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Primary Disease Gradients of Bacterial Blight of Rice

The dispersal potential of Xanthomonas oryzae pv. oryzae, causal agent of bacterial blight of rice (Oryzae sativa), was investigated through measurement of primary disease gradients. Plants within individual hills of rice were inoculated, and the number of new lesions in the primary generation of dispersal from these inoculated sources was counted. Two dispersal models that can describe the number of infections at the source (the Kiyosawa and Shiyomi and the modified Gregory) were fit to the lesion counts. Estimated gradient slopes were similar in the 2 years of the study for both gradient models. However, the Kiyosawa and Shiyomi model gave a better fit in both years, as indicated by higher coefficients of determination and significances of slopes and by a more random pattern of residuals. Primary disease gradients were very steep, with half-distances (distances over which lesion numbers are calculated to decrease by half) estimated from the Kiyosawa and Shiyomi model of 0.091 and 0.081 m in 1994 and 1995, respectively. Splash dispersal of X. oryzae pv. oryzae is the most likely explanation for both the steep slopes measured and the superior fit of the Kiyosawa and Shiyomi model over the modified Gregory model.