Morphological, Cultural, and Pathogenic Variation Among Colletotrichum Species Isolated from Strawberry

Abstract

Conidia, cultural characteristics, appressoria, and setae of 24 isolates of Colletotrichum species from strawberry were compared. The virulence of each isolate on plants of 14 strawberry cultivars and one breeding clone was evaluated. Thirteen isolates identified as C. fragariae produced cylindrical conidia; developed beige to olive to dark gray colonies, generally with dark olive to dark gray reverse colony colors; and did not form the ascigerous state in culture. Two isolates identified as Glomerella cingulata (anamorph: C. gloeosporioides) developed gray or olive-gray colonies with dark gray to dark olive reverse colony colors, produced cylindrical conidia, and formed the ascigerous state in culture. Nine isolates identified as C. acutatum produced fusiform conidia and developed pink, orange, rose, or beige colonies with predominantly cream, pink, or rose reverse colony colors; none formed an ascigerous stage in culture C. acutatum isolates could be easily differentiated from C. fragariae and C. gloeosporioides isolates by their growth rate in plate culture on potato-dextrose agar. The greatest difference in growth rate occurred at 32 C, where the average diameter of 5-day-old C. acutatum cultures was 13 mm, compared with 69 mm for C. fragariae and 63 mm for C. gloeosporioides. All of the C. fragariae isolates induced disease symptoms when wound-inoculated into strawberry leaves and fruit, whereas all of the C. acutatum isolates caused fruit rot but none caused leaf lesions. All 13 of the C. fragariae, four of the five C. acutatum, and one of the two C. gloeosporioides isolates tested caused a crown rot of certain strawberry cultivars. Disease severity ratings after plant spray inoculations resulted in a highly significant isolate .times. cultivar interaction, suggesting that some isolates may represent different races among the tested isolates of C. fragariae and C. acutatum. Overall, C. fragariae isolates caused more severe petiole and crown symptoms than did C. acutatum isolates, which in turn caused more severe symptoms than did C. gloeosporiodes isolates. However, some cultivars were more susceptible to certain C. acutatum isolates than to some C. fragariae isolates than to some C. fragariae isolates, e.g., the cultivar Sunrise was susceptible to C. acutatum isolates Goff and Mil-l but resistant to C. fragariae isolate Fla-2.