EFFECTS OF WIND, RELATIVE-HUMIDITY, LEAF MOVEMENT AND COLONY AGE ON DISPERSAL OF CONIDIA OF UNCINULA-NECATOR, CAUSAL AGENT OF GRAPE POWDERYMILDEW

A wind tunnel was designed to study the effect of wind, relative humid ity, leaf movement and colony age on dispersal of conidia of Uncinula necator. Wind speed as low as 2.3 m s(-1) instantaneously triggered di spersal of conidia from fixed leaf discs of 18-day-old infections. Con idia were observed on sporulating leaf discs even after exposure to 17 m s(-l) wind. The fraction of conidia dispersed ata given wind speed increased with colony age from 12 to 24 days. Conidia of 27-day-old co lonies were less easily dispersed. No gradient of maturation of conidi a along the conidial chain was observed, suggesting that even newly fo rmed conidia were able to germinate after dispersal. Germination of di spersed conidia decreased slightly with greater colony age. Both wind and simulated rain drops caused dispersal of conidia from infected lea ves. Leaf movement at wind speed of 3.5-4 m s(-l) increased dispersal, and the first impact of three simulated raindrops caused release of 5 3% of the total conidia dispersed. Relative humidity had no effect on dispersal of conidia at different wind speeds.