Beginning in 2006 and continuing into 2009, an apparently new disease of strawberry (Fragaria × ananassa) affected commercial plantings (cvs. Albion, Camarosa, and others) in coastal (Ventura and Santa Barbara counties) California. Symptoms consisted of wilting of foliage, drying and withering of older leaves, stunting of plants, and reduced fruit production. Plants eventually collapsed and died. Internal vascular and cortical tissues of plant crowns showed a brown-to-orange-brown discoloration. Differences in cultivar susceptibility were not recorded. Internal crown and petiole tissues, when placed on acidified corn meal agar, consistently yielded Fusarium isolates having similar colony morphologies. No other pathogens were isolated. The Fusarium isolates were subcultured on carnation leaf agar and observed to be producing macroconidia and microconidiophores that are diagnostic of Fusarium oxysporum (1). For two of these isolates, the internal transcribed spacer region comprising ITS1, ITS2, and 5.8S rRNA was amplified using primers ITS-1 and ITS-4 (3). On the basis of a comparison of 515 bp, both isolates had the identical sequence, which was a 100% match for 30 accessions of F. oxysporum in GenBank. This comparison included several formae speciales of F. oxysporum, but F. oxysporum f. sp. fragariae, a previously described pathogen of strawberry (4), was not included. The isolates are archived in the Department of Plant Pathology at UC Davis and are available on request. Both sequenced isolates plus four others were tested for pathogenicity on strawberries. For these tests, spore suspensions of 1 × 105 conidia/ml were prepared separately for six isolates. Roots of strawberry transplants (12 plants of cv. Camino Real) were cut and soaked in spore suspensions for 10 min. Plants were potted in soilless, peat moss-based medium in containers. Control strawberry plants were soaked in water prior to planting. All plants were then grown in a shadehouse. After 8 weeks, inoculated plants began to show wilting and decline of foliage and internal crown tissue was lightly discolored. F. oxysporum was isolated from all inoculated plants. Control plants did not exhibit any disease symptoms and crown tissue was symptomless. To our knowledge, this is the first report of Fusarium wilt of strawberry in California. This disease has been reported from a number of other countries including Argentina, Australia, China, South Korea, Spain, and Japan (2). Since 2006, Fusarium wilt of strawberry has increased in incidence and severity in California. Initial problems in 2006 consisted of multiple small patches (2 to 4 beds wide × 3 to 10 m long) of diseased plants; in these patches disease incidence could range from 80 to 100%. By 2009, in some fields, the disease affected large sections that ran the length of the field.
References: (1) P. E. Nelson et al. Fusarium Species: An Illustrated Manual for Identification. Pennsylvania State University Press, University Park, 1983. (2) H. S. Okamoto et al. Plant Prot. 24:231, 1970. (3) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Application. Academic Press, NY, 1993. (4) B. L. Winks and Y. N. Williams. Qld. J. Agric. Anim. Sci. 22:475, 1966.
