Resistance of Strawberry Cultivars to Crown Rot Caused by Colletotrichum gloeosporioides Isolates from Florida Is Nonspecific

Colletotrichum crown rot of strawberry (Fragaria × ananassa Duchesne) can be caused by any of three fungi, Colletotrichum gloeosporioides (teleomorph Glomerella cingulata), C. fragariae, or C. acutatum (17,27). The closely related species C. fragariae and C. gloeosporioides produce a reddish-brown necrosis of crown tissue following infection that causes plants to wilt and collapse (27). Under greenhouse conditions or in summer nurseries, C. fragariae and C. gloeosporioides also may produce necroses on stolons (4), lesions on fruit (23,26), or black leaf spots (25). Although C. acutatum can also cause crown rot (17), disease symptoms in plants infected with this species are distinct from symptoms produced by C. fragariae and C. gloeosporioides (38). Also, where C. acutatum co-occurs with C. gloeosporioides in Florida, C. acutatum is infrequently isolated from diseased crowns (38). Crown rot is a serious disease in subtropical strawberry production regions such as those in the southeastern United States since infection by C. fragariae and C. gloeosporioides is favored by warm, moist conditions (30,33). Although crown rot is observed in fields during the winter production season, it is most severe in summer nurseries in the southeast and is one of the primary reasons that transplants are produced in temperate regions. There is some uncertainty regarding the initial occurrence of the closely related pathogens C. fragariae and C. gloeosporioides in Florida and historically which of these species was responsible for epidemics in the region. C. fragariae was first reported in Florida nurseries by Brooks in 1935 (5). The first description of crown rot caused by C. gloeosporioides or its teleomorph Glomerella cingulata was reported in 1984 (26). The isolates in this report were homothallic and thus described as G. cingulata. Subsequently, a portion of the historical isolates from Florida previously described as C. fragariae were reclassified as C. gloeosporioides based on conidial and setal morphology (21). This suggests that C. gloeosporioides may have contributed to crown rot epidemics prior to the initial report in 1984, but was misidentified as C. fragariae. At least two of the reclassified isolates produced fertile perithecia when grown together in culture, indicating that they were heterothallic (21). Several of the reclassified isolates have been characterized using molecular methods, and in addition to being distinct from C. fragariae isolates, they also appear to be distinct from homothallic isolates of G. cingulata (18,19). Currently, C. gloeosporioides is the Colletotrichum sp. most frequently isolated from diseased crowns in Florida (38). The isolates of C. gloeosporioides from diseased crown tissue have diverse genotypes (38) and appear to be from a recombining population (38). They have the Cgl-2 genotype A+T-rich DNA banding pattern of C. gloeosporioides isolates originally identified as C. fragariae (18; S. J. MacKenzie, unpublished). The pathogen also appears to have a broad host range, since the population on noncultivated host species growing adjacent to strawberry fields is genetically indistinguishable from that on strawberry and contains isolates pathogenic to strawberry (40). Previous studies examining variation in strawberry resistance to C. fragariae found a broad range of susceptibility among cultivars and aggressiveness among isolates (14,22,34). One study found a significant cultivar by isolate interaction (34). Host resistance studies using homothallic G. cingulata isolates were not as conclusive, as they included only two or three isolates (15,34). One study found significant variation in cultivar susceptibility and isolate aggressiveness, and a significant cultivar by isolate interaction (34). In a later study using a different set of isolates, only cultivar susceptibility varied (15). For C. acutatum, a specific interaction between strawberry cultivars and isolates was observed (15). A single dominant gene appears to be responsible for this interaction (16), with isolates having either an incompatible reaction with cultivars carrying this gene or an intermediate degree of compatibility (15). Assays used to evaluate resistance to C. fragariae and C. acutatum were based on symptom development on petioles or foliage and did not focus on the ability of plants to resist necrosis of crown tissue. Variation in cultivar susceptibility to C. gloeosporioides isolates from Florida has not been investigated. In this study, we investigated the levels of resistance to crown rot caused by C. gloeosporioides in cultivars commonly grown in Florida as well as the patterns of virulence and aggressiveness among C. gloeosporioides isolates under field conditions. This study generated information ABSTRACT MacKenzie, S. J., Legard, D. E., Timmer, L. W., Chandler, C. K., and Peres, N. A. 2006. Resistance of strawberry cultivars to crown rot caused by Colletotrichum gloeosporioides isolates from Florida is nonspecific. Plant Dis. 90:1091-1097.