Corn Crops and Diseases

Symptoms of a few foliar diseases are beginning to appear in cornfields across the state.

Anthracnose leaf blight. This disease was discussed in the June 11 issue of the Bulletin. In some fields, symptoms can still be observed on lower leaves. With the rapid growth of corn, spread to upper leaves has not been seen. Depending on conditions in July and August, anthracnose leaf blight can spread to upper leaves later in the season.

Common rust. Common rust has been observed for about the past two weeks in some fields. Cooler temperatures (<75°F) and high relative humidity favor this disease. High temperatures the past few weeks likely will slow or stop the progression of common rust. In general, most dent corn hybrids have good levels of resistance. Considering the recent warm temperatures and the resistance of most hybrids, applications of foliar fungicides specifically for common rust may not be warranted. Foliar fungicides are effective in controlling common rust, and research at the University of Illinois conducted by Dr. Jerald "Snook" Pataky indicated that the strobilurin fungicides evaluated in his trials (Quadris and Headline) had better "curative" control of rust than Tilt fungicide. Symptoms of common rust and southern rust on corn are pictured below.
Southern rust. Southern rust can cause major yield losses to corn, as nearly all hybrids are susceptible. Southern rust has not yet been reported in Illinois in 2010, but it is progressing northward. The progress can be tracked on the southern corn rust PIPE website. The current southern corn rust map (see below) shows that the disease has been reported as far north as Arkansas.

Gray leaf spot. Conditions in most of Illinois have been very favorable for gray leaf spot so far this season (warm with high relative humidity). Disease symptoms have been observed in some fields on the lower leaves.

In my field research plots, gray leaf spot symptoms are more obvious in corn-on-corn fields than where corn is following soybean. In University of Illinois foliar fungicide trials where gray leaf spot pressure was significant, products with a strobilurin fungicide component have been the most effective. In addition, results from foliar fungicide trials indicate that bigger yield responses with fungicides are more likely to occur on hybrids with low disease-resistance ratings and high yield potential.
Author: Carl A. Bradley

Ear Rots corn diseases is one of the most relatively common corn diseases. Several fungi are able to infect corn ears and lead to decay of corn kernels. Two common ear rots are shown in this photo. The two ears on the left are examples of Gibberella ear rot caused by Gibberella zeae, the same fungus that causes stalk rot of corn. The two ears on the right are examples of Fusarium ear rot, caused by the fungus Fusarium moniliforme. These two ear rots are similar in that both causal fungi produce large of amounts of whitish mycelium on the surface of infected kernels. The two diseases can be distinguished by noting the pattern of infection on the ear. In Gibberella ear rot, infection starts at the tip of the ear and moves toward the base. Typically the husk is also infected and fuses to the ear. In Fusarium ear rot, infection tends to be more uniform, with no real concentration at the tip. Also, fusion of the husk to the ear is relatively less common.
In general, ear rots are of concern because they can lead to yield reduction. In addition, many ear rot fungi, including G. zeae and F. moniliforme, produce toxic compounds (called mycotoxins) that can adversely affect any animal that consumes them. Toxic effects of mycotoxins in domesticated animals vary depending upon the mycotoxin consumed, but can include refusal to feed, loss of weight, vomiting, increased occurrence of liver tumors, loss of kidney function and abortion of fetuses. Some mycotoxins are carcinogens.

This kind of corn diseases, the Eyespot  is one of the relativelycommon corn disease that is caused by the fungusKabatiella zeae.

Symptoms of this disease are very distinctive: small circular lesions/spots surrounded by a red and yellow halo. When the disease is severe (as it can be in cool, humid weather), large numbers of these spots will grow together leading to death of large areas of leaf tissue.

As is the case with many foliar corn pathogens, the fungus that causes eyespot survives in corn debris. Thus use of no-till production practices tends to favor the development of the disease by maintaining a ready source of inoculum. Clean plowing, crop rotation and use of resistant varieties can help in the management of this disease.

Southern corn leaf blight- caused by the fungus Bipolaris maydis (Helminthosporium maydis) occurs worldwide, but is particularly damaging in regions of warm, moist weather. Lesions on the leaves caused by Race 0 of the fungus are elongated between the veins, tan, up to one inch long, with limited parallel margins and buff to brown borders. Lesions produced by Race T (which was very prevalent in the early 1970s) are tan, 1/4 to 1/2 inch x 1/4 to 1 inch, spindle-shaped or elliptical, with yellow-green or chlorotic halos. Later, the Race T lesions often have dark, reddish-brown borders and may occur on all parts of the plants. Race T is commonly found only when hybrids with Texas male sterile cytoplasm are grown. Both races of the fungus overwinter on corn debris in the field. Thus, rotation and destruction of residue will reduce losses due to this disease. Resistant hybrids are also available.

Common smut or boil smut is a highly visible disease found in corn fields and in home-owner sweet corn gardens. The unsightly smut boils enlarge within days after infection of ears, tassels, stalks, or leaves. As the boils mature, the contents change into masses of black spores. Since no toxin is associated with this fungal disease, infected plants can be fed to livestock.

Just check it out this kind of diseases wherein if you peeled open an ear of sweet corn and found corn smut would you:

A) Be disappointed and throw the ear in the trash, or
B) Feel like you hit the jackpot and prepare for a special feast? Your answer probably depends on your culinary heritage and experiences.
Corn smut is a fairly common disease caused by the fungus Ustilago maydis. It's most common on sweet corn but can also occur in pop corn and field corn. Corn smut can infect various parts of the corn plant including stalks, leaves, and tassels, but it is most apparent when it infects the ears. The fungus infects developing corn kernels, causing the formation of galls which can grow to several inches in diameter. These galls have a silvery gray outer coating of corn tissue, so they look like inflated corn kernels. Inside the gall is a mass of black fungus. When the galls mature the outer layer dries and cracks open, releasing powdery black spores to spread by wind and rain splashing. Warm, wet weather promotes the development and spread of the fungus.
Before you throw out infected ears of sweet corn, consider that this corn fungus is a desirable delicacy in Mexican cuisine. Known as huitlacoche (wheat-la-KO-chay) or cuitlacoche, its smoky, mushroomy flavor was appreciated by the Aztecs and continues to be prized for use in soups, sauces, quesadillas, and other dishes. Huitlacoche is becoming more popular in the U.S. and sweet corn growers in several states now purposely inoculate some of their corn with the fungus for commercial harvesting of huitlacoche.