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SUMMARY In three field experiments in Cornwall in 1969–71, spraying 6–8 times with the fungicides zineb or oxycarboxin gave good control of severe attacks of brown rust (Puccinia hordei) in four cultivars of spring barley and increased the persistence of green leaf tissue. The disease developed late in the season, mainly after ear emergence, Other leaf diseases were slight, except for mildew (Erysiphe graminis) in 1971 in one cultivar. Spraying resulted in increases in grain yield ranging from 17 to 31 per cent and these were almost wholly due to increased grain weight (1000-grain weight). One and two-spray programmes of the fungicides gave only a partial control of brown rust and small yield increases. There was a statistically significant relationship between grain yield and brown rust recorded at growth stage 75 (11.1),* an increase of 10 per cent in brown rust on the second top leaf reducing yield by 7.7 per cent.

SUMMARY Frequent spraying with fungicides gave a fairly good control of glume blotch (Septoria nodorum) in winter wheat and increased the persistence of green leaf tissue. Spraying resulted in substantial increases in yield which related well to the degree of disease control achieved; the increase over unsprayed ranged from 18 to 43 per cent, depending on the fungicide used. The whole of the yield increase could be accounted for in terms of 1,000 grain weight.

Summary In nine experiments in 1966–68, fungicides applied on 8–12 occasions to plots of barley resulted in mean grain yield increases of about 40 per cent. Zineb increased yields by an average of 50 per cent in nine experiments and lime sulphur by 19 per cent in six experiments. In single experiments dichlofluanid increased yields but oxycarboxin did not. In 1968, fungicides applied on only two occasions failed to control disease and to increase grain yield. Large yield increases were associated with a good control of leaf diseases including leaf blotch (Rhynchosporium secalis), halo spot (Selenophoma donacis) and brown rust (Puccinia hordei). Mildew (Erysiphe graminis) was excluded by using resistant cultivars. The diseases were severe late in the season, usually at or after ear emergence. Yield increases were wholly due to an increase in grain weight (1000-grain weight).

SUMMARY Two methods were used to investigate the loss in grain yield associated with specific levels of leaf blotch. Yields from plots sprayed with fungicide were compared with those from unsprayed plots and yields of varieties of different susceptibility to the disease were compared with one another. A disease assessment key is presented, which was used to assess the percentage laminar area of the top two leaves affected by the disease. A linear relationship between disease on the upper two leaves and yield was established. Results from nine trials showed a consistent relationship between the disease level, at growth stage 11·1 (Feekes scale), and loss in yield. The loss in yield expressed as a percentage of the yield of an uninfected crop was equivalent to approximately two-thirds of the percentage of the flag-leaf area visibly infected, or one-half of the infected area on the second leaf. The predicted loss in yield is the average of these two estimates.