Hyaloperonospora parasitica (downy mildew)

In California, USA, downy mildew of arugula (Eruca sativa) caused by H. parasitica was so severe that at times it reduced the crop quality to the extent that it could not be harvested (Koike, 1998). Also in California, maneb sprays were used to stop an outbreak of H. parasitica on broccoli, but not before an estimated crop loss of $100,000 had occurred (Kontaxis, 1977).

In Germany, in the autumn of 1996, downy mildew of winter oilseed was so severe that at some locations large areas were totally destroyed at the cotyledon stage (Paul et al., 1998).

In India, Mahajan and Gill (1993) indicated the direct effect on gross weight of cauliflower curd by downy mildew disease severity. Disease rating was negatively correlated with all the components of yield. In the Kashmir Valley, market surveys of kale revealed foliage disease indexes of 20-73 in 1981, and 15-57 in 1982. Indirect monetary losses to consumers were assessed at 8-55 and 5-50%, respectively (Puttoo and Choudhary, 1984). In Shanghai, China, a combination of seed treatment, direct sowing, application of fertilisers and fungicide treatments decreased the incidence of H. parasitica on Chinese cabbages and increased yield by 10-18% (Shao et al., 1991). Application of metalaxyl to Chinese cabbage has also been reported to give good control of H. parasitica, subsequently resulting in a 65% increase in yield (Yang et al., 1983).

In radish, disease index was shown to be positively related with crop density. At dense planting (30 x 30 cm), disease severity was 45% on leaves, 35% on stems, 20% on twigs and 22% on pods and resulted in a 50% decrease in yield compared with healthy control (Lakra, 1999). Vasil'eva (1976) reported that under favourable conditions, H. parasitica could infect up to 50-60% of cabbage seeds, subsequently reducing yield by 16-20%.