With such a wet May, y’all are probably seeing an uptick in leaf spots on corn, but there’s no need for concern. It’s likely holcus leaf spot, a minor bacterial disease that typically shows up early in the season under warm, wet conditions.

What Is Holcus Leaf Spot?

Holcus leaf spot (or holcus spot) is caused by the bacterium Pseudomonas syringae pv. syringae. The disease begins as circular, oval, or irregular dark green, water-soaked spots. As they dry, these lesions turn light tan to white, often with a thin, papery texture, and may develop light brown to reddish margins surrounded by yellow halos (Figures 1 & 2). Spots range from 1/8 to 1/4 inch in diameter and tend to appear randomly across the leaf surface, especially on the upper half of the leaves and near the tips of lower leaves. Disease severity can vary widely from leaf to leaf.

What Can It Be Confused With?

Holcus leaf spot is often mistaken for:

1. Contact herbicide injury, especially from products like paraquat (Figure 3). Herbicide injury typically appears uniformly on leaves at the same growth stage, and new leaves are unaffected. Look for a gradient of symptoms across the field; if symptoms fade from the edge inward or nearby non-corn plants are also affected, herbicide drift is more likely.
2. Eyespot, a fungal disease, which also causes round lesions with a brown border and yellow halo (Figures 4 & 5). However, eyespot lesions are much smaller, more numerous, and more evenly distributed than those caused by holcus leaf spot. They also do not have the water-soaked margins that are more indicative of bacterial leaf spots.

If spots are larger than a pencil eraser tip, the symptoms don’t follow a clear gradient, surrounding vegetation appears healthy, and lesion patterns vary leaf to leaf, holcus leaf spot is the likely culprit.

Why Am I Seeing It Now?

This disease tends to appear early in the season, especially in warm (76–86°F), wet conditions following high winds and thunderstorms. The pathogen thrives in wet conditions and spreads via rain splash and storm-driven wind. It can survive in plant debris and infect new tissue through wounds or natural leaf openings. Holcus leaf spot has a broad host range, affecting many grasses and dicots.

What’s the Impact?

Holcus leaf spot is considered a minor disease in corn. Yield losses are rare, and even when lesions are noticeable, the primary concern is just a slight reduction in photosynthetic area.

How Do You Manage It?

Management is often not required. There are no effective in-season treatments. Fungicides won’t work against this bacterial disease. Most corn hybrids have adequate resistance, and management focuses on cultural practices, including:

• Residue management to reduce bacterial inoculum.
• Crop rotation and weed control.
• Tillage, when appropriate, to break the disease cycle.