Diseases of lettuce (
Fusarium wilt
Fusarium wilt of lettuce is caused by the fungus Fusarium oxysporum f. sp. lactucae. The first symptoms of Fusarium wilt may occur in seedlings that wilt and die. Mature plants have a characteristic red-brown streak extending from the upper taproot into the cortex of the crown (photo 1). Older affected heads exhibit a tipburn that may often be limited to one side of the plant (photo 2). 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. Infected lettuce is not marketable.
Fusarium wilt of lettuce was first identified in central California in 1990. It was observed in commercial lettuce fields in Yuma in fall 2001. The pathogen, Fusarium oxysporum f. sp. lactucae, 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. Measures include removing soil from tractors, implements, harvesting equipment and even workers’ boots.
At this time, the only effective control measure is to avoid planting head lettuce in infested fields for at least several years. In general, romaine lettuce varieties are more tolerant than head lettuce. Tolerance in head lettuce is the focus of on-going research, but degrees of susceptibility have been found. Results of recent trials can be found at: http://cals.arizona.edu/pubs/crops/az1348/az1348_3d.pdf.
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March 31, 2009