A healthy dose of nitrogen fertilizer can coax top yields from rice, but cross the optimum line and diseases can set the crop back and cost producers more than they gain, say University of Arkansas Division of Agriculture scientists.

This interaction with nitrogen fertilizer is true for almost all rice diseases, but it has been seen most commonly with sheath blight, kernel smut and blast, said Dr. Nathan Slaton, director of the Arkansas soil testing and research program.

Dr. Rick Cartwright, Extension plant pathologist, said sheath blight is the number one concern with excessive nitrogen. “When there’s more nitrogen than is needed for maximum yield, sheath blight goes wild,” he said.

Kernel smut is also heavily influenced by excess nitrogen. “Excessive preflood nitrogen applications have the greatest affect on the incidence and severity of kernel smut,” Slaton said. “Application of mid-season nitrogen also tends to increase disease, but not to the same magnitude as the preflood nitrogen.”

Slaton and Cartwright are working with Dr. Rick Norman, professor of crop, soil and environmental sciences, to incorporate studies of nitrogen-disease interaction into rice fertility research. In 2004, Division of Agriculture scientists also began looking at how excess nitrogen influences false smut and panicle blight.

“We’re looking for the optimum application rates and timing that will give each rice variety the best yields without stimulating higher disease levels,” Norman said.

Slaton said research has shown preflood applications of about 120 units of nitrogen, depending on variety and growing conditions, are more effective in improving yields than split season applications of 90 units applied preflood and 60 units in mid-season. The larger preflood application gives rice a shot of early growth that helps build carbohydrates to improve yield.

“Variables like how much nitrogen is native in the soil and the weather conditions at the time of application all have an effect on the results,” Slaton said. “Complete management in an integrated system is important. Producers need to know the disease history of a field and manage it accordingly.”

Slaton said high nitrogen makes the plants lusher and more susceptible to diseases. “The plants grow taller, faster and leafier, filling in the canopy early, so you get higher humidity that promotes more growth of sheath blight.

“When rice is planted very early, you can push nitrogen a little more,” Slaton said. “But when rice is planted later, the threshold for disease becomes more critical.”

Research has also shown that the relationship between nitrogen and potassium is important for plant growth and health, Cartwright said. If those nutrients get out of balance, disease susceptibility increases. Low potassium and high nitrogen is worse than both nutrients being excessive.

“This has become more important in recent years as native potassium levels have fallen in our soils,” Cartwright said. “Producers sometimes decide not to add potassium, but they always apply nitrogen.”

He suggests that producers who are having continuing problems with disease may consider cutting back a little on nitrogen.

“With the costs of nitrogen and fungicides going up, it may be worth considering to cut back maybe 20 units of nitrogen,” he said. “If you throttle back a little, you’ll probably find a happy medium where you can save some cost of nitrogen and fungicide use.”