Grain yield and kernel smut of rice as affected by preflood and midseason nitrogen fertilization in Arkansas

Kernel smut, Neovossia horrida (Tilletia barclayana), of rice (Oryza sativa L.) has been a persistent disease in Arkansas for more than 50 yr. Recently, epidemic levels of kernel smut have occurred on highly susceptible cultivars, which have increased awareness within the rice industry of this disease. Nitrogen fertilization treatments consisting of four preflood (50-152 kg N ha(-1)) and three midseason (0-100 kg N ha(-1)) N rates were arranged in a factorial design and used to delineate the effects of N rate and application timing on rice yield and disease incidence and severity in five site-year-cultivar studies (environments). Significant environment x preflood N rate interactions occurred for yield, incidence, and severity. The minimum preflood N rates, averaged across midseason N rates, that produced maximum yields varied among the environments and ranged from 84 to 152 kg N ha(-1). Depending on the environment, disease incidence ranged from 2 to 93% and severity ranged from <0.1 to 4.8% among preflood N rates. Preflood N rate had no significant effect on smut incidence and severity for three environments receiving optimum-to-excessive N and a fourth environment receiving inadequate-to-optimum N. For the remaining environment that received optimum-to-excessive N, incidence increased linearly (43-93%), and severity increased nonlinearly (0.5-4.8%) as preflood N rate increased. Midseason N rate did not affect severity but caused a positive, linear increase for incidence. Data suggest that excessive preflood N has the greater potential to increase kernel smut but only when environmental conditions are favorable for kernel smut.