Infection of European Hazelnut byAnisogramma anomala: Ascospore Adhesion, Mode of Penetration of Immature Shoots, and Host Response

Ascospores of Anisogramma anomala adhered irreversibly to vegetative internodes of European hazelnut shoots (Corylus avellana) within 5 min of initial contact. The concentration of spores retained was greatest on and at the base of trichomes of the stem internode nearest the apical bud. Reported differences in susceptibility of internodes to infection corresponded to relative differences in spore adhesion and histological maturation in the cuticle and epidermal cells. The infection process was studied with light and transmission electron microscopy on susceptible cv. Ennis. After contact with the host, an adhesive pad developed on the lateral wall of ascospores. Germ hypha penetrated through the pad and host epidermis directly. After breaching the wall, penetration hyphae expanded into vesicles that were similar in morphology to those observed in vitro. However, in all specimens observed, further colonization apparently was restricted by a hypersensitive-like response of the host. Apposition of callose, cell necrosis, and formation of a cicatricial layer isolated the fungus and halted colonization of the adjacent cells. Observations suggest that most attempts by A. anomala to infect C. avellana are prevented by host response and that successful infections are rare.