Abstract
The longevity and infectivity of zoospores of Aphanomyces euteiches and A. cochlioides in soil were studied. The motility of most zoospores was lost in soil within a day, but a small number of them kept for 5 days. The most of zoospores incubated in unsterilized soil disappeared within 5 days, but under low temperatures such as 13C to 20C, a small number of them still remained and kept germinability for 24 days. Zoospores of both fungi kept their infectivity in unsterilized soil for 20 or 30 days. Fifty zoospores per gram of soil were enough to cause damping off of the respective host plant seedling. The maximum zoospore production of A. euteiches per infected seedling attained 9×104 at 75-90% of maximum water-holding capacity of soil and 1.8×105 at 50-60%. Zoospores of A. euteiches and A. cochlioides which are produced on the infected plants survive for 3 to 4 weeks in soil, and surve as inocula of subsequent infections.
