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