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Fundamentals:
Holcus spot is a bacterial disease that occurs occasionally on young corn and is usually not very damaging.
Symptoms:
Symptoms of holcus spot first appear as water-soaked, dark green lesions near the tips of lower leaves. They then develop into round or elliptical, tan to white spots that are 1/8 to ½" in diameter. Red to brown margins develop around the spots, which may be surrounded by yellow halos. Symptoms can be confused with damage from paraquat.
Conditions and Timing that Favor Disease:
This disease typically develops early in the season following high winds and thunderstorms. Disease development is favored by prolonged wet weather and warm temperatures (76 to 86°F).
Causal Pathogen:
The pathogen is a bacterium called Pseudomonas syringae pv. syringae. It overwinters in crop debris. The pathogen has a wide host range including many grasses and dicots. It can have ice nucleating activity that may enhance frost injury to corn leaves.
Disease Management:
This disease is best managed by crop rotation and tillage where appropriate.
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