Host–parasite relationships and development of the ergot sclerotium in Claviceps purpurea

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Abstract
Conidia of Claviceps purpurea germinate on the stigmas of Secale cereale, and intercellular hyphae grow downward through the style and inner wall of the ovary to the tip of the rachilla. Across the tip of the rachilla, hyphae surrounding living host cells form a narrow interface that plunges downward into the central vascular bundle approximately to the level of origin of the lodicules. This biotrophic relationship is well established 5–6 days after infection at anthesis and persists unaltered through maturation of the ergot sclerotium at approximately the 28th day. Tissues in the base of the ovary above this interface are destroyed and replaced by fungus stroma. Xylem vessels in the tip of the rachilla are destroyed and replaced by a hyphal core extending downward from the stromatic foot of the fungus. Hyphae emerging between the epidermal cells of the ovary form an external matrix that contributes to further expansion of the stroma. A palisade of phialidic conidiophores develops over the surface of the stroma, which is involuted into labyrinthine chambers. Conidia ooze from the florets in drops of honeydew by the 6th or 7th day. The ovarian cap and the upper part of the ovule are pushed upward on the apex of the conidial stroma, and some of these isolated host cells remain alive until the 11th day. A layer of vertically arranged parallel hyphae across the base of the conidial stroma forms a generative zone that sharply delimits the basal stromatic foot seated in the floret and by apically directed intercalary growth produces the compact tissues of the ergot sclerotium. The sclerotium begins to protrude from the floret by the 11th day. The withered remnants of the conidial stroma and the ovarian cap are carried upward on the tip of the massive sclerotium. Beneath the dark rind the hyaline tissue of the sclerotium consists primarily of compacted, isodiametric storage cells. A loose hyphal conducting tissue forms a central strand from which narrow vanes radiate irregularly toward the periphery. Ergot of rye is a replacement disease in which the parasite disintegrates host tissues in areas required for development of its own structures while maintaining a compatible relationship with adjacent host cells as a source of water and nutrients.

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