Spatial distribution of Rhizoctonia oryzae and rhizoctonia root rot in direct-seeded cereals

Rhizoctonia oryzae causes root rot and stunting of wheat, barley, and other small grains, and is widely distributed in eastern Washington State. The spatial distribution of both the pathogen and the disease were studied over two seasons in a 36-ha field north of Pullman, Wash. The field was direct seeded with spring barley in 2000, then divided and planted to winter wheat and spring wheat in 2001. The incidence of crown-root rot and root colonization by R. oryzae were measured on plants taken from 95 sites 30-100 m apart on a nonaligned grid, localized by global positioning system. The incidence of crown-root rot was low, averaging 18, 16, and 13% on the spring barley, spring wheat, and winter wheat, respectively. The incidence of root colonization by R. oryzae was even lower, averaging 4, <1, and 2% on the spring barley, spring wheat, and winter wheat, respectively. The frequency distributions were highly skewed, with a high frequency of zero or low values. The distributions were fit to a beta-binomial distribution, indicating an aggregated or overdispersed distribution. Because of the highly skewed data, traditional geostatistical approaches did not show spatial correlation. However, a spatial generalized linear mixed model did show spatial correlation and was used to interpolate values to produce pathogen and disease distribution maps. This large-scale mapping of soilborne pathogens may have applications in precision agriculture.