In California, Fusarium wilt of strawberry is widespread and causes significant yield losses. Resistant cultivars with the FW1 gene were protected against Fusarium wilt because all strains of Fusarium oxysporum f. sp. fragariae (Fof) in California were race 1 (i.e., avirulent to FW1-resistant cultivars) (Henry et al. 2017; Pincot, et al. 2018; Henry et al. 2021). In the fall of 2022, severe wilt disease was observed in an organic, summer-planted strawberry field in Oxnard, California. Fusarium wilt symptoms were common and included wilted foliage, deformed and highly chlorotic leaflets, and crown discoloration. The field was planted with Portola, a cultivar with the FW1 gene that is resistant to Fof race 1 (Pincot et al. 2018; Henry et al. 2021). Two samples, each consisting of four plants, were collected from two different locations within the field. Crown extracts from each sample were tested for Fof, Macrophomina phaseolina, Verticillium dahliae, and Phytophthora spp. by recombinase polymerase amplification (RPA) (Steele et al. 2022). Petioles were surface sterilized in 1% sodium hypochlorite for 2 minutes and plated on Komada's medium to select for Fusarium spp. (Henry et al. 2021; Komada, 1975). The RPA results were positive for M. phaseolina in one sample and negative for all four pathogens in the other sample. Salmon-colored, fluffy mycelia grew profusely from petioles of both samples. Colony morphology and non-septate, ellipsoidal microconidia (6.0-13 μm × 2.8-4.0 μm) borne on monophialides resembled F. oxysporum. Single hyphal tip isolation of fourteen cultures (P1-P14) was done to purify single genotypes. None of these pure cultures amplified with Fof-specific qPCR (Burkhardt et al. 2019), confirming the negative result obtained with RPA. Translation elongation factor 1-alpha (EF1α) was amplified using EF1/EF2 primers (O'Donnell et al. 1998) from three isolates. Amplicons were sequenced (GenBank OQ183721) and found through BLAST search to have 100% identity with an isolate of Fusarium oxysporum f. sp. melongenae (GenBank FJ985297). There was at least one nucleotide difference when compared to all known strains of Fof race 1 (Henry et al. 2021). Five isolates (P2, P3, P6, P12, and P13) and an Fof race 1 control isolate (GL1315) were tested for pathogenicity on Fronteras (FW1) and Monterey (fw1; susceptible to race 1). Five plants per isolate × cultivar combination were inoculated by dipping roots in 5 × 106 conidia per mL of 0.1% water agar, or in sterile 0.1% water agar for the negative control, and grown as described by Jenner and Henry (2022). After six weeks, all non-inoculated control plants remained healthy while plants of both cultivars inoculated with the five isolates were severely wilted. Petiole assays yielded colonies identical in appearance to the inoculated isolates. For Fof race 1-inoculated plants, wilt symptoms were observed in Monterey but not in Fronteras. This experiment was repeated with P2, P3, P12, and P13 on another FW1 cultivar, San Andreas, and the same results were observed. To our knowledge, this is the first report of F. oxysporum f. sp. fragariae race 2 in California. Losses to Fusarium wilt are likely to increase until genetic resistance to this strain of Fof race 2 is deployed in commercially viable cultivars.
Keywords: Causal Agent; Crop Type; Disease management; Fruit; Fungi; Pathogen diversity; Subject Areas; cultivar/resistance; small fruits.