Forecasting barley mildew development in West Scotland

Observations on plots of spring barley cv. Golden Promise during May‐July 1973–79 showed that escalation of mildew commenced in mid‐June 1976, in late June in 1973 and 1974, in early July in 1978 and 1979, and failed to develop in 1975 and 1977. By mid‐July there was 40% infection in 1976, and 13–21% infection in 1973, 1974, 1978 and 1979. Less than 2% mildew occurred in 1975 and 1977. Using as a basis the method of mildew forecasting formulated by Polley & King (1973), it was found that once mildew was established in the crop there was moderately good association between the incidence of high mildew risk according to the Polley weather criteria and the major peaks of spore concentration above the crops. It was not possible to correlate these two features during May and early June each year because of a lack of inoculum, but it is considered that the failure of mildew to develop during the entire growing seasons of 1975 and 1977, in spite of frequent high risk periods, was due in part to low atmospheric humidity during the early part of the season. The relative number of accumulated Polley factors satisfied on days when the relative humidity approx. 1 m above ground was 78% or more (at 0900 GMT) during the period mid‐May to mid‐June, separated the low mildew years of 1975 and 1977 from the moderate ones of 1973, 1974, 1978 and 1979, and the severe mildew year of 1976. This procedure may provide a practical means of forecasting the level of mildew build‐up during the later growth stages of the barley crop.