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Crop advisor’s Casebook

When Ken, a local farmer who owns a 2,900-acre farm in Norquay, Sask., discovered darkened heads appearing throughout one of his wheat fields, he called me right away.

“It has to be some sort of disease, but I’ve never seen these particular symptoms before. I can’t seem to figure out what it is,” Ken explained to me over the phone. He told me about his management strategy on this particular field and it seemed he had conducted good management practices. For example, he had sprayed a fungicide at the flag leaf stage, the seed had been treated and he had a sound fertility program in place. I decided to have a look at the black wheat heads for myself.

When I arrived at Ken’s farm, I noticed the field was starting to turn and would soon be ready to desiccate. The infected heads were located in patches scattered randomly throughout the field. Some of the wheat heads were entirely affected and the most mature heads seemed to be hardest hit. Meanwhile, the younger green heads seemed to be unaffected.

Ken had been right to call me — these were strange symptoms. The first diseases that came to mind were smut, bunt and glume blotch. However, there were no signs of false wheat kernels, the black growth was on the glumes but the glumes themselves were not discoloured and symptoms appeared only on the ripe heads, so those three diseases were ruled out.

I had an idea about what was causing the damage, but I took some pictures and sent them to an agronomy manager for confirmation.

Because the infection only appeared on prematurely ripened wheat heads, the fact that the tissue surfaces of the heads, kernels and glumes were not discoloured, and the feedback from the agronomy manager indicated that Ken’s wheat field had been infected with sooty mould.

Sooty mould is caused by saprophytic fungi that grow on dead tissue and is, therefore, not considered a true crop disease. Sooty mould usually infects heads that are prematurely ripened due to an in-season disease such as fusarium head blight, root rot, take-all, or aster yellows. There is no method of treatment once sooty mould is present in a field.

Fusarium head blight had not been an issue on Ken’s farm and harvest samples didn’t show any levels of concern. However, aster yellows and root rot had been causing problems for producers in Ken’s region. In fact, the hot and wet conditions that summer had caused many diseases to become issues for producers in the area.

In this case, however, the sooty mould was caused by multiple rains on the premature heads at the beginning of August. Aster yellows, take-all or root rot had probably prematurely ripened the heads, all of which was out of Ken’s control.

If the cause of the prematurely ripened heads had been fusarium head blight, a fungicide applied at flowering would have prevented the tissue from being killed early on and the plant’s ensuing susceptibility to sooty mould.

Even though a seed treatment was applied, the long, cool, wet spring allowed the infection of root rot to occur well into the summer, but the level of infection was likely reduced.

Sooty mould is not preventable, but the initial diseases that can make a crop more susceptible to it are, with the exception of plants prematurely ripened due to aster yellows.

Sooty mould can cause kernel discolouration if the infestation is severe. A high incidence of mould will also affect storage — the grain must be stored dry.

Although Ken had conducted good management practices and had taken every precaution, in future he must continue to scout and monitor for diseases such as fusarium head blight to ensure they do not cause prematurely ripened heads. In addition, this field’s grain will be more susceptible to spoilage due to the presence of mould and should be closely monitored during storage. †

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