May 12, 2022
Breeding cucurbits for disease resistance sought

Phytophthora fruit rot and downy mildew have been identified by cucumber commodity groups as the disease priorities for researchers developing resistant lines.

That message has been carried to genetic researchers across the country through USDA’s Specialty Crop Research Initiative Cucurbit Coordinated Agricultural Project (CucCAP).

Rebecca Grumet, professor in Michigan State University’s (MSU) Department of Horticulture, is CucCAP’s project director and works on finding resistance to phytophthora fruit rot in cucumbers. She talked about the process of selecting for disease resistance in the pickling cucumber session of the 2021 Great Lakes Fruit, Vegetable & Farm Market EXPO held in Grand Rapids, Michigan.

Finding resistant genes in cucumbers is a worldwide search with cucumbers grown on nearly 5 million acres in more than 130 countries.

Rebecca Grumet
Rebecca Grumet works on finding resistance to phytophthora fruit rot in cucumbers.

“There’s a lot of variation in what these pickling cucumbers look like around the world,” Grumet said.

The original plant carrying the resistant gene(s) may not look anything like a modern pickling cucumber.

China, by far, grows more cucumbers than any other country in the world. Turkey and Russia are also leading producers. A challenge is that in many local geographies, cultivars evolved with narrow genetic bases and limited capacity to improve through crossbreeding. The demand for uniformity in plant type, fruit type and flowering time is often accompanied by a need for uniformity in disease resistance. This also restricts genetic diversity.

The biology of cucurbits is another challenge. Cucurbits are insect pollinated and can outcross with other large, spreading vines, which can make it difficult to regenerate and maintain genetic lines.

This makes a primary CucCAP objective the collection and analysis of biological data – and, more specifically, the genetic data – of cultivars.

“We have to test the cucumbers for sources of resistance,” Grumet said.

Researchers then identify, map and develop markers for the resistance genes. These molecular marker tags can track genes for specific traits at numerous locations throughout the genome.

“There are regions on the chromosome that show resistance,” Grumet said. “These are the parts of the chain we want to move. We want all of the regions that are important for phytophthora fruit rot to be with the regions that are important for downy mildew.”

Identified resistances must then be introduced into advanced breeding lines, and then the lines evaluated. MSU researchers grow their breeding lines on trellises even though it’s not how cucumbers are commercially grown.

“They’re grown on trellises so there’s no accidental disease inoculation in the field or contamination by mud,” Grumet said.

The fruit is harvested, brought into the lab and inoculated with the target pathogen. The process is slow, but researchers have identified a promising cucumber line from Turkey that has the potential for resistance on multiple chromosomes.

CucCAP has several teams, each with multiple investigators. Yiqun Weng is CucCAP’s team leader for cucumber and is the principal investigator on downy mildew resistance in cucumbers. Weng is with the USDA Agricultural Research Service (ARS) at the University of Wisconsin.

Michael Mazourek of Cornell University is the team leader for squash and pumpkins. Commodity groups have identified phytophthora fruit rot and powdery mildew to be the disease priorities for these crops.

For watermelon, commodity groups have determined fusarium wilt, gummy stem blight, phytophthora rot, powdery mildew, papaya ringspot virus and cucumber green mottle mosaic virus to be the disease priorities. Amnon Levi of USDA-ARS in Charleston, South Carolina, is the team leader for watermelon.

The priorities for melon are powdery mildew, cucumber mosaic virus and cucurbit yellow stunting disorder, and Jim McCreight of USDA-ARS in Salinas, California, is the team leader.

Zhangjun Fei of Cornell University is CucCAP’s team leader for bioinformatics. Bioinformatics is the science of collecting and analyzing complex biological data, such as genetic codes.

Lina Quesada-Ocampo of North Carolina State University is the team leader for integrated disease management. This team seeks to provide timely disease control information and incorporate new genetic materials into disease management programs.

More information on CucCAP and the latest information on developing disease resistance in cucurbits can be found at https://cuccap.org/.




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