By Mike Matheron, UA Extension Plant Pathologist

Fusarium wilt of lettuce, caused by the fungu Fusarium oxysporum, f. sp. lactucae, was first detected on lettuce in Arizona during the 2001-2002 growing season. Fusarium wilt continues to be found in lettuce fields from mid-October through early January.

On the other hand, Verticillium wilt of lettuce, caused by the fungus Verticillium dahliae, was first detected on lettuce in 1995 in the Salinas Valley of California and continues to spread in that region. The disease has not yet been found in Arizona lettuce fields.

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 to brown to black in plants infected with Verticillium and reddish-brown to black in plants infected with Fusarium.

Since symptoms of both wilt diseases are similar, true disease identity only can be achieved by bringing symptomatic lettuce plants to the University of Arizona Yuma Agricultural Center where the causal pathogen can be isolated from infected root tissue and identified by microscopic examination.

Both wilt pathogens are soil inhabitants which can persist in the soil 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 dahlia 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 fungicides are known to provide effective control of diseases caused by Fusarium oxysporum or Verticillium dahlia.

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