In Western-Chosen, four species of Fusaria are found to be the cause of the take-all of cotton seedlings. No less than ninety per cent of cotton seedlings are affected by these fungi every year, which can only be detected when the roots of the seedlings were examined. The great damage of the disease happens once for three or five years. The circumstances which favour the infection of the disease are as follows: (1) the minimum temperature is lower than 12°C, (2) the heavy rainfall which makes the soil water content to its maximum capacity or nearly so. Affected seedlings will easily be recovered from this disease under such condition: (1) when the minimum soil temperature rises above 13°C, (2) when the water content of the soil becomes about 50 per cent of its water capacity. The process of the recovery of the diseased seedlings was studied on the basis of oecology and pathological anatomy. The outbreak of the take-all of cotton seedlings in Western-Chosen in 1938 was explained oecologically on the basis of this study. It is proposed that the causes of the outbreak of the disease in this year are as follows: (1) during the later half of May, the minimum daily temperature fell below 5°C successively for several days, (2) at the same time there was a heavy rain fall which was enough to bring the soil water content to its full water capacity. (3) there occurred, in succession, a continuous rainfall until the begining of June, (4) on the contrary, at the former half of June, the soil became arid rapidly because of the continuous (for more than six days) dry weather and high temperature.