16 The Business of Ontario Agriculture Better Farming | June/July 2024 Tar Spot know what to look for and when to look for it. “Tar spot in corn is identified by small raised black circular spots that resemble specks of tar on the leaf surface,” explains Hooker. “These spots are fungal structures called stromata, which are embedded in the plant tissue and cannot be removed by rubbing.” Thirlwall explains that these black spots cause the yield drag from the fungal infection. “Because tar spot sets in as a dark black spot on the leaf, it reduces the photosynthetic process,” says Thirlwall. “This greatly affects yield. In 2023, there was the highest tar spot pressure so far. We saw a yield difference of 30 to 50 bushels per acre for sprayed versus unsprayed fields because of tar spot. It can be a huge yield robber.” Hooker adds, “Tar spot has reduced yields by up to 70 bushels per acre in fields that were infected early around silking, followed by favourable weather conditions through grain-fill. It was reported on the Crop Protection Network that tar spot was the No. 1 corn disease in Ontario in 2023, causing a loss of an estimated 3.9 million bushels. In severe cases, tar spot can lead to a reduction in test weight and grade due to poor kernel fill.” Tenuta has been involved with monitoring efforts led by the Crop Protection Network in collaboration with U.S. agronomists, OMAFRA, Grain Farmers of Ontario, and the University of Guelph. In recent years he has been finding tar spot as early as the first week of July. “Those first two to three weeks of July are critically important. That’s when we start seeing tar spot build up, and that gives us an indication of what’s going on in Ontario, as well as in the U.S.,” says Tenuta. “In areas with a history of tar spot, it takes six weeks for the initial spores to germinate and build up until we start to see it. If you’re in an area without a lot of tar spot, you’ll likely start seeing it appear higher up on the leaves. The spores would be coming in from other fields or areas, and you’ll see that above the ear leaf.” Tenuta also says that through his surveillance with the Crop Protection Network and their research into tolerant varieties, they have noticed another type of lesion caused by tar spot. The fisheye lesion has a black tar spot with a tan circle of necrosis around it. He says this lesion appears to occur more often in more tolerant hybrids. The evaluation of tar spot tolerance, along with other efforts from researchers and crop scientists in Ontario and the U.S., have developed a strong set of tools for managing tar spot. Emily Croft photo Producers should know the status of tar spot in their area as they’re scouting their fields.
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