Planting disease-resistant varieties remains the most effective and economical way to control diseases in wheat, with resistance being the primary means of managing yield-robbing foliar diseases.

However, few recommended varieties have “good” or “high” resistance to all major foliar diseases, and the fungi that cause leaf rust and powdery mildew are constantly changing. Whenever a new variety is released, it is usually resistant to the most commonly occurring races of the fungi that is prevalent at the time, and the race population can change quickly.

Weather conditions during the winter and spring are one of several factors that affect the severity of wheat diseases. If the winter and spring are cool and/or dry, leaf diseases usually will be of little or no significance regardless of a variety’s resistance. A warm, wet winter and spring are favorable for infection by disease-causing fungi.

The combination of low resistance and warmer than normal winters and springs are favorable for severe powdery mildew, leaf rust, and Stagonospora nodorum leaf and glume blotch, three of the most important fungal diseases.

Seedborne and soilborne diseases are controlled primarily by seed treatments and crop rotation.

One of the more increasingly important and damaging diseases of wheat in the United States is scab or Fusarium head blight.

“Losses are not only due to sizable reductions in the germination, numbers, and test weight of seed from scab-blighted heads, but also to the production of mycotoxins by the causal fungus in diseased seed,” says Austin Hagan, Auburn University Extension plant pathologist. “Other cereal hosts of these causal fungi are barley, corn and grain sorghum. Given favorable weather patterns, wheat drilled behind no-till corn is at highest risk for a destructive scab outbreak.”

Moist weather patterns during flowering through early kernel fill are required for scab development, he adds. Severe scab outbreaks are likely when three to more rain or irrigation events occur from the start of flowering (anthesis) through three to five days post-bloom, particularly on the scab-susceptible wheat varieties grown in Alabama, says Hagan.

Other causes of disease outbreaks

Extended post-flowering rains also contribute to increased disease severity and elevated mycotoxin contamination. “Conversely, dry weather patterns during flowering will suppress scab development. Scab forecasting models, which do not include Alabama, have been developed to help predict the risk of the disease as wheat becomes vulnerable to attack and provide guidance concerning fungicide use,” he says.

One such forecasting model can be found at http://www.wheatscab.psu.edu/.

Survival sites for the causal fungi of which Fusarium graminearum predominates includes fungus-infested seed and host crop residues. No- or minimum-till production systems that leave Fusarium-colonized corn stalks and other crop residues on the surface greatly increase the risk of destructive scab outbreaks in the following wheat crop, according to Hagan.

“Causal fungi attack spikelets in the seed head during flowering, thereby killing the seed embryo. Further colonization and eventual girdling of the rachis by F. graminearum will result in the premature death of a portion or the entire seed head along with poor grain fill. Low test weight, scab-damaged seed typically has a low germ and produces unthrifty seedlings with a poor survival rate.”

Scab is easily recognized in immature wheat by the appearance of bleached or partially bleached heads several weeks after their emergence, says Hagan. Often, a light pink to salmon-colored fungal mycelial mat and spore mass may be seen at the base of the bleached spikelets as well as masses of pepper-seed sized, black fruiting bodies of the causal fungus along the edge of the glumes on diseased seed heads. Shriveled, scab-damaged seed has a chalky white to pink cast.

The fungus Fusarium graminearum produces the mycotoxin vomitoxin and zearalenone that concentrate in the scab-damaged grain, he says. Vomitoxin can cause reduced feed intake and lower weight gain in animals at levels as low as 1 to 3 parts per million (ppm) with swine and other non-ruminant animals being most sensitive.

“Vomiting and feed refusal can occur when vomitoxin levels exceed 10 ppm,” says Hagan. “Vomitoxin may be involved in the human disease alimentary toxic aleukia, so FDA has recommended that toxin levels not exceed 1 ppm in food.

Chickens and adult ruminant animals (non-lactating dairy and beef cattle) are less sensitive to the toxin than swine and can be safely fed grain containing up to 10 ppm vomitoxin as a portion of their daily ration.”

Maximum vomitoxin concentration for pregnant or lactating dairy cattle in grain is 5 ppm. Straw of scab-damaged wheat may be contaminated with vomitoxin, but may be used as bedding from all livestock except for swine.

Zearalenone has estrogenic properties and produces reproductive disorders at concentrations of 1 to 5 ppm, such as infertility, spontaneous abortions and uterine prolapse (hyperestrogenism) in swine and to a lesser extent in feeder (immature) cattle and sheep, says Hagan.

In Kentucky, suggested maximum concentration of zearalenone in swine rations should not exceed 1 to 2 ppm and 0.5 ppm for sheep. In contrast, poultry are fairly tolerant of zearalenone.

Producers concerned about mycotoxin contamination should have grain tested prior to feeding to animals, Hagan recommends.

phollis@farmpress.com