Tar Spot in corn

By Mark McKerrall, CCA-ON
Crop Sales Specialist,
AGRIS Co-operative - Thamesville

Tar spot is showing up in cornfields in Ontario at an alarming rate this year. Weather conditions have been ideal for the spread of this relatively new Ontario corn leaf disease. Favourable environmental conditions that promote infection and disease development are cool 15 to 21 degrees Celsius, humid conditions (85% relative humidity) and long periods of leaf wetness (greater than 7 hours).

Tar spot in corn is caused by the fungal pathogen Pyllachora maydis. The disease is prevalent in Mexico and other Central and South American counties, and the Caribbean. In the Latin America region, Pyllachora maydis and Monographella maydis co-infect corn plants to cause the tar spot complex.

Tar spot was first confirmed on field corn in the United States in 2015 in northwestern Indiana and north-central Illinois. As of 2019, the disease has been found in Florida, Illinois, Indiana, Iowa, Michigan, Ohio, and Wisconsin.

So far, tar spot in the U.S. is only caused by P. maydis, as M. maydis has not been detected on symptomatic plants.

Since tar spot was detected in the U.S., it has caused varying amounts of damage. In 2018 when early infection (R5) occurred, tar spot caused severe lodging and yield losses of 20-60 bu/ac.

Yield loss was associated with an early, rapid senescence that led to reduced ear size and poor grain fill. However, very late detection of the disease in some fields that very same year resulted in no yield loss. The single pathogen P. maydis tar spot complex does not seem to have the same severe yield loss potential as the multi pathogen complex present in Latin America.

Tar Spot. Photo by D. Malvick.

Tar spot appears as small, raised, black spots scattered across the upper and lower leaf surfaces. These spots are ascomatum (fungal fruiting structures). Tan to brown lesions with dark borders surrounding ascomatum can also develop. These are known as “fisheye” lesions.

The keys to managing tar spot are hybrid resistance, residue management, crop rotation and fungicide applications.

Hybrid resistance testing is in its infancy as tar spot is relatively new to the U.S., and presently there are no resistant hybrids available. Some hybrids are more tolerant than others.

Residue management is only moderately effective as tar spot is both residue-borne and wind-borne. The bottom is that if you are practicing conservation tillage/no-till, there is no reason to return to conventional tillage systems.

Similarly, a corn/soy crop rotation is unlikely to have much of an effect on tar spot as these fields were just as severely impacted in 2018 as the corn-on-corn acres.

There are numerous fungicides labelled for tar spot suppression. However, tar spot ramps up reproduction and symptom development rapidly, so application timing is critical. Multiple applications are usually not cost-effective. A single timely application (R1-R3) multiple mode fungicide application is your best option. Once corn reaches R4 or dough stage, we would not recommend a fungicide application any longer.

The last thing to remember is this disease is heavily impacted by weather. Just because you have severe tar spot one season does not mean you will have it the following year. If you had tar spot before and your corn is heading into VT with a forecast of heavy, persistent rains, you may want to consider a fungicide application.

Tar spot in corn was first identified in Ontario last year on September 21, 2020 and confirmed to overwinter in Ontario on July 2, 2021. It was only a matter of time as there were tar spot infected fields as close by as Michigan.

Remember, tar spot is both residue-borne and wind-borne, so scout your fields and take the appropriate action to protect your crop in the future. If you have any questions or concerns, please contact your local AGRIS Co-op Crop Sales Specialists.

Sources: Albert Tenuta OMAFRA
Ref: Purdue.edu/extension
Illinois farmdoc Extension