Chapter 13: Hymenoascomycetes: Erysiphales

Members of the family Erysiphaceae develop superficial mycelium on leaf and shoot surfaces, producing distinctive white powdery colonies composed primarily of conidia. These fungi employ specialized infection structures called appressoria to breach the plant epidermis and establish nutrient-absorbing haustoria within host cells. A defining characteristic of powdery mildew conidia is their ability to germinate and infect without free water, making these pathogens particularly successful in arid environments where many other fungi struggle. The asexual stage dominates the disease cycle, with abundant conidia serving as the primary dispersal mechanism and enabling multiple infection cycles within a single growing season. Sexual reproduction produces chasmothecia, globose fruiting bodies containing one or more asci with distinctive appendages that aid in species identification and provide critical survival structures between crop seasons. The chapter provides detailed examination of Blumeria graminis, a major cereal pathogen with specialized host-specific formae speciales infecting wheat, barley, rye, and wild grasses. Infection by this fungus involves coordinated development of primary and secondary germ tubes, enzymatic penetration of the cuticle, and formation of specialized haustoria for nutrient extraction. Plant resistance operates through a gene-for-gene recognition system triggering hypersensitive cell death that arrests fungal colonization, while the mlo allele in barley confers exceptionally durable and broad-spectrum resistance. The chapter surveys major powdery mildew genera affecting diverse hosts, including Erysiphe on crucifers and legumes, Microsphaera on oak, Uncinula as the historic grape pathogen, Podosphaera on apples and roses, and specialized genera with unusual internal mycelium development. Control strategies integrate multiple approaches including selective breeding for polygenic resistance, chemical fungicides spanning sulfur compounds and modern synthetic classes such as triazoles and strobilurins, and biological control agents including hyperparasitic fungi and phylloplane yeasts, though biological methods show limited field efficacy. This chapter underscores the evolutionary dynamics between pathogenic adaptation and plant defense mechanisms shaping agricultural disease management.