Farmers can shoot themselves in the foot with excessive nitrogen rates Rick Norman recalls a rice farmer once telling him about a black cloud that started coming out of the back of his combine when he started harvesting a field. "The farmer said, 'It looked like the combine was burning up.'"

It wasn't malfunctioning equipment, but an infestation of kernel smut, a quality- and yield-robbing fungus that can leave combines covered in grayish-black soot at the end of the day. The fungus tends to be minor, even on susceptible varieties, but a push for higher yields can cause the problem to explode, researchers have learned.

Norman, a soil fertility professor for the U of A Division of Agriculture, said research indicates the culprit is excessive nitrogen fertilizer applications.

Nathan Slaton, director of soils testing for the University of Arkansas, says researchers and Extension specialists began looking at the nitrogen/kernel smut relationship in 2000.

With the help of funding from the Arkansas Rice Research and Promotion Board, the research looked at many scenarios. Researchers learned that nitrogen timing and rate of application influences kernel smut at any given rate tested.

They also learned that, with new varieties, farmers must apply the recommended amount of nitrogen fertilizer pre-flood to obtain maximum yields, but not go over that amount or risk severe kernel smut, Slaton says.

"Preflood nitrogen sets grain yields," Slaton says. "If you under fertilize at preflood, you've hurt grain yields. Over fertilizing at preflood tends to have a great impact on severity of kernel smut. The mid-season nitrogen application affects the severity but not as great as preflood."

The bottom line, Slaton says, is that if farmers consistently have kernel smut problems, they've likely over fertilized, probably at preflood. The first line of defense for kernel smut, Slaton said, is proper use of fertilizer.

Another consideration is that most varieties are moderately susceptible to kernel smut. However, some rice varieties are very susceptible, including LaGrue, Cocodrie, Jefferson, Cypress and Priscilla. Avoiding using these in historically problem fields can help reduce kernel smut.

Rick Cartwright, a Cooperative Extension Service plant pathologist, has shown that Tilt fungicide is an effective, but expensive, remedy for kernel smut. He notes that it's cheaper to use the right amount of fertilizer and better time the application.

Cartwright, who is working with Slaton on the project, says it boils down to a balancing act.

"You've got to figure out not only the best rate and proper timing for maximum yield but not increase the potential for disease. We want to have our cake and to eat it too, but it can be done."

He says Extension specialists are writing recommendations about timing and nitrogen rates farmers need to use if they have an historical kernel smut problem. Farmers should talk to their county agent if they have concerns.