Tar spot requires seven hours of wetness on a corn leaf to take hold

ARVA – Tar spot can be a big black hole for potential corn yields, in the neighborhood of 60 bushels per acre.

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In 2023 tar spot cost Ontario farmers 3.9 million bushels of corn.

The airborne disease is a fairly recent additional challenge to farmers in Canada. First detected in the US corn belt, it has traveled northward into Canada, detected at Ridgetown in 2020 and in Essex and Simcoe counties in 2021.

“The disease can travel long distances,” said Albert Tenuta, field crop pathologist with the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA) speaking recently at the Syngenta research farm near Arva.

The disease can also be spread by machinery traveling from farm to farm.

“If a neighbor has it, you have it,” added Tenuta of the disease.

Tar spot thrives in the moist conditions that prevailed in the spring and early summer across southern and Southwestern Ontario. It requires seven hours of moisture on a corn leaf to take hold.

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The tiny black dots can first be seen on the lower portion of corn stalks before moving up higher, indicating the plant has become more susceptible.

Eventually brownish areas will emerge on the leaves as the plant is dried out and unable to absorb nutrients, resulting in huge yield loss.

“Its goal is to kill that grain plant,” said Tenuta.

The disease is able to over-winter in the ground. As such, what farmers learn about the disease this year can be applied in guarding against it next year, including how to manage corn stalk trash post-harvest.

The disease likes cooler temperatures, 15-20 C, and is slowed down by hotter temperatures. The hot stretch of weather in July in the London area slowed the disease down there, said Tenuta.

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Since the emergence of the disease in Canada is quite recent, Syngenta and other seed companies have been conducting research to determine which fungicides are most effective.

Twenty different products have been tested. So far the three most effective are Beltyma DLX made by BASF; Delaro Complete made by Bayer; and Miravis NEO by Syngenta.

Tenuta recommended a two-pass fungicide program using multiple active ingredients. Spraying should be done during the silking period of corn or the 10-leaf stage which provides a protection period of two to three weeks. Previous applications have not proven as effective.

Seed companies are working to develop new fungicides. Selecting hybrids with more tolerance can also delay disease development.

But “nothing is immune,” said Tenuta.

Crop rotation and tillage can help to eliminate the disease.

Tar spot normally has a two-week reproduction cycle that can shorten to one week if conditions are favorable. It emerges the first week in July and wraps up in four to six weeks.

Spraying for tar spot should be combined with spraying for other diseases such as northern corn leaf blight or gibberella ear rot.

Farmers have to look at the big picture and just not focus on tar spot, suggested Tenuta.

“Learn what you have now to put into practice in the future,” said Tenuta of the current status of tar spot.

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