Disease Alert! Gray Leaf Spot and Tar Spot

Eastern Iowa, Western Illinois and Southern Wisconsin are seeing heavy Gray Leaf Spot pressure and more reports of Tar Spot.

Gray Leaf Spot (GLS)We have been tracking GLS infections over the last month. The warm weather we had earlier in the summer kept the infection in the lower canopy. However, over the past couple of weeks we have had slightly cooler temperatures, high humidity and several storms that rolled through. This weather has been ideal for the development of fungal disease and the GLS has exploded!

Conditions favoring disease: Wet, warm weather (75°-85°F) and high humidity leading to long periods of leaf wetness, susceptible hybrids, infested corn residue on soil surface.

Heavy mature GLS lesions are being found on the ear leaf and above. The onset of this heavy of infection is troubling as most corn is just now starting to pollinate.

Impact: It is estimated that yield losses of 50% or more can occur with severe infections occur before dent stage.

Now that more mature lesions are out, they are producing spores that will only further infect the plant especially with the upcoming weather forecast of more storms and high humidity. Treatment with fungicide is recommended before more severe infection occurs.

Tar SpotTar Spot sighting continue to show up across the Midwest. Last year the first tar spot was found on August 9th. This year the first tar spot was reported on July 9th. Tar Spot Map 7-21-2020. https://corn.ipmpipe.org/tarspot-2/

Most Tar Spot is currently being found in the lower canopy. However, conditions have been ideal for the development of the disease.

Conditions favoring disease: Wet, cool conditions (67°-72°F) and high humidity leading to long periods of leaf wetness, susceptible hybrids, infested corn residue on soil surface.

Remember the incubation period (the time it takes for a disease to develop lesions) for Tar Spot can be much longer than Gray Leaf Spot. GSL incubation period is 7-10 days. Tar Spot incubation period is 12-15 days so we may be seeing more lesions very soon.

Impact: Yield loss varies but can be severe with infections that occur before dent stage. Late season stalk quality can also be negatively affected due to early plant senescence under severe infection.

Corn fields that have a history of Tar Spot, corn on corn rotation and/or irrigation are at extremely high risk for Tar Spot infection. Treatment with fungicide is recommended before more severe infection occurs.

Management: Headline AMP and Veltyma are labeled for prevention and control of Gray Leaf Spot and Tar Spot. Fungicide applications at the VT (full tassel) or R1 (pollen shed) stage offer good control of GLS and Tar Spot. However, if infection starts early or is severe, earlier applications may be necessary to prevent the disease from advancing to the ear leaf and above.

Use Rates:
Headline AMP: 10-14.4 fl oz/A
Veltyma: 7-10 fl oz/A