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 and continues to be found in lettuce fields, mostly from mid-October through early January. There is another wilt disease of lettuce called Verticillium wilt, caused by the fungus Verticillium dahliae. This disease has occurred in the Salinas Valley since 1995 but has not yet been found in Arizona. The general symptoms of each disease are similar, consisting of internal discoloration of the root cortex and plant wilting often followed by death. Specifically, internal root discoloration ranges from green, 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 Plant Disease Diagnostic Lab, where the causal pathogen can be isolated from infected root tissue and identified by microscopic examination. Both fungal pathogens are soil inhabitants and can persist there for many years. Both pathogens also can be seed-borne. The lettuce Fusarium pathogen can only infect and cause disease on lettuce, although it may sustain itself on roots of other plants without causing disease symptoms. Verticillium dahliae, on the other hand, can infect and cause disease on numerous crops other than lettuce. Management strategies for diseases caused by Fusarium oxysporum and Verticillium dahliae are similar. When available, genetic resistance 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 soil. Currently, no known fungicides applied after planting have consistently and effectively managed diseases caused by either Fusarium oxysporum or Verticillium dahliae.

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.