October 24, 2011
Increased wheat production in the past couple of years in the Southeast has brought with it a few pest problems that commonly can be found wherever wheat follows wheat in a farmer’s field.
Among these problems are the insect pest Hessian fly and diseases problems such as take-all and Fusarium head blight.
“There are insects that can over-summer in wheat fields, so if you grow wheat after wheat, you can run into certain problems,” says Kathy Flanders, Auburn University Extension entomologist.
“So crop rotation is always much better when growing wheat. It’s always better to grow wheat every other year. If you don’t follow a good rotation, the main insect pest is Hessian fly, and it helps to know what it is doing right now.”
Right now, says Flanders, Hessian fly is hanging out in the wheat crop from earlier this year, in the stubble and the top areas of the soil. “They are pupae that are about to hatch out as the first generation of Hessian fly.
“They’ll lay eggs on volunteer wheat, build their populations and be ready to lay eggs in your wheat crop. They over-winter in the previous year’s wheat fields, so if you plant wheat after wheat, you’re just making it easy for them,” she says.
The Hessian fly caused disastrous losses in Alabama wheat in the mid-1980s, says Flanders. In 1985, estimated losses were 21 bushels per infested acre. Since that time, later planting of wheat and the use of wheat varieties resistant to Hessian fly have minimized economic losses to this pest.
In coming years, however, entomologists expect the risk of economic losses from Hessian fly to be as high, or higher, than they were in the 1980s, she says.
Reasons for this include increased wheat acreage, supply shortages of Hessian fly-resistant wheat varieties, widespread occurrence of a strain of Hessian fly that overcomes resistance in most wheat varieties, increased conservation-tillage, and disrupted crop rotation schemes as farmers plant the crop that offers the best promise for an economic return.
A last and very important factor, says Flanders, is that phorate and di-syston insecticides no longer are used at planting to control Hessian flies.
“Historically, Hessian fly has caused the greatest problems in the southern two-thirds of Alabama,” she says.
There are four to six generations per year of this pest in the South, says Flanders. The entire life cycle requires about 35 days at 70 degrees F., longer at cool temperatures. Generations tend to overlap.
“The pest over-summers as flaxseeds in wheat stubble. The first generation in September is generally on volunteer wheat or on wild grass hosts, the most important of which is little barley. Then there are often two more generations in the fall and early winter,” she says.
Feeding by Hessian fly maggots permanently stunts vegetative tillers and can kill seedlings. If fall-infested tillers do produce grain heads, these heads are small and the stems are stunted.
Spring-infested plants have weakened stalks, which can lead to stem breakage and lodging. These plants also have poorly filled, smaller grain heads. Hessian fly reduces forage production of winter wheat, but does not greatly affect wheat forage quality.
The most reliable Hessian fly management strategy in the South is to plant varieties of wheat that are resistant to Hessian fly, says Flanders. Rye or oats may be a better choice for grazing as they are poor or non-hosts for Hessian fly, she says.
Burying stubble slows fly emergence
Burying wheat stubble where Hessian flies over-summer will reduce the emergence of Hessian fly adults.
Burning crop debris also has been recommended for Hessian fly control. However, burning destroys flaxseeds that are present in the straw, but does not kill flaxseeds that have fallen out of the crop debris onto the ground.
Controlling volunteer wheat well before planting is recommended, because Hessian flies are attracted to volunteer wheat when they emerge in September.
Wildlife plantings of wheat, which are often planted early, can be a source of Hessian flies.
Crop rotation helps reduce the chance of infestation by Hessian flies, says Flanders, but it is not infallible, because Hessian fly adults will fly up to a mile in search of host plants.
As far as rotations and diseases are concerned, the main problem with wheat following wheat is take-all disease, says Austin Hagan, Auburn University Extension plant pathologist.
“Historically, take-all has not been as big of an issue in Coastal Plain soils as it has been in the Tennessee Valley region of Alabama. It got to a point to where take-all was so bad growers basically didn’t make any wheat because of it,” says Hagan.
A poor rotation also could lead to fusarium head blight or scab, he says.
“This disease historically has been a problem in the Tennessee Valley where we’ve always had a lot of wheat production. It has been less of an issue in the central and southern parts of Alabama. Corn or sorghum in a rotation can bump up the risk a little more, but we have seen scab outbreaks in wheat behind cotton,” he says.
Losses to fusarium scab blight not only are due to sizable reductions in the germination, numbers, and test weight of seed from scab-blighted heads, but also from the production of mycotoxins by the causal fungus in diseased seed, explains Hagan.
Given favorable weather patterns, wheat drilled behind no-till corn is at highest risk for a destructive scab outbreak, he says.
A variety of control strategies can be employed to prevent scab outbreaks in production fields as well as minimize the risk of contamination of stored wheat and subsequent exposure of livestock to mycotoxins, says Hagan.
“Growers should purchase registered or certified, fungicide-treated wheat seed or apply a hopper box fungicide seed dressing to bin-run wheat seed. Also, avoid sowing wheat directly behind corn or grain sorghum. Growers should plant wheat after a non-host crop of the scab causal fungus such as cotton, peanuts or soybeans.”
The scab risk is higher for no-till compared with conventional-till wheat cropped behind corn or grain sorghum, so plow under or burn stubble and other surface debris associated with either of these crops, he says.
Also, early wheat has an increased risk of scab, says Hagan.
“While considerable advances in the selection of scab-resistant soft winter wheat varieties and breeding lines have been made, none are currently available to Alabama producers. Hopefully, such disease-resistant varieties will be released in the near future.”
Hagan recommends that producers plant rye, oats or a winter legume as a winter cover crop in place of wheat to reduce the risk of scab as well as take-all in subsequent wheat crops.
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