Identification of subpopulations of Colletotrichum acutatum and epidemiology of almond anthracnose in California

In recent years, almond anthracnose has developed into a major problem for the California almond industry. The identification of the causal pathogen a s Colletotrichum acutatum was confirmed using species-specific primers and restriction fragment length polymorphisms of ribosomal DNA in comparative s tudies with isolates of C. acutatum from strawberry and C. gloeosporioides from citrus. Two distinct clonal subpopulations among the almond isolates o f C, acutatum were identified. These two subpopulations differed in their c olony appearance (pink versus gray cultures), conidial morphology, virulenc e in laboratory inoculation studies, temperature relationships for growth, and molecular fingerprints using random and simple-repeat primers in polyme rase chain reactions. Both subpopulations were commonly isolated from the s ame orchard or even the same fruit. In other orchards. one subpopulation pr edominated over the other subpopulation. Using random, simple-repeat, and s pecies-specific primers, isolates of the almond anthracnose pathogen from I srael were very similar to the California isolates that produce gray coloni es. In addition to fruit, the pathogen was isolated from blighted blossoms, water-soaked or necrotic leaf lesions, symptomless peduncles, and spurs an d wood from branches showing dieback symptoms, indicating that the amount o f tissue that may be infected is more extensive than previously considered. Overwintering fruit mummies were identified as inoculum sources for early spring infections. Growth studies using almond kernels with different moist ure contents indicated that postharvest damage of stored kernels likely ori ginates from preharvest field infections.