Drip irrigation can reduce mildew threat for lettuce By Bob Johnson
From the day a downy mildew spore lands on a lettuce leaf, growers have a brief window to take action to reduce crop damage caused by the next gener- ation of spores. Once mildew infects the leaves, noth- ing can prevent cosmetic damage that makes lettuce or spinach unfit for market. The disease spreads quickly because each generation produces many air- borne spores that can move the infection throughout a field. “These airborne spores can be car- ried by wind; they land on a healthy leaf and penetrate it,” said Alex Putman, University of California Cooperative Extension assistant plant pathology specialist. He discussed the downy mil- dew threat during an online seminar last month, part of the UC Ag Experts Talk series. Spinach and lettuce growers contend with this disease every season, and they have made downy mildew a priority is- sue. They financed numerous studies on the disease, including some that were shared in the recent 2022 Leafy Greens Research Conference held in Pismo Beach. Variety resistance is the brass ring of control tools because it is the easiest,
The climate in the Central Coast region is ideal for lettuce—and for development of diseases such as downy mildew. Researchers suggest new irri- gation strategies.
most economical and reliable way to manage downy mildew. But the disease shows a troubling ability to evolve and confound efforts to maintain resistance, experts say. Seventeen races of spinach downy mildew have been identified and at least seven lettuce downy mildew, as the dis- ease has overcome varieties that were once resistant. Monitoring shows that spinach and lettuce downy mildew spores are in the air year-round in the Salinas Valley, as host crops and favorable weather for the
diseases are usually present. Unfortunately, the cool, moist climate that makes the Central Coast region ideal for lettuce and spinach is also favorable for the development of water molds such as downy mildew. While researchers are working fe- verishly to catch up with the disease by discovering new genetic sources of resis- tance, they are also developing new tools to monitor mildew and devising irriga- tion strategies to slow its spread. Drip irrigation on lettuce is a way to reduce the leaf wetness that makes for
mildew infection and spread. Spinach, however, is sprinkler irrigated in densely seeded 80-inch beds that can be machine harvested, which creates a microclimate ideal for downy mildew. Recent trials of organic spinach in the desert showed that drip irrigation can significantly reduce mildew pressure. “Downy mildew was two to seven times lower with drip than sprinklers,” Putman said. “Drip reduces direct wet- ting of the leaves.”
See MILDEW, Page 18
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10 Ag Alert June 8, 2022
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