Corn foliage diseases

Wednesday, July 15, 2015

Not only has this been a difficult year to get the crop planted, but the weather is also conducive to disease development on the crop that we did get planted. Overall, the crop is behind normal in development. There is a wide range in growth stages of corn across the state as well as within fields. Recent weather events are conducive to foliage disease development.

Anthracnose leaf blight is showing up on lower leaves but does not appear severe at this point. Anthracnose leaf blight usually occurs early in the season on the lower leaves of young corn plants. Anthracnose lesions tend to be brown, oval to spindle-shaped lesions with yellow to pinkish to reddish-brown borders. Lesions may be 0.2 to 0.6 inch in length. Lesions may merge or coalesce to kill larger areas of leaf tissue. Lesions may be concentrated towards the leaf tip, or portion of the leaf that was emerged when rain occurred, giving the leaves a fired appearance that might be mistaken for nutrient deficiency or herbicide injury. Generally the disease stops at this point because of drier, warmer weather conditions and is not considered a significant problem.

The lab has received samples with symptoms suggestive of either Holcus leaf spot or herbicide injury. Holcus leaf spot produces lesions that are usually oval to elliptical and range in size from 0.25 to 1 inch. Initially, they are dark green and water-soaked. Later they become dry and turn light brown with a reddish margin. The bacteria that cause holcus leaf spot are spread by wind-driven rain or splashing rain, as outbreaks frequently occur several days after a rainstorm or storm with strong winds. Since holcus leaf spot is caused by a bacterium, common corn fungicides will have little effect on this disease.

The lab is also beginning to receive samples of corn with northern corn leaf blight, gray leaf spot, and common rust. At this point. these corn diseases do not appear to be as severe as other states in the Corn Belt are reporting. It will be important to scout for these and other corn foliage diseases over the next few weeks.

Northern corn leaf blight lesions are long, elliptical grayish green lesions ranging from 1to 6 inches in length.

As the lesions mature they may become more tan in color. Northern corn leaf blight usually begins on the lower leaves of the plants.

Gray leaf spot has become a serious problem. Initially, small, round to oval, reddish brown lesions with yellow halos develop between the leaf veins. These lesions increase in size, but since growth of the causal fungus is restricted to leaf veins, the lesions develop parallel edges which give them a rectangular or blocky appearance.

Common rust begins as small, circular, light green to yellow spots in the leaf tissue. These lesions develop into circular to elongated, golden-brown to reddish-brown, raised pustules.

The rust pustules may be in bands or concentrated patches on the leaf as a result of infection that occurred while the leaf was still in the whorl.