Effect of wind on the dispersal of oospores of Peronosclerospora sorghi from sorghum

The effect of wind on the dispersal of oospores of Peronosclerospora sorghi, cause of sorghum downy mildew (SDM) is described. The oospores are produced within the leaves of aging, systemically infected sorghum plants. These leaves typically undergo shredding, releasing oospores into the air. Oospores are produced in large numbers (6.1 x 10(3) cm(-2) of systemically infected leaf) and an estimate of the settling velocity of single oospores (0.0437 m s(-1)) of P. sorghi indicated their suitability for wind dispersal. In wind tunnel studies wind speeds as low as 2 m s(-1) dispersed up to 665 oospores per m(3) air from a group of leaves previously exposed to wind and displaying symptoms of leaf shredding. The number of oospores dispersed increased exponentially with increasing wind speed. At 6 m s(-1), up to 12890 oospores per m(3) air were dispersed. Gusts increased oospore dispersal. A constant wind speed of 3 m s(-1) dispersed a mean of 416 oospores per m(3). When gusts were applied the mean was 15 592 oospores per m(3). In field experiments in Zimbabwe, oospores were sampled downwind from infected plants in the field and at a height of 3.8 m above ground level immediately downwind of an infected crop. These data indicate that wind could play a major role in the dispersal of oospores from infected plants in areas where SDM infects sorghum, perhaps dispersing oospores over long distances.