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Cucumbers: Hunting Down Pathogens and Fighting Back

Michigan’s multimillion dollar cucumber crop has two enemies: Phytophthora capsici and downy mildew.

These create significant problems with growing cucumbers and all vegetables in the cucurbit class, such as watermelon, cantaloupe, gourds, squash and zucchini. “These two problems — Phytophthora and downy mildew — truly threaten the viability of growing cucumbers in Michigan. They could take out the industry,” said MSU AgBioResearch scientist Mary Hausbeck, who has worked effectively to help growers understand and control these two diseases for more than 20 years.

One of her research team’s first discoveries was that Phytophthora was resistant to Ridomil, a key fungicide that used to be effective at controlling the fungus. To help cucumber growers with this problem, Hausbeck, a professor of plant, soil and microbial sciences, and her research team developed best growing practices, including subsoiling, raised beds, drip irrigation and early destruction of infected crops.

Downy mildew entered the Michigan vegetable scene in 2005. It is a water mold pathogen that affects cucurbits. Before that time, cucumbers had strong genetic resistance to downy mildew, which has since returned every year to trouble growers. 2014 proved to be a very difficult year for growing cucumbers and other cucurbits because the wet, cool weather promoted downy mildew.

One aid for growers with downy mildew is spore trapping, which Hausbeck’s research group has done for many years to alert growers when the pathogen is present in the state. Growers use this early warning system to time their crop protection programs, saving money and helping to prevent fungicide resistance.

“This is a huge effort for our lab,” Hausbeck said. “But it has been accurate each year in getting the warning out to growers.”

MSU researchers are also looking for long-term solutions to Phytophthora and downy mildew. MSU AgBioResearch scientist Rebecca Grumet is screening for sources of Phytophthora resistance that could be useful for breeding. In an initial screening round, Grumet discovered an age-related resistance.

“The young fruit are very susceptible, but as the fruit develops it becomes resistant to Phytophthora,” explained Grumet, a professor of horticulture. That fact allowed growers to be better informed about when to use sprays to protect the fruit.

Since then, Grumet’s research team has identified other germplasm with distinct patterns and changes in gene expression as fruit goes through stages of early development. Her research group has also identified three cucumber accessions (unique identifiers) that may serve as sources of resistance in young fruit.

“These are important steps forward in helping to eventually pinpoint individual genes that may lead to conventional breeding for resistance or other resistance mechanisms,” Grumet said.

Futures, Fall 2014

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