SELECTIVE ISOLATION PROCEDURES FOR DIFFERENTIATION OF 2 STRAINS OF COLLETOTRICHUM-GLOEOSPORIOIDES FROM CITRUS

Citrus postbloom fruit drop is caused by a slow-growing orange strain of Colletotrichum gloeosporioides (SGO). A semiselective medium was developed to isolate the pathogen and differentiate it from the ubiquitous fast-growing gray saprophytic strain of C. gloeosporioides (FGG). Addition of streptomycin to potato-dextrose agar (PDA) at 300 mg/L suppressed contaminants without affecting growth of either strain Of C. gloeosporioides. When plates were incubated for 4 days at the optimum temperature of 27 C for growth of the two strains, FGG colonies often overgrew SGO colonies. Incubation of plates for 4 days at 18 C restricted colony size of both isolates. When incubated for one more day at 27 C, SGO colonies produced abundant orange conidia, whereas FGG colonies developed characteristic dark pigmentation of hyphae. Addition of copper hydroxide (42 mg/L Cu) to PDA + streptomycin reduced contamination only slightly but enhanced orange color development of SGO colonies, facilitating counting of colonies. Plating of citrus tissue washings on PDA + streptomycin + copper hydroxide and incubation for 4 days at 18 C plus 1 day at 27 C optimized recovery of the SGO isolates.