By Mike Matheron, UA Extension Plant Pathologist

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. The first confirmed appearance of Fusarium wilt on lettuce was recorded the week of Oct. 11.

The primary diagnostic features of this disease include yellowing of leaves, plant wilting, and a brown-to-black necrosis of the internal taproot and crown tissue. 

Disease incidence can range from a few plants 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, bring plant samples to Mike Matheron at the Yuma Agricultural Center (YAC) in Yuma for analysis. The 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 the planting date and the type of lettuce grown. The main determinant of disease severity with respect to the planting date is soil temperature.

Experimental data demonstrated that lettuce planted in early September resulted in high levels of Fusarium wilt. Plantings in mid-October or early December in the same naturally-infested field sustained moderately low and trace levels of disease, respectively.

Of the many crisp head and romaine cultivars tested, crisp head cultivars generally were significantly more susceptible to Fusarium wilt than romaine lettuce.

The lettuce Fusarium wilt pathogen can survive in the soil for many years so minimizing the spread of infested soil within and especially between fields is of paramount importance. 

Two comprehensive research reports are available from Matheron concerning disease development and management of Fusarium wilt of lettuce.

Contact Matheron: (928) 726-6856 or matheron@ag.arizona.edu.