EMERGING DISEASE PROBLEMS IN INTENSIVE RICE SYSTEMS IN ASIA
H KATOl, S WAKIMOTO2 and TW MEW3
lKwansei Gakuin University, Nishinomiya, Hyogo 662-0891 Japan; 2Tokyo University of Agriculture, Setagaya, Tokyo,156-0054 Japan; 3International Rice Research Institute, PO Box 933, 1099 Manila, The Philippines
Modern rice cultivars were distributed and grain productivity increased in Asia. By this change, tungro disease covered rice cultivating tropical areas and was conquered by newly bred cultivars. Breakdown of resistance to blast disease was experienced in Indica-Japonica bred lines in Japan and Korea. Non race specific resistant cultivars were bred, and disease forecasting systems and new fungicides have been developed. After such "Green Revolution", what are our problems of recent and future systems of intensive rice production? We would like to select three topics here and explain how they are affected by the disease occurrence.Labor-saving technology Because of economic growth in the rice cultivating area, urbanization caused shortage of labor. There are two directions, mechanization and other devices of production skills. At the stage of nursery (i)the adoption of a nursery box for mechanization and (ii) the adoption of direct seeding in three different ways has progressed. In the case of (i)seed borne diseases such as blast, Helminthosporium brown spot, bacterial diseases caused by Burkholderia glumae and B. plantarii and soil borne diseases like seedling blight caused by Fusarium spp., Pythium spp., Rhizopus spp., Trichoderma viride, Rhizoctonia solani, Corticium rolfsii and Mucor fragilis occurred. In the case of (ii), blast occurrence is observed because of the high density of plants affecting the duration of wetness. When the seeds are preserved under high density and no soil covering conditions, various pathogens are born from seeds because of aerobic and wet conditions. B. glumae and Acidovorax avenae produce the seedling rot. Higher fertilizer use
High rate of nitrogen, phosphorus and potassium fertilizer is applied to achieve high yield in modern cultivars. High nitrogen input results in predisposition favorable for blast, sheath blight and bacterial grain rot caused by B. glumae. Phosphorus has the same effect on blast occurrence. Quality improvement
Yield enhancement is still our major objective, but quality improvement also becomes important issue. (i)The main diseases like blast, sheath blight, Helminthosporium brown spot, bacterial leaf blight (Xanthomonas oryzae pv. oryzae) and bacterial grain rot not only affect yield loss but quality deficiency. Newly noticed sheath blight syndrome caused by Sclerotinia fumigatum, S. hyderohilum, Rhizoctonia zeae, S. oryzae-sativae, R. oryzae, R. solani AG2-2 shows similar symptoms to Rhizoctonia(AG-1) sheath blight. The last three pathogens produce sclerotia mainly within the tissues of the host sheath. The others produce sclerotia on the surface of the host tissues. The sheath rot complex and grain discoloration syndrome involves a complex group of Pseudomonads and Sarocladium oryzae. All these diseases appear in the later stage of growth, giving rise to the suspicion about effects on the quality of grains. (ii) The minor diseases, even saprophytic fungi cause quality deficiency. At flowering,the fungi such as Alternaria padwickii, A. alternata, Curvularia lunata, C. intermedia, C. clavata and Epicoccum purpurascens invade into the gap between the hulls, infecting the seed coat and causing discoloration of kernels. The contaminant such as Tilletia barclayana and Ustilaginoidea virens results in disturbance of processing and causes low quality.