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 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 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 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 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, 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. 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. On the other hand,
no known fungicides applied after planting have consistently and effectively managed
diseases caused by either Fusarium oxysporum or
Verticillium dahliae.
Fusarium
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