Distinguishing Spots On Corn: Holcus Leaf Spot vs. Gramoxone Drift

Round, pale, necrotic lesions have been reported on corn in Indiana. Two issues can cause this type of symptom: a disease called Holcus leaf spot, and herbicide drift.

Symptoms of Holcus leaf spot, a disease caused by the bacterium Pseudomonas syringae pv. syringae, are characterized by round, discrete lesions that are initially pale yellow to white and then enlarge and turn gray or brown. Lesions have a water-soaked halo and on certain hybrids, the margin of the lesion may appear brown or purple (Figure 1). Lesions are typically distributed from the center to the tips of leaves (Figure 2). Gramoxone injury will produce similar symptoms on plants (Figures 3 and 4). To determine if it is Holcus leaf spot or drift, the following questions must be answered:

Is there a field nearby that received a burndown herbicide application?

Gramoxone is a broad-spectrum contact herbicide, meaning that it has efficacy on all plant species and only affects the parts of the plant that are contacted by the spray solution. Gramoxone can only be used as a burndown product in agronomic crops because of the broad-spectrum activity, so producers need to first look around at neighboring fields to determine where a burndown herbicide application may have been applied.

Is there an obvious pattern to the symptoms observed in the affected field?

If a field with a recent burndown application is identified, and a drift event is suspected, symptoms in the affected field should appear in a gradient of severity from the edge of the field moving inward. The highest density of symptomatic plants should be observed closest to the target site (field with the burndown application) and symptoms will decline in frequency and density as you move away from the target site. The pattern should also follow the same direction as the prevailing winds on the day of the drift event. In a drift event, necrotic lesions should also be observed on all plants present in ditches, road banks, and fence rows between the target site and the cornfield in question.

If symptoms do not follow a gradient across the field, and plants surrounding the field are not affected, it is possible that the symptoms are caused by the Holcus leaf spot bacterium. Plants diagnosed with Holcus leaf spot will be randomly scattered throughout the field, and severity may vary across the field and within a plant.

There is no concern of symptom spread from Holcus leaf spot or gramoxone drift. In a gramoxone drift event, only the parts of the plants that were exposed at the time of drift will be affected. Leaves emerging after the drift event will not exhibit symptoms. With Holcus Spot, the bacterium that causes the leaf spot enters the plant through wounds or through stomates during warm, rainy and windy weather. The bacterium will overwinter in infected residue, and also on other grass species, including weeds. It is not known to spread from infected leaves to healthy leaves.

Holcus leaf spot is seen sporadically in Indiana, and is not known to limit yield. Although the disease may cause concern based on symptom appearance, no in-season treatment is available or necessary. Fungicide applications will not have efficacy against this bacterial disease.

Samples may be submitted to the Purdue Plant and Pest Diagnostic Lab (PPDL) <http://www.ppdl.purdue.edu/contacts.html> for assistance with diagnosis.

Figure 1. Yellow and white lesions with a water-soaked halo are indicative of holcus leaf spot in corn.

Figure 2. Holcus leaf spot observed on centers and tips of leaves.

Source : Purdue.edu