INFLUENCE OF BLAST DISEASE IN UPLAND RICE ON THE CONCENTRATION OF AIRBORNE CONIDIA OF PYRICULARIA-GRISEA

The relationship between blast disease and the airborne concentration of conidia of Pyricularia grisea was quantified based on measurements from three upland rice field experiments. The concentration of conidia measured with a volumetric spore trap showed large day-to-day fluctua tions and a mid-season maximum. Disease severity was assessed quantita tively and partitioned into components: leaf blade lesions of four age categories, collar lesions, and panicle lesions. Crop density was als o measured. Young leaf lesions of age classes 1-3 were positively corr elated with airborne inoculum, and the correlations were highest when disease severity was adjusted for crop density (r = 0.52 to r = 0.60). Old leaf lesions (class 4) and collar lesions did not correlate with inoculum density, and panicle lesions showed a small negative correlat ion. Spore catch data were regressed on disease data by forcing all di sease components and crop density variables into a multiple regression analysis. The adjusted R2 of the regression equations for each season ranged from 0.65 to 0.69. The regression equations were used to predi ct the spore catch, and a good fit between observed and expected spore catch was obtained, with R2 ranging from 0.66 to 0.72. Thus, most of the variability in spore catch data could be explained by the disease severity, the age stratification of the lesion population, and crop de nsity. The usefulness of these findings for forecasts is explained.