What researchers call “a potentially new crown rot disease of lettuce” turned up in Monterey and Santa Cruz counties in 2000 and again in 2001 and is under study to determine its cause and control.
Steve Koike, Monterey County farm advisor, and Krishna Subbarao, University of California plant pathologist at Salinas, plan fungicide trials on the early-spring disease that shows symptoms of stunting, wilting, and collapsing similar to Sclerotinia minor and Botrytis cinerea.
In a recent progress report in Seaside to the California Lettuce Research Board, which is funding their studies, Koike said the disease goes mostly to romaine but it has been found on individual plants of iceberg and leaf varieties. In some romaine fields it occurred only occasionally, while in others 75 percent of the crop was infected 10 days before harvest.
Plant base as key
“Since it resembles other diseases, it is important to look closely at the base of the plant to determine the problem,” Koike said.
One sign is dark brown, dry, sunken lesions at the base of the plant. Even though foliage may show little yellowing, the plant will break off easily when struck. Infected plants show a lop-sided growth as the diseased side lags in development.
Koike and Subbarao recovered a gray-green fungus, possibly a species of phoma, from samples at all locations but did not find the common pathogens sclerotinia, botrytis, rhizoctonia, or verticillium. Potted laboratory plants exposed to the recovered isolates later developed typical symptoms found in the field.
“One important pattern we've seen the last two seasons,” Koike said, “is the disease occurs in early spring through early summer. By June and July, it is hard to find fields with this problem, even fields that had it in the preceding spring were planted again to romaine. That argues for a climatic factor.”
In another project for the research board, Koike and Subbarao evaluated fungicides for Botrytis crown rot, another disease that strikes mainly at early-season romaine transplants, during the 2001 season.
Botrytis cinerea, which lurks in crop debris, numerous crops, and the soil, is thought to be carried to damage lettuce tissue in an airborne form.
The pathogen ignites with cool temperatures and free moisture and seeks tissue that is wet or in contact with the soil. A rot sets in at the crown and advances to the main stem and attached leaves. Severely infected plants become girdled at the crown and collapse.
Although several fungicides show promise, BAS 516 and Botran showed the most consistent control of the disease in their trials. Results were better when transplants were drenched, rather than sprayed twice in the field later.