Epidemiologia dell'elmintosporiosi e del brusone e difesa del riso

Epidemics of rice brown spot and rice blast incited by Cochliobolus miyabeanus (Ito e Kurib.)Drechsler ex Dastur and Magnaporthe grisea (Hebert)Barr, respectively, were monitored in two sites for three years. At the same sites, we evaluated the efficacy of two new strobilurins, azoxystrobin and trifloxystrobin and the plant activator acibenzolar-S-methyl. The fungicides were applied at different doses of a.i. at late booting and at heading or only at beginning of heading. Efficacy of the new compounds was compared to efficacy of tricyclazole and the mixture of tricyclazole and propiconazole applied at beginning of stem elongation and at late booting. Severe epidemics of rice brown spot and rice blast were seldom observed in our field trial. Brown spot was always observed on leaves, whereas rice blast appeared only on flag leaf of japonica rice cv Balilla in 2000. Neck blast was observed approximately two wks after heading, but disease incidence never exceeded 15%. Panicle blast was due to infections of C. miyabeanus and M. grisea, and symptoms were not visually distinguishable. Panicle blast incidence increased exponentially in late August, when kernels were dough stage, and by harvest was found on almost 80% of panicles with severity of 68% on cv Balilla in 2000. Efficacy of fungicides against brown spot was low and varied between 25% on the susceptible cv. Balilla and 60% on tolerant cvs Thaibonnet, or L202, and Fragrance. Two treatments of azoxystrobin and trifloxystrobin at 250 and 125 g/ha a.i., respectively, were more effective than tricyclazole at 225 g/ha a.i. However, the difference of efficacy between two treatments of strobilurins and one treatment of tricyclazole at 450 g/ha a.i. was not significant. Both strobilurins and tricyclazole were highly effective against leaf blast and neck blast and reduced incidence and severity by 90-100% and by 75-90%, respectively. Tricyclazole at 225 g/ha a.i., applied at beginning of stem elongation alone or in a mixture with propiconazole at 125 g/ha a.i. did not significantly improve efficacy against brown spot and blast. Acibenzolar-S-methyl did not sufficiently control brown spot and blast. The yield increase for fungicide treated plots ranged between 0 and 8%, with the exception of cv Balilla in 2000 when reached 40% because of early epidemic of panicle blast. Fungicide treatments increased the average milling quality by 2.5%, whereas the percentage of grain discoloration did not increase significantly. Because severe epidemics are infrequent, fungicide treatments are cost-effective only when the risk of epidemic at late booting is high. Therefore, when severe epidemics are predicted, we suggest two treatments at late booting and at heading with azoxystrobin or trifloxystrobin at 250 and 125 g/ha a.i., respectively, or one treatment with tricyclazole at 450 g/ha a.i. followed by one treatment with strobilurin.