THE EFFECTS OF TEMPERATURE ON ZOOSPORES OF THE CROOK ROOT FUNGUS SPONGOSPORA-SUBTERRANEA F SP NASTURTII

The effect of temperature on the release, survival, encystment and inf ectivity of Spongospora subterranea f.sp. nasturtii zoospores released from diseased watercress roots was studied. There was rapid, synchron ized release of zoospores at 20 degrees C followed by a steep decline in zoospore numbers. A similar trend occurred at 10 degrees C and 15 d egrees C, although fewer zoospores were released and the subsequent de cline was more gradual. In contrast, at 5 degrees C very low numbers o f zoospores were released over the first 5 days. Subsequently, zoospor e release increased rapidly over the following 3 days and zoospore num bers were maintained at relatively high levels for a further 6 days. T his duration of high zoospore numbers at 5 degrees C was due to prolon ged survival rather than continual release. When a uniform zoospore su spension released at 10 degrees C was transferred to 20 degrees C zoos pore numbers were found to decline rapidly over 6 days; the decline wa s less rapid at 15 degrees C and 10 degrees C, taking 9 and 15 days, r espectively. At 5 degrees C zoospore numbers decreased slowly. Few zoo spores encysted on roots at 20 degrees C but increased numbers were fo und at 15 degrees C and 10 degrees C. There were significantly more di seased roots at 10 degrees C than at 5, 15 or 20 degrees C. The lowest number of diseased roots was found at 20 degrees C. The correlation b etween increased severity of the disease in the field during the winte r months and the effect of temperature on zoospores of S. subterranea f.sp, nasturtii is discussed.