In October and November of 1996, typical downy mildew symptoms appeared on rose plants cultivated in many areas in KwaZulu Natal, South Africa. White mycelial mats were observed on the lower surface of young and mature leaves and on stems at the base of the plants and shoot apices. Leaves and stems developed purplish to black spots. Flowers and buds had downy mildew-like growth on peduncles, calyces, and petals. Plants were poorly developed and generally chlorotic, and several were stunted. The most severe symptoms included premature leaf drop followed by plant death. The fungus from leaf and stem lesions was identified microscopically as Peronospora sparsa based on the following characteristics: conidia, sub-elliptical ranging from 20 to 23 μm long and 14.0 to 18.0 μm wide; conidiophores 150 to 350 μm long and 6 to 10 μm wide. Conidiophores protruded through the stomata as bunches of slender structures bearing conidia at the tips. The optimum temperature for sporulation and germination was 18°C. Artificially inoculated healthy mature leaves and stems developed purplish to dark brown lesions similar to those described from the field samples. The fungus was confirmed on the artificially inoculated tissues to complete Koch's postulates. The disease has been reported in all rose growing regions in KwaZulu Natal and causes up to 100% losses in some areas since its first appearance towards the beginning of winter 1996. The president of the Natal Rose Society, KwaZulu Natal, predicts that in the year 2000 rose plants may be eliminated if immediate control measures for Peronospora sparsa are not implemented. Rose is grown for local needs and has high export value in South Africa. This is the first report of the downy mildew of rose caused by Peronospora sparsa in South Africa.
