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, a nd Virginia in 1992 and 1993. Colletotrichum and Botryosphaeria spp, w ere recovered from 78.8 and 21.2% of the sampled fruit, respectively. Conidial and colony morphology, growth rate, and perithecial productio n 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, ell iptic-fusiform conidia tapered at one or both ends, whereas isolates o f C. gloeosporioides produced hyaline oblong conidia with obtuse or ro unded ends. Among the C. gloeosporioides isolates, 42.1% of monoconidi al cultures produced the Glomerella cingulata teleomorph. None of the isolates identified as C. acutatum produced an ascigerous stage. All i solates of C. acutatum had a significantly slower growth rate on potat o dextrose agar (PDA) than did isolates of C. gloeosporioides and G. c ingulata. Among the isolates of C. acutatum, 90% were characterized as ''chromogenic types'' producing a distinct ruby red pigmentation on P DA. However, the ruby red colony color was not diagnostic for species identification as there were some isolates of C. acutatum that produce d colonies orange to dark brown in color on PDA. The chromogenic isola tes were only found among the isolates identified as C. acutatum. Of t he 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, th ree 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, recover ed.