This season has started out a little slow as far as diseases are concerned. However, things that might start appearing in the field with the cool wet weather are root rots and seedling blights. These diseases have been covered in Bulletin issue no. 2 and issue no. 6.|
Windy, raining weather that causes leaf tearing can produce significant wounding of leaf tissue and encourage the spread of bacterial diseases. Not many bacterial pathogens infect corn. The most famous, of course, is Stewart's bacterial wilt, whose symptoms are yellow streaking along leaf tissue. Another minor bacterial pathogen of corn causes spotting of leaf tissue. Pseudomonas syringae pv. syringae causes holcus spot. Dave Feltes, IPM educator for the Quad Cities area, and Tom Beveroth of Farmers Elevator and Supply in Morrison, reported possible holcus spot in a cornfield. Symptoms reported from Whiteside County were small tan-to-buff-colored spots, with a darker outer edge. The spots started appearing on the lower leaves after storms with heavy winds and rain. The spots looked similar to paraquat drift, but no known application was made in the area. These symptoms are consistent with those that can be observed with holcus spot. Holcus spot lesions initially are dark green and water soaked before the centers turn creamy white to tan. Eventually they dry out and turn brown.
Holcus spot on corn.
Most spots will be surrounded by a reddish or brownish margin. Lesions may be surrounded by a yellow halo.
Other things can cause spots to occur on young corn leaves that can be confused with holcus spot. Early in the season, Anthracnose can appear during wet weather on the lower leaves of corn that has reached at least 5 to 6 inches in height. These lesions often appear at the base of the plant in places where blowing soil has been hitting the plant. Lesions caused by Colletotrichum graminicola appear as small, oval or elongated, water-soaked spots on the leaves. Typically the spots have a tan or light-colored center surrounded by a reddish to purple-brown border.
Anthracnose on corn.
Fruiting bodies (acervuli) form in the centers of the dead tissue and appear as small dark spots. Close examination with a hand lens will reveal the setae, or dark, hairlike structures, that will grow upward out of the acervuli. Symptoms of anthracnose are fairly common early in the season on lower leaves and later in the season on upper leaves. However, it is normally not an economic problem.
Drift or contact from chemicals, such as paraquat or glyphosate, can also cause spots on corn leaves.
Paraquat injury on corn.
These spots can also be confused with holcus spot, but an obvious pattern should be visible throughout the field. Disease organisms occur more randomly throughout a field or in hot spots.--Loretta Ortiz-Ribbing