First Report of Pseudoperonospora rumicis Causing Downy Mildew on Rumex vesicarius in India

Bladder dock (Rumex vesicarius L., family Polygonaceae) is an important biennial bushy vegetable in southern India. The young leaves have a piquant flavor and sour taste, and are eaten boiled or raw in salads. The dried roots are ground into powder and used to treat diarrhoea and bleeding. From September to November 2007 and 2008 symptoms and signs consistent with downy mildew were observed on R. vesicarius at the AVRDC-RCSA field plots in Hyderabad, India (Fig. 1). Initial symptoms included chlorotic lesions on the leaf blade, curling of the affected leaf tissues, and later in the season blighting of the entire plant foliage. During wet weather or high relative humidity, sporangiophores with sporangia were abundantly produced on the abaxial and occasionally on the adaxial sides of the middle and lower leaves (Fig. 2) resulting in a purple-gray appearance. Sporangiophores, 313-573 μm long, emerged in groups of 3 to 15 through stomata. They were dichotomously branched, tree-like, straight to sub-straight, slightly swollen at the base and ended with sterigmata at acute angles (with the tips of the branches acute) bearing single sporangium. Sporangia were thin-walled papillate, poroid, operculate, ellipsoidal, hyaline to olivaceous brown in color, with a mean length of 46.7 μm (32.9-60.5 μm) and a mean width of 30.4 μm (17.9-42.9 μm) (Fig. 3). The sporangia germinated indirectly by producing 40 to 60 globose zoospores (diameter 9-15 μm) per sporangium. Fig. 1. Downy mildew symptoms and signs on infected leaves of Rumex vesicarius.