Corn School: Managing late-arriving tar spot

Field research shows that the best time to tackle tar spot with a fungicide is the VT/RI stage of corn development — when the tassel is visible and silks are emerging.

That's the case again this year, says Ontario Ministry of Agriculture, Food and Agribusiness plant pathologist Albert Tenuta. He's been testing fungicide treatments to control the leaf disease in his disease nursery at Rodney, Ont., since 2020. On this episode of RealAgriculture Corn School, Tenuta and host Bernard Tobin tour the plots to assess the efficacy of different products applied at VT/RI.

The duo also take a close look at the performance of two-application strategies where the fungicide is applied at the VT/RI stage and at the R3 stage, typically three to four weeks later. Tenuta's research trials, sponsored by Grain Farmers of Ontario and the Sustainable Canadian Agriculture Partnership, show growers can get strong efficacy and control with this strategy, depending on the product combination.

But what happens when tar spot shows up late in the summer? Across Ontario, many growers are reporting the arrival of tar spot in fields at the dough (R4) or dent (R5) stage of development. Tenuta notes that tar spot typically begins in the second week of July, but due to the hot summer conditions, the disease appeared later than normal in many fields and is now progressing rapidly.

Tenuta has been getting plenty of calls from growers asking whether a late application will help protect grain yield from the late-arriving leaf disease. "At that point, you're really not going to see benefits," he says. "You may see green up, and a more cosmetic appearance, but not the return, the payback that we would expect if you came in with that R3 application."

In the video, Tenuta advises growers to get out and scout their corn crop and determine whether tar spot is setting up shop in their fields and whether the hybrids they plant are showing good defence against the disease. That information will be key in helping growers develop a tar spot strategy for future crops.

Website: https://www.realagriculture.com/
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