Tar spot is a relatively new disease facing corn growers in southwestern Ontario and it is spreading fast! Tar spot was first identified in deepest southwest Ontario in late 2020. Only a year later, it was found right up and through to fields north of Toronto. Yield losses from tar spot can range from five to 40 bushels an acre, depending on how early infection sets in. If you grow corn in Eastern Canada, you need to read up fast on this disease — what to look for, when to look for it and how to manage it.

What is it?

Tar spot is caused by a fungus called Phyllachora maydis that infects corn leaves. It creates tiny black, tar-like specks, called stroma, on both sides of the leaf. These fruiting bodies produce spores that continue to infect corn plants throughout the season. As the infection spreads and stroma continues to take up more real estate on the leaves, it reduces their ability to photosynthesize, which can lead to stalk cannibalization, premature drydown and poor standability.

What factors affect outbreak?

Tar spot thrives in temperatures from 15 C to 21 C and with relative humidity of around 85%. Warmer temperatures don’t seem to stop infection, though, and it continues to spread with just seven hours of leaf wetness per day. This disease overwinters very well on crop debris and in the soil and, once the infection takes place, it moves up the plant toward the canopy.

How do I identify it?

Tar spot infections can start before tassel, so begin scouting early, and be thorough — stromata are often mistaken for insect frass and the way to tell for sure is to wet the leaf and rub it between your thumb and forefinger. If the spots rub off, it’s frass, if they don’t, it’s tar spot — stroma will not scrape off a leaf surface.

How do I manage tar spot?

Protecting the canopy is the main goal when it comes to treating crops for tar spot. It’s about keeping those upper leaves as clean as possible so they can direct the sun’s energy into grain fill. Make a proactive application of Delaro® Complete fungicide when tar spot is first detected. This broad-spectrum fungicide contains three active ingredients, including fluopyram, a Group 7 fungicide that is highly mobile in the plant so you get some residual protection. You can also apply Delaro Complete with a half rate of Proline® fungicide at R1 to control leaf diseases and protect against DON.

A visual example of multiple small black tar spots throughout the corn leaf.

Black, tar-like stroma on corn leaves.

Want to become a tar spot specialist?

To become a tar spot specialist, click the link below
Corn Fungicides: Become a Tar Spot Specialist - YouTube.