Conidial morphology, host colonization, and development of shot hole of almond caused by Wilsonomyces carpophilus

Morphology and ultrastructure of shot hole disease of almond infected by conidia of Wilsonomyces carpophilus were examined using light, scanning, and transmission electron microscopy. The multicelled conidia of the fungus were thick-walled and darkly pigmented. The conidial wall was multilayered and mainly consisted of an electron-dense, outer-wall layer and an electron-translucent, inner-wall layer. Septa of conidia were also multilayered. An electron-translucent zone separated the electron-dense, septa-wall layers of adjacent cells, and this zone extended to the inside of the outer-wall layer of a conidium. Conidia lacked true septa (distoseptate) and germinated by rupturing the outer-wall layers. Germination hyphae penetrated indirectly through stomata or directly through the cuticle into leaf tissue from appressoria that were produced terminally or on lateral branches of germ tubes. An extracellular mucilaginous matrix was commonly observed around hyphae and germ tubes in contact with leaf surfaces. Within leaf tissue, hyphae ramified throughout intercellular spaces and degraded cell walls of epidermal cells apparently without cuticle degradation. Diseased host tissue was tan brown and collapsed; ultrastracturally, diseased leaf cells were reduced in size, nonvacuolated, and had disrupted chloroplasts. A healthy host tissue response, adjacent to an infection site on leaves of potted plants, was the formation of a wound periderm. Within 10–14 days after an infection period (16 h of wetness), the periderm became a lignified–suberized barrier at 15 °C or a suberized abscission layer at 22 °C based on the histological stains Safranin O, Sudan III, and Sudan Black. At 15 °C, no abscission occurred and meristematic cells remained isodiametric but their walls became suberized and lignified, whereas cells adjacent to diseased tissue became lignified. At 22 °C, abscission occurred as cells adjacent to diseased tissue became vacuolated, enlarged, and suberized. Subsequently, the epidermis ruptured and the enlarged cells separated along the middle lamella to form an abscission layer. Hyphal growth was limited to the boundaries of the walled-off diseased tissue. Under favorable environmental conditions, hyphae replaced host epidermal cells and aggregated above the palisade layer to form pulvinate sporodochia. Sporodochia consisted of hyphae, numerous sympodially developing conidiogenous cells, and conidia that ruptured the host cuticle. Key words: fungal morphology, fungal foliar diseases, wound periderm, host–parasite interactions.