Holcus Spot or Gray Leaf Spot?

A quick guide to telling two similar corn leaf diseases apart.

When scouting corn this time of year, Holcus Spot and Gray Leaf Spot can be easy to confuse. But knowing the difference matters when deciding whether a fungicide application is warranted.

HOLCUS SPOT

Bacterial

Typically produces round to oval lesions with tan or white centers, reddish-brown edges, and a yellow-green halo.

Because it is bacterial, fungicides generally won't provide control.

GRAY LEAF SPOT

Fungal

Lesions are usually longer and rectangular, with the edges restricted by the leaf veins.

When disease pressure becomes severe, it can reduce healthy leaf area and potentially impact yield. Fungicide can provide control.

What to look for in the field:

• Lesion shape

• Whether the lesion is restricted by the leaf veins

• Whether there is a yellow-green halo

The biggest takeaway is simple: identify what you have before deciding how to treat it. A few extra minutes scouting can prevent an unnecessary fungicide application and help make sure the dollars you spend are actually addressing the problem.

— Dwayne