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Odd Plant Diseases Due to Odd Season

June 21, 2002
Head Scab of Winter Wheat

"Crazy Top, Common Smut, and Head Scab" would be a great name for a new-wave band. Head scab of winter wheat is being seen already, and the others should be fairly common this season, given our wet and stormy spring.

I wrote about head scab of wheat earlier in the season (issue no. 9, May 24, 2002), and now it is evident in the field. Matt Montgomery, Extension crop systems educator, notes scab showing up in many fields in Sangamon and Menard counties.

Scabby wheat heads. (Photo courtesy of Matt Montgomery.)

Scab is seen easily in the field because of its characteristic bleached or partially bleached heads. Examine the affected heads, carefully peel back bleached glumes, and you will likely see the diagnostic salmon-to-pinkish-colored fungus growing right beneath it. Abundant scab may lead to significant quality reduction, as well as direct yield loss. Blower speed on the combine should be turned up high to blow as much of the scabby wheat out of the back of the combine. Yes, this provides a source of the fungus in the field, but the less scabby wheat you bring in, the better the price.

Odd-Looking Field Corn Diseases

Why will we see crazy top and common smut of field corn this year?

Let's start with crazy top. Crazy top is caused by the fungus Sclerophthora macrospora, a soilborne fungus. The fungus is ubiquitous in our soils but only incites infection under very favorable conditions for the fungus. Under highly saturated or flooded conditions, the fungus produces motile spores. Typically, the soil must be saturated for 24 to 48 hours for the fungus to develop. Spores are either splashed up into the whorl or, in a flood situation, washed right into the whorl. The spores then infect the growing point of seedling corn, and the fungus grows systemically within the plant. When the plant should be tasseling, instead a great proliferation of short, stunted leaves is produced in response to the fungal infection.

Crazy top symptoms. (Photo courtesy of the University of Illinois.)

This looks like a tight bouquet of leaves, which makes it easy to see why this disease was named crazy top. Other than avoiding flooded conditions, no control measures are known for crazy top. It is usually very limited in its appearance and doesn't cause substantial yield loss as a whole.

Common Smut

The second part of the question deals with common smut. Common smut is also a soilborne fungus. Caused by the fungus Ustilago maydis, it is a very recognizable disease. This fungus, like the crazy top fungus, also infects the growing point, as well as other embryonic tissue, of the corn plant. It's a fairly lazy fungus, in that it typically needs some type of wound in the seedling corn to successfully infect the plant. The wounding usually takes place in the form of tissue damage to the seedling corn leaves from blowing, infested soil during heavy storms. The fungus is ubiquitous in our soils and corn is susceptible (sweet corn even more), so the right environmental conditions are all that is needed for infection. I have observed through the years that corn that received a shot of growth-regulator herbicide, such as 2,4-D, late in the season is likely to be affected by smut. Interestingly, variation exists in different varieties' responses to growth-regulator damage and subsequent smut infection.

When an ear should be emerging on the corn plant, instead, a proliferation of galls (smut balls) are produced that later form the familiar black, powdery spore masses as they mature.

Smut galls of common smut. (Photo courtesy of the University of Illinois.)

Common smut can also be found on leaves or tassels, but usually the ear is most affected. Yield loss is direct.

So, why will we see more than the usual amount of crazy top and common smut this year? Because we have had highly saturated soils from numerous storms and local flooding; the fungi are throughout most of our soils; and, of course, we have a susceptible crop. That's all the disease needs to develop, besides a bit of time.--Suzanne Bissonnette

Author: Suzanne Bissonnette

The Pest Management and Crop Development Bulletin
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