The first symptoms
of fusarium wilt may occur as early as thinning, when some seedlings
wilt and die. Infected plants display a characteristic red-brown streak
extending from the upper taproot into the cortex of the crown. Older affected
heads exhibit a tipburn that may often be limited to one side of the plant.
Yellowing of leaves and a brown to black streaking of the foliar vascular
tissue is often present. Infected plants may be stunted or fail to form
a head. The cortex of the crown turns a reddish-brown in color, and vascular
darkening extends into the root tissue on the affected side of the plant.
At this time, the only effective control measure is to avoid planting
lettuce in infested fields. This soil-borne fungus can be transported
in contaminated soil and water. To minimize the spread of the pathogen
to clean fields, it is very important to avoid the movement of soil and
water from an infested field into a field free of the pathogen.