April 2013
Fighting Late Blight By Everett Brazil III, Spudman correspondent

Late blight isn’t an annual problem for Wisconsin growers, but it does make a presence in potato fields occasionally. Researchers at the University of Wisconsin-Madison (UW-M) have recently seen new strains of the disease developing in the state, prompting them to undertake a comprehensive study to find resistance before it becomes a serious epidemic.

Amanda Gevens, UW-Extension potato and vegetable pathologist, is leading a project on pathogen tracking and characterization of late blight.

We had late season detection in 10 counties, but the incidence was very low. So while it was present, we didn’t have a severe problem of late blight across the entire state,” Gevens said about the 2012 growing season.

Wisconsin grows about 63,000 acres of potatoes annually, making it the largest value crop in the state. That contrasts with tomatoes, which account for about 4,000 acres. Both crops can influence the disease, Gevens said, and once in the fields, the pathogen can remain active and potentially cause problems for several seasons.

“History has shown us when we have late blight problems in one year, it lingers in the system for a subsequent year. It could be in volunteers, cull piles or seed potatoes,” Gevens said. “We’re not seeing the same strains which predominated in past years, and what this is telling us is that we have multiple sources of pathogen coming into the production fields.”

Researchers have identified three different late blight strains in the past four years in Wisconsin: US 22, US 23 and US 24. Only US 23 was identified in the state for 2012.

The strain was first seen in the northwest corner of the state, but slowly moved eastward to larger potato concentrations as the weather changed at the end of the summer.

“The strains we have now are new. As such, scientists need to continue to study this pathogen to further enhance management,” Gevens said. “Late blight management is multifaceted and can be expensive; our producers have done a great job of management.”

The new strains make it difficult to combat the pathogen, so researchers are focusing on building resistance in new potato cultivars. Dennis Halterman, a USDA-ARS geneticist at the Vegetable Crop Research Unit in Madison, is conducting his work in conjunction with UW. Halterman is studying the disease at the molecular level to find what genes make some potatoes resistant, and using them to create new varieties.

“When pathogens interact with plants, they use an arsenal of molecules they insert into the plant cell to modify it,” Halterman said. “The new gene recognizes only one of these and can respond by cellular suicide and stops the pathogen from getting nutrients. It’s like an alarm system, and the plant can respond in ways that prevent the pathogen from spreading.”

All varieties currently grown in the United States and Canada are susceptible to the disease, Halterman said. The research team is looking beyond U.S. borders for varieties that are naturally resistant. Their search has taken them to Mexico because of a large variety of potatoes and the disease is present south of the border, providing a fertile ground for research.

“Most of the good resistance people have identified came from Mexico and that’s because there is the highest development of the pathogen, and they differ in all those molecules they inject into the plants,” Halterman said. “When we get a new strain of the pathogen in the U.S., we think they come out of Mexico, or it changed in the U.S.”

One gene that has shown resistance promise is RB. Four potato varieties currently contain the gene and it has been tested in Mexico, where it seems to show positive results in both field and greenhouse systems. The researchers are looking at new ways to introduce the gene, including inserting it into new varieties that can then be crossbred with conventional cultivars.

“We’re trying to give the breeders something to cross. We’re trying to use multiple resources to introduce resistance into potatoes,” Halterman said. “We’re using both conventional and biotech methods at this point. We’re just trying to figure out how resistance is working.”

Researchers are also looking at a wild cultivar, Solanum verrucosum, which is resistant to the disease. Researchers are crossing it with susceptible varieties for a more natural means of resistance. It has shown the same effects as the RB gene, but may be more viable in breeding efforts.

“We can use this species to cross with cultivars and other wild potatoes better than varieties with the RB gene. That makes it a lot more useful as we move forward with germplasm development,” Halterman said.

Even as research continues to work toward a solution to the new strains the pathogen continues to alter and develop its own resistance to the resistant genes and rendering them useless. There may come a day when even RB is no longer resistant, which is why search for resistant varieties seems to never end.

“What is happening is the pathogen is very good at what it does and overcomes the resistance very quickly, so we’re looking for longer resistance,” he said. “We have the deck stacked up against us, but we’re making do with what we’ve got, and if we can reduce fungicide use, that’s something, at least,” Halterman said.

The disease remains a serious concern for Wisconsin growers, but having active late blight research projects in their backyard could be a boost because it will allow the new cultivars to be tested in their fields, putting the state on the map for relief of late blight woes.

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