Symptoms and Signs
Varnish spot affects only the inner leaves of head lettuce varieties. To see the symptoms, remove the outer leaves of the head. Varnish spot appears as dark brown, shiny, firm, necrotic lesions on these inner leaves. Lesion size can range from small to very large. Lesion borders are not delimited by veins, but areas along veins are most often affected. In severe cases, an infected leaf may have 90% or more of its surface area diseased. A notable feature of these lesions is that affected leaf tissues are initially intact; however, as the lesions become old, they can be soft or broken down due to the activity of secondary soft rot bacteria.
Comments on the Disease
This disease occurs in places where Pseudomonas cichorii contaminates water in reservoirs. When such water is used to sprinkle irrigate head lettuce crops at the rosette stage, the bacteria are introduced into the developing head. There is some evidence that P. cichorii also survives for brief periods in the soil and is splashed up onto plants via sprinkler irrigation or rain. Varnish spot does not develop on cultivars that form an open head such as romaine and leaf types.
Avoid using contaminated reservoir water when sprinkler irrigating head lettuce at susceptible stages. Rotate away from susceptible crops (e.g., chicory, endive, cabbage, cauliflower, and celery) for at least 1 year.