Tar Spot in Corn

There was a lot of discussion and questions surrounding tar spot even before it was found this year in Ontario. Tar spot is a relatively new corn disease in the US Midwest and since its initial confirmation in northern Indiana (2015) it has moved into new areas including up to the Michigan-Ontario border last year (Figure 1- map). Due to wind patterns and close proximity to infected areas in Michigan, it was not unexpected that tar spot was confirmed in Chatham-Kent (Ridgetown) late in the 2020 season (September 21). Since then tar spot has been confirmed in Essex, Lambton, Elgin and Middlesex counties. OMAFRA has been working closely with US pathologists and a real-time tar spot tracking system, which includes Ontario, has been developed. Please visit Corn.ipmPIPE for updates.

Figure 1. Tar spot distribution in 2020 (yellow) and previous years (grey) as of August 12, 2020. For updates visit Corn ipmPIPE.

Figure 1. Tar spot distribution in 2020 (yellow) and previous years (grey) as of August 12, 2020. For updates visit Corn.ipmPIPE.

The concern with tar spot has led to many samples and pictures being submitted for examination this summer. Until the Ridgetown detection all these samples had been negative for tar spot and the vast majority of samples had been confused for insect frass (poop)! It is easy to distinguish insect frass from tar spot lesions, you just need some water or "spit". Wet the leaf spot and rub the area between your fingers - hence the scientific procedure name of "spit-test"! If the spot rubs off the leaf, it is not tar spot which produces raised black lesions which are embedded in the leaf and don't rub off.

What does Tar Spot look like?

Tar spot appears as small, raised, black 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.

Figure 2. Tar spot produces raised, black lesions (stroma) on leaves and husks.

Figure 2. Tar spot produces raised, black lesions (stroma) on leaves and husks.

Tan to brown lesions with dark borders surrounding stroma can also develop. These are known as "fisheye" lesions. In Latin America, where tar spot is more common, fisheye lesions are associated with another fungus, Monographella maydis, that forms a disease complex with P. maydis known as the tar spot complex. M. maydis has not been detected in the United States.

Figure 3. Tar spot lesions including "fish-eye" lesions.

Figure 3. Tar spot lesions including "fish-eye" lesions.

Scout!

Corn during the mid to late grain stages (R3-R6) is when tar spot is most commonly found. Keep in mind, a few other diseases such as rust and physoderma brown spot can be confused with tar spot. Rust forms orange/red lesions which erupt through the leaf surface (volcano-like) and as they get older can turn black/dark brown but when you rub the lesions the spores do rub off leaving a smudge on your finger. Physoderma form flat brown lesions primarily on the leaf mid-rib or near leaf base unlike tar spot which usually occurs from the middle toward the tip of the leaf.

If you suspect tar spot, please contact OMAFRA field crop plant pathologist Albert Tenuta at albert.tenuta@ontario.ca.

For more resources and information please visit:

Ontario Diagnostic Days Episodes #2 and #8 at
Fieldcropnews
Realagriculture/ontariodiagnosticdays

Grain Farmers of Ontario - Agronomy Alert and webinar
Agronomy-resources

Crop Protection Network
CropProtectionNetwork

Corn IPM Pipe - tar spot tracking and reporting
Corn.ipmPIPE

Ontario Agricultural Conference January 6 - 7, 2021
On demand session #11 Dynamic Disease
OntarioAgConference


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca