Abstract

A bioassay was used to monitor the release of inoculum in drainage water from strawberry plants inoculated with zoospores of P. fragariae var. fragariae. The fungus was detected in drainage water from plants that had been held at temp. between 2 and 20C, but not from plants held at 26C. The lag phase before secondary inoculum was first released, the max. and total amounts of inoculum released, and the length of time over which inoculum was released were all greater at the lower temp. regimes, especially those below 10C. The results were consistent with observations on the effect of temp. on zoospore production from agar discs and on zoospore motility: more zoospores were produced at lower temp. and they remained motile for longer. From this it is concluded that the inoculum detected consists mainly of motile zoospores. In most experiments with standardized suspensions c. 10-15 were sufficient to initiate infection of the plants in the bioassay. In general, more inoculum was produced by host genotype/fungal isolate combinations in which there were marked root rot symptoms than in combinations in which the host was resistant.