Farm Talk

Crops

July 1, 2014

Watch for corn leaf diseases

Parsons, Kansas — In general, corn in southeast Kansas looks about as healthy as any reasonable producer might hope.

However, Doug Jardine, K-State Research and Extension plant pathologist, recently reported in the Kansas River valley that an outbreak of gray leaf spot has occurred. With that announcement, it is probably a good idea to discuss common corn leaf diseases in Kansas.

Anthracnose leaf blight

Anthracnose leaf blight is the earliest of these diseases to appear in the season. Anthracnose appears as tan, irregular-shaped lesions and can be up to a half-inch wide on the lower leaves as early as the third leaf appears. The borders of the lesions may appear orangish-yellow to a reddish brown. In what will be a recurring theme, the disease is most common in fields where old corn debris is present.

Anthracnose is most commonly found in northeast Kansas under no-till conditions. Compared to the other diseases, it is intermediate in yield loss potential. The best management practice for anthracnose is to plant resistant hybrids, tillage to bury past crop debris (if possible), and rotation. It must be noted that the leaf blight anthracnose is completely different than the anthracnose stalk rot and hybrids resistant to one may not be resistant to the other.

Common rust

The next disease that can show up in corn is common rust which I have already seen in corn fields this year. Symptoms of common rust begin as small golden-brown pustules that turn darker as they mature. Infection from the disease is very likely under high humidity and moderate temperatures conditions.

One interesting diagnostic tool is that the pustules form on both sides of the leaf, differentiating it from another disease that will be discussed. The good news about common rust is that it does not impact yield much at all. It very commonly found (well named, I guess) and fungicides are not recommended as a management practice since yield impacts are so low.

Goss’s bacterial wilt

The next disease that could show up in a corn field in Kansas is Goss’s bacterial wilt. As the name suggests, this disease is caused by a bacteria rather than the fungal infection that most producers are used to. Bacteria need a wound or opening to cause infection so this disease often requires hail or a sand blast to occur.

Initially, the disease was only found in northwest Kansas, northeast Colorado and southwest Nebraska. It has begun to spread which is quite worrisome. The disease is moderate to high in yield loss, and the best management practice for control is to plant resistant hybrids. Reminder, since it is a bacterium, fungicides will have zero effect on the disease.

Gray leaf spot

Gray leaf spot is the next disease that can show up in a year. As mentioned earlier, it is already in northeast Kansas so the fungus is active. Of these foliar diseases, gray leaf spot can cause the most yield loss. No-till, continuous corn is the most likely cropping system to be infested.  

Symptoms begin on the lower leaves and progress upward. Tiny lesions appear with a yellow halo which progress into pale brown to gray, rectangular spots. The fungus survives on infested plant debris and infection is increased under high-humidity conditions. Crop rotation and tillage are two methods of management that can provide some control but, since it causes so much yield loss, a fungicide application is likely to be recommended.

Southern rust

Southern rust must not be confused with the common rust mentioned earlier. The disease appears very similar to common rust with a couple of exceptions.

Southern rust appears only on the upper side of the leaf, and there are many more pustules than common rust. The disease usually blows up from the southeastern United States in mid-to-late July which is usually too late to cause yield damage to southeast Kansas corn. However if it arrives early enough, it is second only to gray leaf spot in terms of yield loss. Resistant hybrids are the best management practice, but a fungicide application may be warranted if the disease conditions are favorable.

In summary, the corn leaf diseases arrive at certain times of the year which can help in the identification of which disease is present. Yield loss is very dependent upon which disease is present and because of this, a fungicidal application may or may not be needed.

If you have questions or would like more information, please call me at the office 620-724-8233, e-mail me at jcoltrain@ksu.edu, or visit the Wildcat Extension District website at www. wild catdistrict.ksu.edu.

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