STRIPE RUST ON wheat occurred sporadically in North Carolina fields last spring

STRIPE RUST ON wheat occurred sporadically in North Carolina fields last spring.

Weed resistance becoming a problem in Southeast wheat crop

• ALS-inhibiting herbicides, (such as Harmony Extra, Osprey, and PowerFlex), have been used for the past several years to manage these weed problems, but those days of spray and go are rapidly going away.

The Carolinas and Virginia planted an extra 250,000 acres of wheat this past fall, compared to the fall of 2011 to take advantage of continued good prices for wheat and reflective of continued high prices for soybeans that can be planted in a double-crop/double-value economic scenario.

Getting wheat planted in the fall came with a few hitches, starting with finding enough seed in the desired varieties. Less than optimum seed supplies in the fall could have some yield-limiting consequences when wheat is harvested during May and June in the region.

Virginia Tech Small Grains Specialist Wade Thomason says seed size on some of the more popular varieties may contribute to thin stands and increase the chances of yield loss to a number of pests this year.

“Getting a good stand with the optimum number of plants per acre is an important part of achieving high wheat yields. A large amount of research in Virginia and the mid-Atlantic region supports the statement that we need about 25 vigorous seedlings per square foot in the fall to achieve these high yields. Since germination and emergence are not 100 percent, we recommend seeding at 30-35 seeds per square foot,” Thomason says.

Wheat growers in Virginia have been coping with weed resistance in hard-to-manage chickweed and other small-seeded weeds for the past couple of years.

ALS-inhibiting herbicides, (such as Harmony Extra, Osprey, and PowerFlex), have been used for the past several years to manage these weed problems, but those days of spray and go are rapidly going away.

In 2009, Virginia Tech Agronomist Scott Hagood first noted the problem in Virginia wheat. “In terms of plant weight, using .5 up to 32 times the labeled rate of Harmony GT we saw 98 percent or better control of the wild chickweed. On two samples from grower fields back in 2009, the best we could get was 50 percent control and on some of these plants we got virtually no control,” Hagood says.

Developing situation

In chickweed collected from one particular field in New Kent County, Hagood recalls 32 times the labeled rate of Harmony had no visible effect. Of more concern, he says, samples from this field were sprayed with a number of other sulfonylurea herbicides, with a similar lack of control.

“We also tested these resistant plants to other ALS herbicides other than the sulfonylurea herbicides. The New Kent site was resistant to both Pursuit and Arsenal.

“Arsenal, in particular, is a very persistent and effective herbicide and resistance to this material should give growers some insights as to how difficult multiple herbicide resistant chickweed will be to manage,” Hagood says.

Since Hagood’s initial discovery, resistance problems have become an ever-increasing problem and continue to be a significant threat to the big wheat crop planted in the Upper Southeast last fall.

North Carolina State University Small Grains Specialist Randy Weisz says a good option for growers this spring to ALS-inhibiting herbicides, like Harmony Extra, Osprey, Powerflex and Finesse; may be to use 2,4-D or dicamba either alone, or in a tank-mix one of these ALS-inhibitors.

He points out that dicamba may be a better choice if henbit and chickweed are the major weed problems.

The North Carolina State specialist says that growers using either 2,4-D or dicamba should remember that both of these herbicides should only be applied after tillering is complete and before wheat plants begin jointing.

In future years Weisz suggests rotating or mixing chemistries.

An example of rotation across seasons might include using a pre-emergence or spike stage herbicide (like Axiom, or Valor) one year, Harmony Extra, Osprey, or PowerFlex the next year, and 2,4-D or dicamba in the third year.

Chemistries can also be mixed within a season by using a pre-emergence or spike stage herbicide followed by Harmony Extra, Osprey, or PowerFlex a few months later.

An easy approach to preventing resistance might also be to just tank-mix two herbicides with different modes of action such as Harmony Extra and dicamba.

He says that a rotation of using Axiom or Valor one year followed by one of the ALS herbicides the following year should also be good for preventing ryegrass resistance.


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“There are many ways growers can implement a resistance management program. The examples above are only examples, not absolute recommendations. Harmony Extra is still an excellent broadleaf weed control option in North Carolina and Osprey and Powerflex will provide control of small broadleaf weeds and grasses,” Weisz says.

Stripe rust threatens

Other than some cold weather the last week of January, conditions across much of the Upper Southeast this winter have been good to ideal for development of stripe rust in wheat. Last year the disease caused sporadic, but severe damage to wheat in some areas of North Carolina, Weisz says.

It can be particularly dangerous because it doesn’t occur every year, in fact it doesn’t occur in most years in the Upper Southeast, and growers are not always diligent in looking for the disease.

Unlike leaf rust, stripe rust is bright yellow, the pustules line up in stripes along the leaves, and the fungus prefers cool to mild temperatures.

Stripe rust spores are blown into the U.S. from Central America and infections begin in the gulf states.

If winters are mild and cool, these infections can get started early, and stripe rust spores from the Gulf state infections then blow into Georgia. Usually, by the time spores can blow north from Georgia into North Carolina, weather has turned hot and the newly arrived stripe rust spores die. This year stripe rust infections have occurred early in the Gulf states and weather has been mild and favorable for the fungus.

“If stripe rust does occur again this year, it will begin showing up in fields from February until late April. ‘If a grower sees a yellow area in a wheat field, he should scout the field and determine whether he has stripe rust or some other disease, or nutritional problem,” Weisz says.

Stripe rust will look a lot like leaf rust, but the disease pustules will be yellow and will line up in distinctive row patterns,” he notes. 

If the grower determines he has stripe rust, he should spray a triazole containing fungicide as soon as possible, the North Carolina State specialist stresses.

Wheat stripe rust often causes severe grain yield and quality loss. Cool, mild, and moist, springtime weather across the Southeast last year created a good environment for the disease to spread.

If these conditions occur again this winter and spring, growers should take special care to protect their crops, because this particular disease can cause a great deal of damage in short amount of time, Weisz adds.

This year there will likely be about 250,000 more acres of wheat than last year. Knowing what varieties were planted and watching disease, weed and insect pressure will go a long way toward providing a top yielding and quality crop, in addition to being one of the biggest in terms of acreage in recent years.

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