During October 2011, wilted and dead strawberry (Fragaria × ananassa cv. Albion) plants from two commercial fields in South Carolina were sent to the Clemson University Plant Problem Clinic in Pendleton, SC. Symptoms consisted of wilting and chlorosis of foliage, scorch and dieback of older leaves, and stunting of plants. Internal vascular and cortical tissues of plant crowns showed a distinct reddish brown discoloration. To isolate the causal agent, necrotic crown tissue selected from two symptomatic plants from one location and four symptomatic plants from the other were placed on acidified potato dextrose agar (APDA) and on quarter strength acidified PDA (QPDA). Colonies with light purple mycelia and beige or orange reverse colony colors developed on APDA after 5 days of incubation at 25°C. Colonies on QPDA were light purple. Morphology, growth, and development of macroconidia and microconida were consistent with descriptions of Fusarium oxysporum Schlechtend emend. Snyder & Hansen (3). Genomic DNA from 3 isolates (11-1246A, 11-1247A, and 11-1247B) was extracted and purified according to Chi et al. (1). The internal transcribed spacer region comprising ITS1, ITS2, and 5.8S rRNA was amplified by primers ITS1 and ITS4 (4). The sequence comparison revealed a 100% match with F. oxysporum sequences in GenBank. To confirm the pathogenicity of the fungus, roots of 15 strawberry plants (cv. Albion) were cut and then five plants were soaked for 10 min in either 500 ml of conidial suspension (104 conidia/ml) of one of the two isolates or in sterile distilled water. All were then potted in 15-cm pots with artificial peat-based soil mix and maintained at 25°C in the greenhouse. After 6 weeks, all plants inoculated with isolates 1247A and B were stunted and developed wilt symptoms similar to those observed in the field, while the control plants remained healthy. Support roots on all affected plants were soft and flaccid and new feeder roots had brown lesions. Crowns of three plants inoculated with isolate 1247A and four plants inoculated with 1247B showed vascular discoloration. To reisolate, crowns were plated as above and roots were surface sterilized in 10% bleach for 1 min and rinsed in sterile distilled water prior to plating on QPDA. F. oxysporum was isolated at frequencies of 70 and 100% from crowns and 100% from roots of all inoculated plants. To our knowledge, this is the first report of the occurrence of Fusarium wilt caused by F. oxysporum on strawberry plants in South Carolina. The presence of Fusarium wilt in South Carolina should alert growers, county agents, and specialists to properly identify Fusarium wilt symptoms, which may be confused with Anthracnose or Phytophthora crown rot of strawberry. The disease has been reported previously in other countries including the United States (2).
References: (1) M. H. Chi et al. Plant Pathol. J. 25:108, 2009. (2) S. T. Koike et al. Plant Dis. 93:1077, 2009. (3) W. C. Snyder and H. N. Hansen. Am. J. Bot. 27:64, 1940. (4) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Application. Academic Press, NY, 1993.
