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Bacterial leaf stripe research moves to the front burner

Bacterial leaf stripe outbreaks in Red River Valley spring wheat are becoming enough of a concern that University of Minnesota extension plant pathologist Charla Hollingsworth and two of her colleagues are applying for state research funding to s...

Bacterial leaf stripe outbreaks in Red River Valley spring wheat are becoming enough of a concern that University of Minnesota extension plant pathologist Charla Hollingsworth and two of her colleagues are applying for state research funding to start identifying resistant wheat varieties.

Also referred to as black chaff, the disease can cause significant yield losses if it becomes severe early enough, according to information from the Northwest Research and Outreach Center in Crookston, Minn. Yield losses as high as 40 percent have been reported in Idaho, though the typical loss is about 10 percent or less.

Hollingsworth's concerns are twofold. Because the disease is bacterial, rather than fungal, it is unaffected by fungicides. It also seems to have dropped anchor in the Red River Valley.

"We see it at some level every year," she says. "In 2005, we had an epidemic, but we also had an epidemic of Fusarium head blight that same year, so it was nigh impossible to separate the loss from one disease from the other."

Bacterial leaf stripe was reported along much of the Red River Valley, stretching from the Canadian border down to the Morris, Minn., area, some 250 miles away.

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It also was detected in North Dakota and South Dakota. Pest management scouts for North Dakota State University reported bacterial leaf stripe in 25 percent of the wheat fields scouted in east-central and southeastern North Dakota.

Another epidemic occurred last year, this time without other diseases emerging to mask its effects, she says.

"It became apparent that there were losses, and that's when this became more of a front-burner issue than a back-burner issue," Hollingsworth says.

In 2008, crop loss was observed by growers from as far north in Minnesota as Roseau County down 200 miles along the Red River Valley and into the Fergus Falls, Minn., area. It did not, however, reach farther south into the Morris area as it did in 2005.

In the field

Bacterial leaf stripe is hard to see and hard to recognize, Hollingsworth says. Symptoms generally appear during the heading growth stage.

"It starts out as a water-soaked strip that's delimited by the veins," she says. "As the severity increases and the disease develops, it turns into a blotch, so now you have water-soaked leaves when there's dew on them. They look water-soaked."

When the leaf dries, it has a glazing on the surface that looks like glazing on a doughnut, she says.

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"That is the bacterial layer that's dried on the leaf surface. There's so many bacteria, you're seeing it," she says.

When that leaf gets wet again and the bacteria reconstitutes, it can become mobile. The bacteria can move from leaf to leaf, plant to plant, field to field.

"It has to be rubbed and transported by something, whether it's rain splash, insects, tillage equipment or wind," she says. "If the variety is susceptible and the weather supports the disease, chances are the leaf is doomed."

It is soil- and residue-borne as well, so year-to-year outbreaks are more likely, though crop rotations can hinder the return and spread of the disease. Seed infection also is a concern for growers who are fighting recurring outbreaks.

Resistance

Certain spring wheat varieties may be more resistant to bacterial leaf stripe, though the research to discover them has yet to be conducted. Granite hard red is one that seems to have been susceptible, according to the Northwest Research and Outreach Center.

"We have seen a couple susceptible varieties percolate to the top over the last couple of years that we've had these epidemics," Hollingsworth says. "But we don't know if there are resistant varieties because we don't know if they are resistant or if they are just escaping the disease."

There currently are no chemical applications available to fight it. Hollingsworth did test a bactericide against bacterial leaf stripe last year, but those results were "fairly inconclusive," she says. Besides, bactericides are few, expensive and controversial.

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Hollingsworth thinks that approaching the problem as a resistance challenge will bear more fruit. She and has applied for a grant with the Minnesota Small Grains Initiative to do so.

"If we get funded, it will be for two years, and our whole purpose will be to identify how we can inoculate and produce disease in the greenhouse and the field," she says.

This will allow the scientists to set up disease nurseries and start screening germplasms and the commercial varieties to find which ones are resistant and which ones are susceptible.

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