Damage by aogare (wilt disease) to paddy rice and nutritional and physiological conditions

On September 13, 1982, aogare (wilt disease) occurred in the northern part of the Kanto Region right after typhoon No.18 had passed. The damage extended over 3,700 ha of paddy rice in the northern area of Saitama prefecture. To investigate the actual conditions of damage and its cause in these areas, such as meteorological factor, fertilizing practices on paddy field, and nutritional and physiological conditions of different varieties of paddy rice were studied. The results are summarized as follows: (1) The day was bright with high temperature of 28 degrees C, wind speed of 6.0 m s(-1), minimum humidity 36% at 9:00 AM on the 13th; at 15:00 the ground surface was dehydrated and acute dehydration disease occurred in paddy rice. (2) Aogare occurred about 20-30 d after heading. The physiological symptoms were partly attributed to high levels of total nitrogen application especially in the later growth stage. (3) The average of physical and chemical values of the soil in the aogare area, which decreased less than in other areas, were as follow: the depth of the plow layer was 10.4 cm; soil hardness exceeded 2.0 MPa; hydraulic conductivity was 10(-6)-10(-9) ms(-1). These areas obviously had compaction, and adverse effects on the soil environment were also found. (4) The photosynthate content of starch and sugars in the leaf sheath plus culm tended to be lower in the aogare paddy rice. The carbohydrate content in the stem was less than 14% in the aogare plants. (5) The potassium and silicon contents in the straw of aogare rice wee reduced. The carbohydrate content of the culm decreased to the minimum level 20-30 d after heading. This was also confirmed by the starch iodine test. (6) The basic factors in aogare is thought to be related to nutrient conditions during the ripping stage, climatic conditions, the amount of nitrogen application, and changes in the soil environment in recent years