The Search for Resistant Cultivars

By Emily McKay Johnson, Associate Editor

In the second part of our series with Steve Koike, plant pathology farm advisor for UC Division of Agriculture and Natural Resources, Cooperative Extension in Monterey County, the focus is on the critically needed research on resistant cultivars. Koike, who has focused his research since 1989 primarily on the understanding of disease systems and the investigation of new methods of disease control, said, “The role or the need for resistant cultivars is tremendous.”

“Some good case studies of resistant cultivar research address soil-born problems on lettuce,” Koike explained. “For example, Fusarium wilt and Verticillium wilt on lettuce could be managed lightly; but in order to overcome those diseases, resistant lettuce varieties need to be in place.”

Steve Koike, UC Cooperative Extension, Monterey County.

“First discovered in 1995 on the Central Coast,” said Koike, “Verticillium wilt has the potential to infect and damage numerous different crops. And although Fusarium wilt was typically unseen in the San Joaquin Valley, it has recently begun to appear on the Central Coast.”

Continued improvement of management techniques upon discovering initial disease symptoms is necessary, according to Koike. “Symptoms of the disease usually appear on the lower leaves of plants, around the edges, and the areas between the veins can turn a yellowish-brown.”

“Strawberries currently have three really important disease pressures state-wide: Verticillium wilt, Macrophomina (charcoal rot) and Fusarium wilt,” Koike commented. “Even the fumigation tools we have are not cleaning them up 100 percent, so we have problems.”

“We will continue to have problems,” Koike elaborated, “until there are truly resistant strawberry varieties to those pathogens. Plant breeders understand that IPM management of these diseases is so dependent on developing resistant varieties,” he said, “but we’re not there yet. We do not yet have truly resistant lettuce or strawberry varieties out in the field,” Koike said.

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Koike and his fellow researchers discovered a new race of the downey mildew pathogen in spinach that has been designated race 16. While there are some varieties that are supposedly resistant to race 16, Koike noted that there is still more research to be done.

To read the first part of our series on Downey Mildew, click here.