Among the bitter rot pathogens, three taxa, C. acutatum, C. gloeosporioides, and G. cingulata, could be distinguished. Based on this survey, there was considerable orchard-to-orchard variation in the frequency of the Colletotrichum spp, recovered.