Trap plant method to predict the occurrence of rice blast

Managing rice blast requires a forecasting system which is simple and easy to use. To develop and test such a system, experiments were conducted at the Supanburi Field Crops Research Centre (SFCRC) during Aug 1992 to Mar 1993. Experimental plots were planted monthly under irrigated and rainfed conditions with a variety, RD23, susceptible to blast disease. The disease severity and climatic conditions were recorded monthly. Leaf blast was first seen on 7 Sep 1992. Lesions appeared over the whole plot (1 m square) but severity was low. During Sep 1992, average daily temperature showed a general decreasing trend while dew incidence increased. The disease severity was higher in the following month. The climatic conditions and disease severity through the crop were statistically analyzed and results indicated a strong correlation between disease severity, dew, temperature and humidity. It can be concluded that as average temperatures declined with the dry winter season, dew incidence increased, relative humidities exceeded 60 percent and the disease incidence and severity exceeded the local economic threshold level. In the same field, nine rice varieties with different reactions to Pyricularia oryzae were also planted. Each row was 50 cm long; SPR60 was used as border rows. At the end of Sep 1992, all varieties showed symptoms of rice blast (BL). Slides coated with gelatin were attached to lesions with size and symptoms typical to each variety, and approximately the same across varieties. The number of conidia on each slide from each of the nine varieties was determined. There was no significant difference in lesion size among the nine varieties but the number of conidia/lesions was highly significant (P0.01). Though the resistant and moderately resistant varieties showed typical blast lesions, the number of conidia/lesion was not as high as in the susceptible variety. This indicated that the occurrence of rice blast requires not only favorable microclimatic conditions but also a susceptible rice variety