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Making hay

CROOKSTON, Minn. - Northwestern Minnesota producers looking for tools to help them produce bountiful alfalfa crops this year learned the latest news on pests and diseases at the University of Minnesota's recent Crops College.

CROOKSTON, Minn. - Northwestern Minnesota producers looking for tools to help them produce bountiful alfalfa crops this year learned the latest news on pests and diseases at the University of Minnesota's recent Crops College.

U of M extension plant pathologist Charla Hollingsworth, working with Deb Samac, a research plant pathologist at the USDA-ARS plant science research unit in St. Paul, is keeping tabs on a slow-progressing but potentially damaging disease.

Brown root rot

"We've been working on brown root rot for four years now," Hollingsworth says.

She first noted the disease in Minnesota's Red Lake and Pennington counties in 2002 and soon enlisted Samac's help to study and track it.

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"We're trying to figure out what crops can be used in rotation with alfalfa to lower the inoculum level and also how well it survives over the wintertime," Hollingsworth explains.

She says between her test plots and Samac's farther south, they're able to gauge the disease's response to a variety of climate conditions.

Brown root rot is caused by the phoma sclerotiodides pathogen, which lives in the soil and can fester in fields where alfalfa is grown again and again. Above ground, plants with brown root rot will be slow to green up in the spring and may suffer from winterkill.

Below ground, brown root rot produces dark, dry, flaky lesions on the plant's roots, gradually cutting off its nutrient supply.

"We're dealing with a pathogen that's very slow growing, and for the first three years, it doesn't generally kill plants unless there's an epidemic," Hollingsworth explains.

She says the University of Wyoming has developed an experimental alfalfa line that seems to show some resistance to brown root rot, but since the disease develops so slowly, it may be quite some time before the new line proves itself.

Crown rot

Crown rot tends to be more aggressive in alfalfa, but it also is proving to be a research challenge.

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"It is a complex disease, so the resistance would have to be for at least four pathogens," says Hollingsworth. "That's a hard one to look at."

The disease often enters a plant when it's been damaged by machinery wheels or animal hooves. Once infected, symptoms below the ground include wedge-shaped lesions on the plant crown and sometimes a hollow crown core. Above ground, plants with crown rot will be lopsided or have stunted growth, and wilting may occur.

Since alfalfa collects carbohydrates and proteins through its crown, the disease can reduce crop quality.

Alfalfa weevils

In addition to disease, alfalfa fields are prone to insect infestations, according to regional extension crops educator Phil Glogoza.

"We have a lot of insects that show up in an alfalfa field," he says. "It's an excellent field laboratory for entomology."

The two that present the biggest problems for alfalfa producers are each less than an inch long, but capable of taking a significant bite out of yields and productivity.

"Alfalfa weevil is the No. 1 thing," Glogoza says.

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The pests often winter in plant material left on the field and produce one generation of offspring per year.

""You're going to see the adults come out in the spring, laying eggs, and you're going to see more adults in midsummer, but the larvae are the ones doing all the harm," he warns. "That's what's going to defoliate the plant."

Glogoza says female alfalfa weevils chew a hole in the plant stem, then back into that hole to deposit her eggs. The larvae hatch and, usually by late May, begin feeding on the plant material.

"When they feed, they're going to be up in the tips, the growing points of the plant," Glogoza says. "They like to feed on those leaf buds that haven't expanded yet. They like to be in there defoliating those young leaf buds that haven't unfurled yet."

Take time to count

Glogoza recommends checking fields during the larvae stage by using a sweep net, or pulling a few stems and counting the larvae, then comparing those numbers with published threshold information.

If a producer doesn't collect and count alfalfa weevil larvae, but waits until the field shows signs of damage, "you're already too late," Glogoza says.

"The alfalfa field will take on a 'whiteish' cast, like it's been nipped by frost. If you take a closer look at the field, you'll see the ragging of the leaves. They'll look like lace, or kind of be eaten from the edge."

Glogoza says the first cutting of alfalfa is typically most vulnerable to weevil damage, though weather is a big factor in determining when the larvae will hatch and start feeding. He says timing will influence a grower's course of action as the crop matures.

Timing is everything

"If you're at flowering and you're thinking about cutting, what happens if you've got a lot of larvae in there?" he asks. "You're going to think, 'well, I need to spray it, but I also need to cut it.'

"The thinking is, cut it," he advises. "Get your yields out of there, don't let the weevils get it, but don't spray it so that you'll have to wait 10 to 14 days because of the spray. You're much better off to cut it, get your hay, then assess how well those larvae survived."

Glogoza says larvae that survive the cutting often hunker down under the swath. Once that it is baled and picked up off the field, some farmers hook up a spray boom to their four-wheeler, targeting the swath rows.

"You can just spray that small area, and it's less expensive than treating the whole field," Glogoza says, reiterating the need to keep tabs on larvae counts throughout the early season.

"There's a lot of nice things you can do to manage this insect."

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