Relatively speaking, this has been a dull year for disease scoutingso far, at least. Recent torrential rains can change that situation. Leaf blights will likely take advantage of the high moisture and humidity. Look for them when you are out getting pelted with Japanese beetles. Nevertheless, a few disease oddities are starting to pop up now as the corn begins to tassel.
Steve Ayers of the Champaign Extension unit brought in a sample to me of a fungal disease that was developing on leaves; it was common smut. His question to me was "Why is this on the leaves? The ears haven't emerged."
Well, a bit about smut. Common smut is actually a soilborne fungus: Ustilago maydis. Common smut is a very recognizable disease. It infects the growing point as well as other embryonic tissues of the corn plant. Infection actually occurs early in the season. In one sense, this a fairly lazy fungus, in that it typically needs some type of wound in seedling corn tissue to successfully infect the plant. The wounding usually takes place in the form of tissue damage to the seedling corn leaves from blowing spore-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 may have gotten a shot of a growth regulator herbicide, such as 2,4-D, a bit late in the season is likely to be affected by smut. Interestingly, different varieties respond differently 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. Yield loss is direct.
"Smut galls of Common smut", photo University of Illinois
So why does common smut show up on leaves and occasionally tassels, too? Well, meristematic tissue is in those plant parts, too, so the fungus hops in for the ride and galls develop as those tissues mature.
As tassels emerge, you will likely be seeing a fair amount of a very odd-looking disease called crazy top. Crazy top is caused by Sclerophthora macrospora, a soilborne fungus. It also is ubiquitous in our soils but only incites infection under conditions very favorable 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, just washed right into the whorl. The spores then infect the growing point of seedling corn, and the fungus grows systemically within the plant, just like common smut. When the plant should be tasseling, instead a great proliferation of short stunted leaves are produced in response to the fungal infection. It looks like a tight bouquet of leaves, which makes it easy to determine why this disease is named crazy top. Other than avoiding flooded conditions, there are no control measures for crazy top. It is usually very limited in its appearance and doesn't cause substantial yield loss as a whole.
"Crazy top symptoms" photo University of Illinois
Will we see more than the usual amount of common smut and crazy top this year? Only in those parts of the state that had highly saturated soils, numerous storms, and local flooding in the spring when seedling corn was present.--Suzanne Bissonnette