2019 ASHS Annual Conference

Beginning in the spring of 2017, lettuce plants symptomatic of Fusarium oxysporum f. sp. lactucae (Fol) were detected for the first time in iceberg lettuce production fields of the Everglades Agricultural Area. Affected plants were chlorotic, wilted, and displayed vascular discoloration along the taproot when sectioned. Plant samples were obtained from four infected fields and isolates were cultured on Komada’s semi-selective media. Seventy-eight Fol isolates were harvested and used for pathogenicity testing on the susceptible iceberg cultivar ‘Chosen’. Koch’s postulates were fulfilled by re-isolating the pathogen from diseased plants on Komada’s media and verifying conidia morphology microscopically. Four disease rating experiments were also conducted by planting differential cultivars in infected fields to determine the pathogen’s race. Fifty-two of the isolates caused disease in pathogenicity tests, twenty-nine of which were re-isolated to confirm Koch’s postulates in 37% of the original isolates. The differential experiments showed a significantly lower disease incidence and severity (P<0.05) between the Race 1 resistant cultivar ‘Costa Rica 4’ and the Race 1 susceptible cultivars ‘Patriot’ and ‘Banchu Red Fire’. The results from our experiments show evidence for a new occurrence of fusarium wilt of lettuce Race 1 in Florida and these results are also being confirmed using Race 1-specific PCR primers. This information will help manage a new threat that could jeopardize around 15,000 acres of lettuce planted just south of Lake Okeechobee, as has occurred in many other lettuce-growing regions worldwide.