EFFECTS OF ENHANCED UV-B RADIATION ON THE GROWTH OF RICE AND ITS SUSCEPTIBILITY TO RICE BLAST UNDER GLASSHOUSE CONDITIONS

The effects of enhanced UV-B (280-320 nm) on the susceptibility of 18 irrigated lowland rice cultivars to rice blast (Pyricularia grisea) were investigated. The rice cultivars were irradiated with UV-B 313 lamps that were either filtered with cellulose acetate of 0.13 mm thickness (light transmission greater than 290 nm) or with Mylar D of 0.13 mm thickness (light transmission greater than 320 nm). Irradiation was for 6 h daily for 21 days starting at 9 days after planting. After irradiation, plants were inoculated with one or two blast isolates. In 18 out of 36 measured interactions the number of lesions per plant was higher in the presence of UV-B than in its absence. However, only two interactions were statistically significant. In a second experiment the dose-response relationship between the cultivars IR30 and IR72 and UV-B was established in the presence and absence of disease. Increasing levels of UV-B significantly reduced leaf areas, dry weights, and heights of both cultivars. Disease severity was either unaffected or decreased by UV-B. However, disease significantly changed the effects of UV-B on plant growth and recovery from UV-B damage, IR30 but not IR72 recovered from UV-B damage in the absence of disease within 5 days. No recovery occurred when inoculated. IR72 suffered even greater reductions of leaf area and dry weight by UV-B when inoculated. Although the effects of UV-B on disease severity may be small, it appears that the tolerance of plants to disease is decreased by UV-B radiation.