Studies on environmentally founded rice [Oryza sativa] plant disease management

For establishment of low chemical input agriculture, this thesis describes various methods for integrated plant disease management against major rice diseases including rice blast disease, rice sheath blightm and seed disinfection of some seed-borne rice diseases. The following summarizes the thesis. 1. Characteristics of rice blast disease occurrence in Shonai, Yamagata prefecture, from 1971 to 2001 This study addressed characteristics of occurrence of rice blast disease from primary occurrence of leaf blast after transplanting to final incidence of panicle blast just before harvest in paddy fields. Research data for forecasting rice blast disease were used : data at the Shonai Plant Protection Center were accumulated over 31 years from 30 locations in the Shonai district. Results of this study indicate that the area of disease occurrence of rice blast disease in the Shonai district has decreased markedly from 1981. The amount of leaf blast development has been closely dependent on occurrence of panicle blast in the paddy fields : when the amount of panicle blast was low, leaf blast disease either did not occur or was delayed from the primary occurrence time. 2. Behavior of Pyricularia oryzae in brown rice seed as inoculum source of rice seedling blast The following were investigated to establish effective control methods for seedling blast caused by infected seeds : the status of Pyricularia oryzae infestation of rice seeds harvested from seed production fields ; the role of infected seeds on seedling blast occurrence ; and effective methods for disinfecting infected seeds. Rice blast infection intensity of seeds was tested by the Blotter method. About 20% of seeds that were sporulated on the seed surface showed fungal invasion into the husk. Results of periodical observation on invasion into the inner husk showed that the rate of seeds invaded the fungus into the husk increased concomitant with advancing of reaping, Blastinfected seeds harvested at late reaping were not only removed by specific gravity of salt, but also disinfected with some seed disinfectant chemicals. The inner infected seed by the fungus developed seedling damping-off and sporulated abundantly at the base of the seedling stem at 40 d after sowing. Vacuum filtration methods with disinfectant chemicals and hot water immersion method allowed effective control of pathogens in the seeds without conventional commercial chemicals. 3. Seed disinfection using hot water immersion to control several seed-borne diseases of rice plants The author designed a seed disinfection method using hot water to control seed-born fungal and bacterial rice disease to establish low-chemical-input disease protection for rice production. Three sets of 10-kg weight seeds infested with Pyricularia oryzae, Fusarium moniliforme and Pseudomonas plantarii were immersed in 200 l of hot water at 58-60 deg C for 10-20 min using an experimental machine that maintained water temperature accurately (+-0.1 deg C). Occurrence of each disease and the rate of seedling emergence in nursery beds were observed after hot water treatment. Hot water immersion was as effective as conventional chemical seed disinfection for disease control. In a germination test, greater than 90% seed germination was obtained with 20 min at 58 deg C or 15 min for 60 deg C for eight nonglutinous rice cultivars, but not for three glutinous rice cultivars used in this study. These results indicate that hot water immersion for seed disinfection is a promising method compared to conventional chemical seed disinfection methods. 4. Suppression of rice seedling blast by application of silica gel as a silicon source to nursery soil Silica gel (250-1000g ; 99.8% SiO2, pH5.0) was mixed into nursery soil (3000g) in a nursery box (30 X 60 x 3cm) before sowing blast-infested rice seeds (100g per nursery box) to clarify its effects on the incidence of seedling blast, soil pH, SiO2, and nitrogen content of seedlings grown in such soil. The relative frequency of diseased seedlings on the soils with 250g of silica gel (the lowest dose) was 80% lower than that of the soil without silica gel. The silicon content of rice seedlings from soil with silica gel (500g) increased starting at 5 d after sowing. The value was 235% higher than that of control plants at 7 d. The pH of all soils with added silica gel did not change; it was maintained in the optimal range for nursery growth of rice plants with respect to other commercial silicon materials that engendered unsuitable pH values. Furthermore, silicon absorbance in rice plants of soils with silica gel was more effective than that of seedlings grown in soils with other commercial silica materials. Nitrogen content in the seedlings from the silica-gel-mixed nursery soil did not decrease. These results show that silica gel application to the nursery soil was an effective method for rice seedling blast control. Subsequently, the author tried to determine suitable silicon contents in rice plants for suppressing blast occurrence in the seedling stage. The number of blast lesions on leaf sheathes decreased when blast infested seeds were sown and cultivated on Si-deficient volcanic ash soil to which increased amounts of silica gel had been added. In addition, SiO2 content in the rice plants increased linearly with increased amounts of silicon applied to the soil. Nevertheless, total nitrogen content did not vary among plants grown in soils with different silica gel concentrations. Next, the author cultivated nine rice cultivars with different complete resistance gene and different degrees of partial resistance in nursery soils mixed with silica gel at different rates. Rice seedlings were inoculated with a compatible strain of Pyricularia oryzae to allow estimation of their resistance against seedling blast. The degree of resistance to rice blast disease differed markedly among rice cultivars regardless of silicon levels in the plants and soil. However, no significant difference was found within each rate of SiO2 among the rice cultivars themselves. Notwithstanding, in all rice cultivars used. The number of sporulating lesions per leaf was reduced significantly with increased SiO2 content of the seedlings : the number of lesions was reduced 5-20% from the initial number when the SiO2 contents reached 5%. Moreover, the author compared susceptibility to blast disease of rice seedlings (cv. Haenuki) grown in eight soils from different districts, to which varying amounts of silica gel were added. The number of blast lesions on rice seedlings cultivated in 'Aso' and 'Gosei' nursery soils remained very low regardless of the amount of silica gel added to the soils. In other soils, the number of susceptible lesions decreased significantly when SiO2 content of the rice plants reached 5%. These results suggest that suitable SiO2 content for control of this disease at rice plants' seedling stage under any conditions would be at least 5%. This information may further elucidate constituents of good soil forrice seedlings. 5. Occurrence of rice sheath blight and development of the action threshold of the disease Another important rice disease is ride sheath blight, caused by Rhizoctonia solani. Chemical fungicide is apt to be overused to control this disease in this district. Damage to three rice cultivars - 'Domannaka', 'Haenuki' and 'Sasanishiki' - affected by the fungus was investigated by the percentage of yield loss and the percentage of whole grains. Using relevant results, the author attempted to estimate the action threshold of the disease to reduce the use of chemical fungicide. The action threshold was estimated by the percentage of the above two items of diseased rice hills at heading time. The degree of the disease incidence (Y) at the maturing stage was anticipated by the percentage of affected hill at heading time (X). It was calculated by the following regression equation : Y = 0.971X+- 2.306. The percentage yield loss was estimated from the degree of disease incidence at the maturing stage of rice plants. The degree of disease incidence of 'Domannaka' was 23.6, 'Haenuki' was 20.0, and that of 'Sasanishiki' was 16.5 ; in economic injury level was 5% of yield loss. The author inferred that the action thresholds of both 'Domannaka' and 'Haenuki' were 15% of affected hills at heading time. In addition, the author anticipated a 10% action threshold for 'Sasanishiki' because the rice cultivar was liable to lodge. 6. Distribution of rice sheath blight in a paddy field and yield loss by the disease Distribution of rice sheath blight disease in a single paddy field and yield loss by the disease were investigated to establish a simple investigating method and an efficient control method for the disease. Results of investigation showed clearly that rice sheath blight disease occurs intensively near ten rows from a levee in a field where the disease occurs frequently. The percentage of hills affected by the disease declines abruptly with distance from the levee. Moreover, the influence on yield was small. Consequently, by control at the field of about ten rows from the levee, where the disease occurred frequently, the source of infection was reduced efficiently the following year.