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.

“As a soil fertility scientist, my inclination is to put on plenty of nitrogen to be sure you have enough to get the best yield,” said UA agronomist Rick Norman. “But plant pathologists come along and say, ‘whoa!’ Beyond the optimum amount, nitrogen may increase the incidence and severity of several common diseases that can reduce yield and quality.”

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 Rick Cartwright, Extension plant pathologist. “We're seeing this especially with kernel smut,” he said.

Norman, Cartwright and Nathan Slaton, director of the Arkansas soil testing and research program, have incorporated studies of this nitrogen-disease interaction into rice fertility research. Producers toured their test plots in August during field days at the Rice Research and Extension Center near Stuttgart, the Northeast Research and Extension Center at Keiser and the Pine Tree Branch Experiment Station near Colt.

“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.

At mid-season, producers use a rice gauge to measure the plants to see if they have grown as much as expected. If so, a second application of nitrogen may not be necessary, reducing fertilizer costs.

“Preflood application makes better yields than split applications at equal rates,” Slaton said, “but if you exceed the optimum amount, you not only waste money on fertilizer you don't need, you may cause higher levels of disease that could rob yields or add to input costs for control. You end up paying more and getting less.”

The causes of this nitrogen-disease interaction are largely unknown, but Cartwright said higher nitrogen makes the plants lusher and, likely, 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. This disease can also use excess nitrogen in the plant tissues for its own growth.

“High levels of nitrogen also delay maturity of the panicle and increases the length of time that the grain is tender,” he said. “This is when kernel smut does its damage.”

“The good news,” said Chuck Wilson, Extension rice specialist, “is that we have sound research data and good recommendations. The research is ongoing and each year we have better information to find that happy medium where we get the optimum nitrogen rate for better yields without kicking in the disease.”