Genetic diversity of Colletotrichum coccodes in the United States using amplified fragment length polymorphism analysis

Black dot disease of potato, caused by Colletotrichum coccodes (Wallr.) Hughes, is widely distributed in the United States. However, little is known regarding the population biology of this fungus. A total of 370 single-spore isolates of C. coccodes were collected from naturally infected potato plants in nine states and analyzed using amplified fragment length polymorphism (AFLP) markers and three primer pairs, yielding 190 polymorphic bands with 90.7 % polymorphism. The isolates were assigned to four vegetative compatibility groups (AFLP/VCG): AFLP/VCG1, AFLP/VCG2, AFLP/VCG4/5, and AFLP/VCG6/7. No isolates tested belonged to AFLP/VCG3. The United States C. coccodes population structure was confirmed with a high differentiation value (G (ST) = 0.30) among VCGs. AFLP/VCG2 was the dominant group in the population (n = 262) and was the most frequent AFLP/VCG among states, fields, farms, and plants. However, in several instances, more than one AFLP/VCG was isolated from the same plant, field, farm, and state, indicating variability within the C. coccodes population in the United States. A geographic pattern was found for isolates originating from Texas, Montana, North Dakota, and Wisconsin. Diversity within states accounted for 73 % of the total genetic diversity, and among populations accounted for 27 %. These results suggest that several AFLP/VCGs are widely distributed in the United States and that they form a single large population of C. coccodes.