Continuing studies supported by the California Lettuce Research Board are accumulating data for development of an integrated disease management program for anthracnose, which struck Central Coast fields for a time this spring.
Steve Koike, Monterey County farm advisor, said outbreaks of the disease, which produces lesions on inner leaves of romaine and leaf lettuce, were severe in some fields from late March until early May and associated with cool temperatures and rain.
“It was more severe in the Santa Maria area and low to moderate around Salinas, it eased up when the rains stopped, and it had disappeared by May,” he said.
Frank Laemmlen, Santa Barbara County farm advisor, said the disease was found in many heads in the Santa Maria area, causing the stripping down of heads and in some cases abandonment of fields.
Noting that Quadris is approved for use against the disease on lettuce, Laemmlen said treatments must be made preventively, since prophylactic applications to do not prevent lesion formation and head loss.
Anthracnose, also known as ringspot and shothole, is caused by Microdochiuim panattonianum, a soil-borne fungus. Koike recalled it was particularly severe and prolonged during the 1998 “El Nino” spring, but infections have been comparatively mild since.
The CLRB has allocated funds for compiling research data on refined spray fungicide application programs, detection, how the disease develops, and tests for cultivars having resistance. This will be used for an integrated program to manage the disease.
Due to moderate rainfall and comparatively light pressure from the disease this year and the preceding two seasons, Koike's research plots, which were poised for any outbreak, had little to show.
Survive in soil
Ring spot occurs in most cultivars of lettuce, and romaine is particularly sensitive to it. It has the ability to survive in soil for as many as four years without a lettuce host.
It is conveyed by water from sprinklers or rain splashing from the soil onto lettuce leaves. Another source of inoculum can be diseased, undecomposed lettuce crop residue.
Koike said studies on the fungus at the University of California, Davis showed it has at least five races of varying severity against lettuce. Field and greenhouse trials demonstrated that Folicur and Quadris effectively control the disease.
Laemmlen said varnish spot disease also caused more losses than usual in lettuce during the spring. The disease, caused by Pseudomonas cichorii, may not show symptoms until near harvesttime when several leaves of a head have shiny black necrotic spots, freckles, or blotches.
Little is known about how it spreads, but varnish spot is associated with sprinkler irrigation or rains. Other than avoiding sprinkling lettuce after thinning, no management strategies are known at this time, Laemmlen said.