Since the first detection of Fusarium wilt on lettuce in Arizona during the 2001-02 growing season, the disease has been found yearly in lettuce fields from mid-October through early January. This year is no exception, as the first confirmed appearance of Fusarium wilt on lettuce for the current growing season was recorded this week. The initial visual suggestion of the disease is yellowing of one or more older leaves, followed by leaf wilting and death. The external root surface is unaffected; 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. Plants can become infected and display symptoms at any age, ranging from very young plants just after thinning to those ready for harvest. The symptoms of Fusarium wilt resemble two other lettuce disorders, ammonia toxicity and the early stages of lettuce drop. To confirm disease identity, it is necessary to bring plant samples to me at the Yuma Agricultural Center for analysis. Confirmation of disease identity is achieved by isolation and identification of the causal fungus of Fusarium wilt of lettuce, Fusarium oxysporum, f. sp. lactucae, from symptomatic root tissue. Disease development is strongly affected by planting date and 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 early 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 cultivars tested, crisphead cultivars generally are significantly more susceptible to Fusarium wilt than romaine lettuce. 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 comprehensive research reports concerning disease development and management of Fusarium wilt of lettuce are available. Please contact me and I will email these reports to you.

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To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.