Corn School: Distinguishing tar spot from common rust


The good news is tar spot — a new corn leaf disease — has yet to be found in Ontario cornfields.

The bad news is it’s only a matter of time before the yield robber is confirmed in the province, says Ontario Ministry of Agriculture, Food and Rural Affairs plant pathologist Albert Tenuta.

On this episode of the RealAgriculture Corn School, Tenuta explains that tar spot has made a rapid trek through the U.S. Midwest and into Michigan, but OMAFRA surveys, researchers, agronomists, and growers have yet confirm presence of the disease in Ontario.

High industry and grower awareness of tar spot has produced a steady stream of suspected cases, but so far Tenuta says those suspicious-looking leaves have turned out to be nothing more than common rust. Characterized by tar-like speckling on the upper surface of corn leaves, tar spot has typically caused losses of up to 20 to 40 bu/ac in the midwest.

In the video, Tenuta explains how growers can distinguish between the two diseases, noting that common rust lesions tend to break open on the leaf. (Story continues after the video.)

Because it’s a new-to-North America disease, genetic resistance will be key to keeping tar spot at bay, says Michigan State University field crop pathologist Martin Chilvers.

At the 2019 SouthWest Agricultural Conference, Chilvers noted that some hybrid resistance to the disease has been observed, but more research is required. Fungicides are an option, preferably those with two or more modes of action, he says, but there’s still much work to be done on assessing the best product and ideal application.

Click here for more Corn School episodes.

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