The effects of temperature on zoospores of the crook root fungus Spongospora subterranea f.sp. nasturtii

Abstract

The effect of temperature on the release, survival, encystment and infectivity of Spongospora subterranea f.sp. nasturtii zoospores released from diseased watercress roots was studied. There was rapid, synchronized release of zoospores at 20°C followed by a steep decline in zoospore numbers. A similar trend occurred at 10° C and 15° C, although fewer zoospores were released and the subsequent decline was more gradual. In contrast, at 5°C very low numbers of zoospores were released over the first 5 days. Subsequently, zoospore release increased rapidly over the following 3 days and zoospore numbers were maintained at relatively high levels for a further 6 days. This duration of high zoospore numbers at 5°C was due to prolonged survival rather than continual release. When a uniform zoospore suspension released at IOC was transferred to 20·C zoospore numbers were found to decline rapidly over 6 days; the decline was less rapid at 15° C and IOC, taking 9 and 15 days, respectively. At 5°C zoospore numbers decreased slowly. Few zoospores encysted on roots at 20° C but increased numbers were found at 15°C and 10°C. There were significantly more diseased roots at 10° C than at 5,15 or 20° C. The lowest number of diseased roots was found at 20° C. The correlation between increased seventy of the disease in the field during the winter months and the effect of temperature on zoospores of S. subterranea f.sp. nasturtii is discussed.