is a common plant disease prevalent on many barley varieties and causing yield losses of up to 40%. It is caused by the fungus Erysiphe graminis f.sp. hordei and appears as a distinctive fluffy white mass on leaves, initially as isolated spots but progressing into a complete covering which reduces light penetration resulting eventually to leaf death. Overwintering cleistothecia (closed spore-bearing structures) are eventually produced.

The fungus infects new plants from infected plants nearby or from field residues from previous seasons, often blown or splashed onto leaves by rainfall. Growth of the fungus is predominantly on leaves and less extensive on the flowering head but yield is reduced by its effect on stunting plant growth, reducing the number of tillers produced and the number of corns in each head.

During growth the fungus penetrates the leaf to obtain nutrients and saps energy from the plant. However, the majority of the fungus remains on the upper surface of the leaf. Warm, moist conditions enhance the growth of mildew. Rainfall is not essential but extensive leaf growth due to high nitrogen fertilizer application may encourage growth and enhance its spread.

Resistance to mildew, conferred by various genetic mechanisms, is present in some barley varieties. For example, the mlo gene, which controls cell wall development in the barley leaf, can limit the penetration of the fungus. Such resistance genes are important factors in limiting the impact of the disease on current barley production and are under investigation with the hope that their effectiveness can be enhanced.