Fruits & Vegetables A SPECIAL GROWERS’ REPORT OF AG ALERT ® CALIFORNIA
A new strain of fusarium wilt disease was discovered in Oxnard strawberry fields in 2022, prompting researchers to scramble to develop resistant strawberry varieties and short-term solutions.
Remedies sought for new strawberry disease strain By Bob Johnson
planted strawberry field in Oxnard. The field was planted with strawberries considered resistant to the plant virus. But they were no match for the new disease strain, which wilted foliage, and deformed and discolored strawberry crowns. Once a strawberry plant is infected with fusarium wilt, fungicides will not help manage the disease. It eventually kills the plant. Henry said researchers screened almost 200 varieties of strawberries, “includng heir- loom and wild types, and none of them showed promising resistance to race 2.” He told the Strawberry Production Research Meeting in Watsonville in February that a study team is screening 300 wild and heirloom varieties in collaboration with UC Davis and the strawberry producer Driscoll’s. “We’re very hopeful at least one of these will provide a meaningful source of resistance,” Henry said, adding researchers will reach out to the National Clonal Germplasm Repository plant bank in Corvallis, Oregon, for potential breeds that may be resistant to the disease. While the process of identifying sources of resistance remains slow and painstaking, researchers have made progress helping growers protect against the new strain. Henry said polymerase chain reaction tests can be conducted on soil and plants to determine how much of the race 2 fusarium wilt pathogen is present.
A resistance-breaking strain of fusarium wilt discovered in five Oxnard area fields could upend the strawberry disease management program carefully crafted since soil fumigation with methyl bromide was banned. University of California plant breeders have 36 precommercial strawberry varieties with resistance to four soil-borne diseases—fusarium wilt, macrophomina charcoal rot, verti- cillium wilt and phytophthora crown rot, according to Mitchell Feldman, director-elect of the strawberry breeding program at the University of California, Davis. Fusarium and macrophomina first became problems after strawberry growers stopped fumigating their ground with methyl bromide and chloropicrin. Strawberry growers have had success preventing crop diseases that can be caused by the race 1 fusarium wilt strain. But an emerging variety of the disease—fusarium race 2—had devastating impacts in Oxnard strawberry fields in the fall of 2022. So far, researchers have been unable to find or develop strawberry varieties that are resistant to race 2. “It’s going to take time to breed resistance to race 2, but resistance to race 1 remains ef- fective in most fields,” said Peter Henry, a U.S. Department of Agriculture plant pathologist. “This is the new resistance-breaking strain we found a little over a year ago.” Henry is one of six authors of a study published last May in the journal Plant Disease that reviewed the impacts of the race 2 strain in an organic, summer-
See DISEASE, Page 7
6 Ag Alert May 1, 2024
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