Abstract
Continuous cropping fields as an environmental factor on cucumber wilt pathogen were discussed. In cucumber or soybean field infested with Fusarium oxysporum f. sp. cucumerinum where crops were double cropping annually, chlamydospores of the pathogen changed to active form after cultivation of crops. In sorghum field the number of active forms was less than in cucumber field. In non-cropping field activities of the pathogen were not ensured. Although the pathogen grew and formed conidia in each crop rhizosphere, they turned more quickly to dormant stage in natural soil of non-host plant fields than the soil of host plant field. When cucumber was cultivated in each continuous cropping field, Fusarium wilt occurred only in cucumber field, but root damage more or less appeared in all fields. The damage, however, were slight in sorghum field, moderate in soybean field and severe in cucumber field.
The virulent pathogen was found more on the roots of cucumber than those of soybean and sorghum. However virulence of the pathogen in a field was subjected to the kind of cropping field. The cucumber field would offer favorable environments for the pathogen, especially for virulent isolates. On the other hand, the sorghum field would offer bad conditions. Accordingly, it is suggested that there were two types of pathogenic isolates, sensitive type and non-sensitive one as a suppressive factor, and this suppressive factor was obliged to ineffective when the population of an isolate in the field increased.
