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 this growing season was recorded last week. The primary diagnostic features of this disease include yellowing of leaves, wilting of the plant, and a brown to black necrosis of the internal taproot and crown tissue. 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 young plants just after thinning to those ready for harvest. The symptoms of Fusarium wilt resemble two other lettuce diseases, ammonia toxicity and the early stages of lettuce drop. To confirm disease identity, it is necessary to bring plant samples to Mike Matheron 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. Experimental data demonstrated that lettuce planted in early September resulted in high levels of Fusarium wilt, whereas plantings in mid-October or early December in the same naturally-infested field sustained moderately low and trace levels of disease, respectively. Of many crisphead and romaine cultivars tested, crisphead cultivars generally were 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. If you are interested, please contact me, Mike Matheron, for a copy of the reports.

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.