Comparison of two lettuce wilt diseases

By Mike Matheron, UA Extension Plant Pathologist

Fusarium wilt of lettuce, caused by the fungus Fusarium oxysporum, f. sp. lactucae, was first detected on lettuce in Arizona during the 2001-02 growing season. It continues to be found in lettuce fields from mid-October through early January.

There is another wilt disease of lettuce, called Verticillium wilt caused by the fungus Verticillium dahliae, which has occurred in the Salinas Valley since 1995 but has not yet been found in Arizona.

The primary symptoms of each disease are similar and consist of internal discoloration of the root cortex and plant wilting followed by death.

The internal root discoloration ranges from green, brown, and black in plants infected with Verticillium and reddish-brown to black in plants infected with Fusarium.

Since symptoms of each wilt disease are similar, true identity only can be achieved by bringing symptomatic lettuce plants to the UA’s YAC. The causal pathogen can be isolated from infected root tissue and identified by microscopic examination.

These wilt pathogens are soil inhabitants which can persist for many years. Both pathogens also can be seed-borne.

The lettuce Fusarium pathogen, Fusarium oxysporum, f. sp. lactucae, can only infect and cause disease on lettuce, although it may sustain itself on roots of other plants without initiating disease.

Verticillium dahliae, in comparison, can infect and cause disease on numerous crops other than lettuce.

Management strategies for diseases caused by Fusarium oxysporum and Verticillium dahliae are similar. Genetic resistance, when available in host crop plants, can provide effective disease control. Soil fumigation and soil solarization can reduce disease levels by lowering viable populations of both pathogens in the soil.

No known fungicides provide effective control of diseases caused by Fusarium oxysporum or Verticillium dahliae.

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