Take-all disease sporadic in 2007 wheat

Take-all disease is a highly destructive disease of wheat that may be compounded in the upper Southeast by reduced and no-tillage systems.

In the 2006-2007 crop season for wheat the disease may have been more widespread than reported because the symptoms of take-all closely mimic drought damage. In many areas of the Southeast drought, especially late in the growing season, was common.

Wheat plants infected with the take-all fungus appear to be drought stressed because they are — even if plenty of water is available. The disease destroys the root system of wheat, preventing the uptake of water or nutrients.

“Take-All is aptly named, because when wheat plants are infected with the fungus that causes the disease the plant will not produce seed,” says Erik Stromberg, a plant pathologist at Virginia Tech University.

In light infections, wheat plants may show no visible signs of the disease. However, in severe cases, yield losses of up to 80 percent have been recorded.

Gaeumannomyces graminis var. tritici is the fungus that causes take-all of wheat. Two other varieties of the take-all fungus are known and they are distinguished by their virulence on various crop and grass species. G. graminis var. avenae is pathogenic on oats and turfgrasses. G. graminis var. graminis is the least pathogenic of the three varieties and is pathogenic on common weed grasses.

The take-all fungus cannot survive in the soil. It can only survive in infested plant debris or host plants. If plant debris is buried in conventional-tillage systems, the plant debris rots and the take-all fungus will die, unless weeds or other host plants occur in the field.

However, in a typical no-till system wheat is harvested, then soybeans are drilled into the wheat stubble. At the end of the summer soybeans are harvested, but frequently volunteer small grains come up and these are infected with the take-all fungus. As long as the tissue is present, the disease will survive.

The next crop planted is corn. Grasses, like panicum and foxtail, in corn support the take-all fungus. The corn is harvested and wheat is planted into the corn stubble. At that point the disease cycle starts all over again, Stromberg explains.

In areas of the country where wheat is planted year after year, take-all tends to be less of a problem, which makes it somewhat unique among fungal diseases. After a period of years planted consecutively to wheat, the soil builds up microorganisms that are antagonistic to the take-all fungus. However, when a grower breaks that wheat cycle with a legume, the fungus comes back strong.

“Any time you change the micro-flora of the soil, you can create an environment that is favorable to take-all. I suspect that no-till systems with corn, soybeans and small grain may favor a build-up of take-all, and it may be a bigger problem in the future, if we continue to stay in these systems,” Stromberg says.

Take-all is usually more severe in lighter soils, with higher pH, and lower fertility. Generally, the earlier infection takes place, the more disease and the greater the yield loss at harvest. Thus, infections of young plants soon after planting result in the most severe yield losses. Moisture retaining, poorly drained soils or abnormally wet weather, especially in the second half of the growing season, favors the development of the disease.

The first symptoms of take-all on wheat or barley occur on young seedlings. Infected root and stem tissue darkens to a nearly black color and the lower leaves typically are chlorotic. If plants are not killed at this stage of growth, they tiller poorly or not at all, and black lesions develop on roots and extend up into the crown tissue as the disease progresses.

The disease tends to spread in soils that remain moist for long periods during the growing season. In high rainfall areas or in irrigated fields, the disease may develop in patches, and eventually plants in patches develop white heads and die prematurely.

Take-all affects plants by sequestering manganese, converting it to manganese oxide, a form the plant cannot use. Ultimately, the wheat plant cannot protect itself from the disease-causing organism, because it doesn’t have access to manganese that is a key component in the plant’s ability to fight fungal diseases.

Recent research has shown that ammonium (anhydrous ammonia and urea) and the use of denitrification inhibitors will suppress take-all. Slow release forms prevent nitrogen loss by leaching and denitrification. Nitrate forms of nitrogen tend to increase the severity of the disease.

Leaving soil fallow in the summer, creating temperatures too hot for the causal fungi to survive may be another way to reduce the incidence of the disease. Cultural practices best suited to managing take-all in wheat are often in contrast to best use of no-till systems, which may not bode well for increasing wheat production in the Southeast.

“Wheat has little resistance to the take-all fungus and fungicides are not effective in combating the disease, making it even more deadly to wheat.

The answer to take-all is simple. On the one hand, destroy the soil debris, kill host weeds and rob the fungus of a place to live, but doing that is going to be very difficult in our current cropping systems,” Stromberg concludes.

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