Holcus Leaf Spot Identified in Kentucky Corn Fields

By Blake Jackson

Holcus leaf spot is an occasional bacterial disease found in Kentucky corn fields but identifying it can be difficult because its symptoms often resemble herbicide injury.

According to University of Kentucky specialists Kiersten Wise, Kelsey Mehl, and Carl Bradley, understanding the disease's characteristics can help growers make accurate diagnoses.

The disease typically appears as small, round lesions on corn leaves. Early symptoms are pale yellow or white spots that gradually enlarge and become gray, tan, or brown.

Many lesions are surrounded by water-soaked halo, while some corn hybrids may develop brown or purple borders around the affected tissue.

In certain cases, neighboring lesions merge into larger irregular patches with a dry appearance. Symptoms are most often seen from the middle of the leaf toward the tip and generally develop before pollination. Unlike many plant diseases, holcus leaf spot does not typically spread from infected leaves to healthy ones.

Diagnosing the disease can be challenging because its symptoms closely resemble damage caused by contact herbicides such as paraquat.

Herbicide exposure often creates white or yellow spots bordered by brown tissue, making visual identification difficult. To distinguish between the two issues, growers should examine symptom patterns across the field.

Herbicide drift usually affects nearby vegetation, including weeds, grasses, and plants along roadsides, fence lines, and ditches. Injury from drifts also tend to decrease gradually as the distance from the application site increases.

When symptoms appear randomly throughout a field and surrounding vegetation remains unaffected, holcus leaf spot may be the more likely cause. Disease severity often varies among plants and across different areas of the field.

The disease is caused by bacterium Pseudomonas syringae pv. syringae, which can survive in crop residue and alternate hosts such as wheat, sorghum, and grassy weeds.

Wet weather before tasseling encourages infection by splashing bacteria onto leaves. Plant injuries caused by hail, wind, or other factors can also aid infection.

Although holcus leaf spot may appear alarming, it is not known to reduce corn yields. No in-season treatment is recommended, and fungicides are ineffective because the disease is bacterial rather than fungal.

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