The causal agent of the disease is the highly specialized fungus Blumeria graminis f. sp. hordei. The pathogen infects all above-ground plant organs, forming a superficial mycelium that attaches via appressoria and directs haustoria into epidermal cells for nutrient uptake. In mild climates, the fungus survives on winter crops, while in harsh conditions, it persists in the form of cleistothecia on plant debris.
Symptoms appear as a white, web-like coating that eventually densifies into cotton-like patches, which take on a yellowish-grey hue. At later stages, black dots—cleistothecia—develop within the coating. The infection leads to yellowing and wilting of leaves, impairment of photosynthesis, and grain shriveling.
Genetic protection of barley is based on the use of more than 100 identified resistance alleles, including the Mla and Ml genes and the mlo locus. Of particular importance are the mlo genes (e.g., mlo1, mlo11), which provide effective resistance by preventing the penetration of fungal haustoria into plant cells.
General information about the subject — on the page
Powdery mildew.