The effect of relative humidity during production and germination of Erysiphe graminis f. sp hordei was examined in the laboratory. High humidities at both stages promoted the infectivity of the conidia. Experiments with different soil depths showed that deeper soils resulted in higher humidities around the plants and significantly higher levels of disease. When differential humidity build - up around the plants during different watering regimes was prevented, there was no significant difference in disease development. This confirms that the effects observed were due to changes in atmospheric relative humidity, not to alterations in the water supply to the host plant. Field trials confirmed the importance of relative humidity to disease development in the crop, and a clear relative humidity gradient was discovered within the stand. As with the laboratory experiments, deeper and heavier soils resulted in higher humidities around the plants and an accelerated rate of disease development. By constantly monitoring relevant environmental factors, a forecasting system was devised to predict best spray date for fungicide treatment. A comparison of a range of commercially available fungicide treatments was undetaken in both glasshouse and field experimlents to assess control of mildew, Puccinia hordei and Rhynchosporium secalis on spring barley. The two aspects of the work were combined in 1984, where the most effective fungicide was used with the developed forecasting system showing that by using efficient chemicals, properly timed, impressive yields can be obtained at minimum cost.