chasmothecium

Chasmothecium is the sexual fruiting body of powdery mildew fungi, members of the order Erysiphales in the Ascomycota. It corresponds to the cleistothecium of other fungi but is distinguished by its ostiole, a small opening that allows the discharge of ascospores. Chasmothecia form on the surfaces of host plants, commonly on leaves, stems, or fruits, and are typically visible as small white or tan pustules.

Development begins after fertilization in the fungal life cycle. Inside the chasmothecium, asci form and produce eight ascospores each. The peridium, or wall, is usually multilayered and constructed from hyphal tissue; the outer surface may bear ornamentation such as appendages or hairlike structures in some species. Sizes are variable but most commonly range from a few tenths of a millimeter to about 0.5 millimeter in diameter.

As chasmothecia mature, a narrow ostiole at the apex opens to release ascospores, often in response to humidity and temperature conditions. This sexual stage contributes to genetic recombination and can provide overwintering inoculum, persisting on plant tissue until environmental conditions favor secondary infection.

Chasmothecia are diagnostic features for identifying powdery mildews, as their size, shape, wall characteristics, ornamentation, and the presence and structure of ostioles and appendages differ among species. The term chasmothecium emphasizes the ostiolated, closed nature of these structures in powdery mildews, while analogous structures in other fungi are usually called cleistothecia.