July 11, 2023
Kansas State University row crop plant pathologist Rodrigo Onofre reported in the July 6 Agronomy eUpdate that tar spot has been confirmed in four northeast Kansas counties: Doniphan, Atchison, Jefferson and Brown.
Tar spot is a disease in corn caused by the fungus Phyllachora maydis. The tar spot spores can overwinter on infested corn residue on the soil surface until the next growing season. Then, wind and rain splash can spread the spores to nearby fields.
Onofre writes that recent rains likely promoted tar spot development in these confirmed fields. Irrigated corn can be at higher risk for yield or silage loss from tar spot. Rainfall and high humidity conditions are ideal for tar spot to take hold and spread.
Tar spot develops as small, black, raised spots that can appear on one or both sides of the corn leaves, leaf sheaths and husks. They can be found on healthy green tissue as well as on dying brown tissue. Scouts might confuse tar spot with insect poop, which can also appear as black spots on the surface of the leaf.
Fungicides can be effective to control tar spot, and the best timing is to apply on corn when fungal diseases are active in the corn canopy. Onofre recommends also applying when the corn is at least nearing VT/R1 (tassel/silk) or R2 (blister) stages. The National Corn Disease Working Group has a list of effective fungicides labeled for tar spot at bit.ly/cornfungicide.
Onofre warns that higher disease pressure may call for a second application. And remember, there are no benefits from fungicides after the R5 stage of development.
It’s important to call a local county Extension office or the K-State Plant Disease Diagnostic Lab at 785-532-6176 to help you confirm tar spot. Onofre also encourages farmers who suspect they have tar spot to call him at 785-477-0171 to help track the disease.
Read more online at bit.ly/eupdatetarspot.
Kansas State Agronomy eUpdate contributed to this article.
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