Downy mildew of onion is one of the major threats to onion cultivation, especially in subtropical and dry temperate zone, caused by an oomycetes Peronospora destructor (Berk.) Casp. ex Berk. A study was undertaken to investigate into the epidemiological factors that affects the initiation and dispersal of disease. Under in vitro conditions suitability of different temperature and relative humidity regimes, on germination and relative growth of germ tube has been studied. The pathogen preferred a temperature of 15°C as optimum for sporangial germination where the favourable relative humidity was above per cent with maximum sporangial germination at 100 per cent. Correspondingly the effect of different meteorological parameters on the appearance and dispersal of the disease were also studied. | Epidemiological studies of downy mildew of onion
