June 24, 2008
Nebraska wheat fields are plagued for the second straight year by a bad case of scab, or Fusarium head blight, thanks to an unusually wet spring. The disease is especially prevalent in south central and eastern Nebraska but also has shown up in wheat fields as far west as Imperial, says Stephen Wegulo, University of Nebraska-Lincoln plant pathologist in the university's Institute of Agriculture and Natural Resources.
It is too late in the growing season for wheat growers to combat the problem this year, Wegulo adds.
Symptoms of scab include premature bleaching of one or more wheat spikelets or the entire immature head. Bleaching can start anywhere on the head and spread until the entire head is bleached. With the continued wet weather, bleaching of infected wheat heads is expected to continue, which will increase severity of the disease.
Bleached spikelets are sterile and contain shriveled and/or discolored kernels, commonly referred to as Fusarium-damaged kernels. A high proportion of these kernels results in downgrading of contaminated wheat grain. Fusarium-damaged kernels are lighter than healthy kernels and can be removed or reduced by cleaning or setting the harvest combine to blow out "chaffy" grain.
During humid conditions, white or pink fungal growth with orange spore masses may be seen on the bleached spikelets. Blue-black fruiting structures also can form, giving the head a scabbed appearance, hence the name scab.
Scab has not been common in Nebraska in recent years due to drier than normal conditions, Wegulo says. However, excessive rainfall in 2007 caused widespread occurrence of the disease in south central and eastern Nebraska. Similar weather conditions this year have favored development of scab in many wheat fields.
The disease is caused by several species of the fungus Fusarium, with one species predominant in North America, and is favored by wet weather during the growing season.
"It can survive in soil or on corn, wheat and grass stubble. Scab also is more severe in reduced or no-till fields, especially if wheat follows corn," he explains.
The disease can take over an entire field in severe cases.
Spores of the scab fungus are carried to wheat heads by air currents. Most infections occur during flowering because anthers and pollen serve as a food source for the fungus. Infected seed can transmit the fungus to emerging seedlings.
"This can cause severe seedling blight under favorable conditions for disease development," he said. "During warm temperatures (between 77 and 86 degrees), blight symptoms on heads appear within 2 to 4 days following infection. Therefore, a crop that appeared healthy a few days earlier can suddenly show widespread symptoms."
Scab can be managed by planting tolerant cultivars, crop rotation out of cereal grains or grasses and foliar fungicide applications at early flowering. Most of Nebraska wheat already has gone through the flowering stage, so it's too late to apply a fungicide to suppress the disease this year.
In addition to lowering yields and grain quality, the scab fungus also can produce mycotoxins, said Michael Carlson, UNL diagnostic toxicologist. "Most likely the fungus will produce deoxynivalenol, or DON, commonly referred to as vomitoxin, and zearalenone," Carlson says.
The disease can result in significant yield loss - potentially 100% - if grain is rejected because of the toxins the fungus can produce, Wegulo says.
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