EFFECT OF TEMPERATURE AND HOST GENOTYPE ON THE PRODUCTION OF INOCULUMBY PHYTOPHTHORA-FRAGARIAE VAR FRAGARIAE FROM THE ROOTS OF INFECTED STRAWBERRY PLANTS

A bioassay was used to monitor the release of inoculum in drainage wat er from strawberry plants inoculated with zoospores of Phytophthora fr agariae var. fragariae. The fungus was detected in drainage water from plants that had been held at temperatures between 2 and 20 degrees C, but not from plants held at 26 degrees C. The lag phase before second ary inoculum was first released, the maximum and total amounts of inoc ulum released, and the length of time over which inoculum was released were all greater at the lower temperature regimes, especially those b elow 10 degrees C. The results were consistent with observations on th e effect of temperature on zoospore production from agar discs and on zoospore motility: more zoospores were produced at lower temperatures and they remained motile for longer. From this it is concluded that th e inoculum detected consists mainly of motile zoospores. In most exper iments with standardized suspensions c. 10-15 were sufficient to initi ate infection of the plants in the bioassay. In general, more inoculum was produced by host genotype/fungal isolate combinations in which th ere were marked root rot symptoms than in combinations in which the ho st was resistant.