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Since the first discovery of Fusarium wilt of lettuce in Arizona during the 2001-02 growing season, the disease usually has been detected from late September through early January and occasionally at the end of the production season. Initial visual indications of the disease are yellowing of one or more older leaves and stunting of infected plants. These symptoms are usually followed by leaf wilting and can culminate in plant death. The external root surface is not affected; however, a brown to black necrosis of the internal taproot and crown tissue will be apparent. Disease incidence can range from a few plants up to large areas or zones of infected plants within a field. Symptoms can first appear on plants at any age, ranging from very young plants just after thinning to those ready for harvest. Symptoms of Fusarium wilt resemble two other lettuce disorders in Arizona, ammonia toxicity and the early stages of lettuce drop. To confirm disease identity, it is necessary to bring plant samples to The University of Arizona Yuma Agricultural Center Plant Disease Diagnostic Lab for analysis. Confirmation of disease identity is achieved by isolation and identification of the causal fungus, Fusarium oxysporum, f. sp. lactucae, from symptomatic root tissue. Disease incidence and severity are strongly affected by planting date and the type of lettuce grown. The main determinant of disease severity with respect to planting date is soil temperature. Research data demonstrate that lettuce planted in September can result in high levels of Fusarium wilt, whereas plantings in the same naturally-infested field started in mid-October or early December sustain moderately low and trace levels of disease, respectively. Of many crisphead and romaine varieties tested, crisphead varieties generally are significantly more susceptible to Fusarium wilt compared to romaine lettuce. There are also significant differences in susceptibility among romaine cultivars. The lettuce Fusarium wilt pathogen can survive in soil for many years, so minimizing the spread of infested soil both within and especially between fields is of paramount importance. Two field trials have been established this year in mid-September within growers’ fields to evaluate some current lettuce varieties, experimental lines, fungicides and biofungicides for potential efficacy in managing Fusarium wilt of lettuce. The objectives of ongoing field trials are to find promising genetic resistance or tolerance to Fusarium wilt within one or more tested lettuce varieties as well as to identify products that can help reduce the incidence and severity of the disease. For more information about Fusarium wilt of lettuce, you are invited to consult the publication entitled “Biology and Management of Fusarium Wilt of Lettuce”
To contact Mike Matheron go to: matheron@ag.arizona.edu.
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For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
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