Ontario Grain Farmer November 2021

A NETWORK OF Ontario crop specialists and U.S. extension specialists monitored the real-time movement of tar spot in 2021. In September, the disease spread significantly east and north from the original infections confirmed in southwestern Ontario’s lower five counties in 2020. Tar spot appears as small, raised, black tar-like spots scattered across the upper and lower leaf surfaces. These spots are stroma (fungal fruiting structures). If viewed under the microscope, hundreds of sausage-shaped asci (spore cases) filled with spores are visible. When severe, stroma can even appear on husks and leaf sheaths. Tan to brown lesions with dark borders surrounding stroma can also develop. These are known as "fisheye" lesions. Tar spot is here to stay; and based on the U.S. experience, seasonal weather patterns (especially rainfall and relative humidity) will determine the impact each year. Corn farmers need to be proactive in determining if their fields have been affected in order to ensure proper management decisions are made in 2022. Make note of fields which have tar spot, plant tar spot tolerant hybrids if available, scout early, and pay attention to weather and tar spot alerts to determine if in-season management such as fungicide are needed. How well various fungicides work against tar spot depends on the product efficacy against tar spot, timing, weather, hybrid susceptibility, disease levels at application timing, and spore load in region. Based on what we have seen and observed in corn hybrid trials, there is some degree of tolerance or partial resistance. A 64 hybrid OMAFRA trial in Rodney, Ontario does show some good candidate hybrids. You can see where the latest confirmations of tar spot have been reported through the Corn ipmPIPE initiative ( https://corn.ipmpipe.org /tarspot/). l Tar spot CROP SIDE WITH... Albert Tenuta, Field Crop Plant Pathologist, Ontario Ministry of Agriculture, Food and Rural Affairs Crop side TAR SPOT DISTRIBUTION - SEPTEMBER 29, 2021. EARLY INFECTED FIELD, SEPTEMBER 13, 2021. PHOTOS COURTESY OF ALBERT TENUTA. 22 Tolerant Susceptible Tolerant Susceptible Tar spot in on August 31, 2021 Tar spot in on September 10, 2021 2020 FUNGICIDE TRIAL TOLERANT VERSUS SUSCEPTIBLE HYBRID (RODNEY, ONTARIO).