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On October 8th we received our first confirmed sample of Fusarium wilt on lettuce
for the 2014-15 production season. Since the first discovery of this disease in
Arizona during the 2001-02 growing season, Fusarium wilt has been found yearly in
some lettuce fields from mid-October through early January. The initial visual indication
of the disease is yellowing of one or more older leaves, followed by leaf wilting
and eventually plant 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,
Fusarium oxysporum, f. sp. lactucae, from symptomatic root tissue.
Disease incidence and severity is 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 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. 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 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.
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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