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Holcus Spot or Plant Injury?

Green corn leaf with a few white lesions throughout the leaf.
Figure 1. Holcus spot lesions on the corn leaf.

Is your corn developing spots? Corn fields have been found with what appears to be Holcus spot, a bacterial disease. Upon further investigations, the leaves were found to be negative for any plant pathogens. Although very low levels of Holcus spot have been found in corn fields this season (Figure 1), these should not be confused with heavy leaf spots seen on the leaves and/or a large number of plants, which are more likely the result of crop injury (Figure 2). Holcus spot usually develops as a few creamy white, round-to-elliptical spots on mid-to-low canopy leaves. It is considered a minor disease and does not spread between leaves.

How to Differentiate Between Crop Injury and Holcus Spot

Corn leaf with white spots with brown halos scattered throughout.
Figure 2. Corn leaf with paraquat drift injury.

Plants with injury due to fertilizer burn, herbicide drift, or adjuvants such as surfactants in pesticides are usually widely distributed in the field or have a gradient starting from the point of the initial pesticide drift. Check several locations in a field to determine if there is a gradient or if the entire field is affected. Plants with pesticide injury have symptoms appearing on leaves at the same level (Figure 2) and new leaves tend to be injury-free. Holcus spot on the other hand develops usually on the top half of the leaf and the level of disease will vary from leaf to leaf.