POSSIBLE NEW CROWN ROT OF LETTUCE

A potentially new crown rot disease of lettuce has been found in Monterey and Santa Cruz Counties. It was first noted during the spring of the 2000 growing season. First observed in fields of Romaine lettuce, the early symptoms consist of yellowing and wilting of the lower leaves on affected plants. As the disease progresses, plants become lopsided because one side of the developing head grows normally, while the diseased side is stunted and short. As plants mature, there is an overall stunting of the entire plant, and such plants eventually wilt and collapse. The aboveground symptoms are somewhat similar to lettuce drop (Sclerotinia minor, S. sclerotiorum), and grey mold (Botrytis cinerea).

A close examination of the basal stem and crown of affected plants shows distinct, sunken, black cavities on the crown and roots. This results in a weak root-shoot interface, and diseased plants can be broken off easily at ground level. The plant tissues in the black cavities are firm and lack any sign of fungal growth or spore bodies (a dry rot). These latter symptoms differentiate this disease from Sclerotinia or Botrytis, which both cause a slimy, wet, soft rot, and also usually show an abundance of fungus growth on the affected tissues.

The new disease has caused significant crop losses in a few Romaine fields. Disease incidence in affected fields ranges from less than 5% to up to 50%. Romaine lettuce has, to date, been the most severely affected, but the disease has also been found in iceberg and green leaf lettuce fields. Only a small number of fields are involved so far.

Monterey County farm advisor Steve Koike has isolated a gray-green sterile fungus from all diseased plant samples from all affected fields. Tests are now underway to identify this fungus and to establish if this fungus is indeed the causal agent.

If Santa Barbara/San Luis Obispo County growers/PCAs find lettuce showing symptoms similar to the above description, please contact the Cooperative Extension office (805.934.6240) immediately, so samples can be collected for identification. There is every indication that this disease is soilborne, so use good sanitary procedures when moving equipment, pipe, or harvesting machinery from field to field, and especially if you have machinery or pipe coming in from outside the Oceano, Santa Maria or Lompoc areas. Some growers /PCAs may go to Salinas to see the new disease. Pickup tires and field shoes should be thoroughly cleaned before you return to your own farm.

Information adapted from article by Steve Koike, Farm Advisor, Monterey County.

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