Light and electron microscopic observations of the infection of Camellia sasanqua by the fungus Exobasidum camelliae var. gracilis

Only a very small percentage of the shoots of the numerous Camellia sasanqua Thunb. plants examined in this study were infected by Exobasidium camelliae var. gracilis Shirai, and prior to the onset of new leaf formation in the spring, it was impossible to distinguish these shoots from healthy shoots with the naked eye. However, beginning in early April, an infected shoot gave rise to a group of tightly clustered, pale-green leaves that were significantly larger and thicker than the dark-green healthy leaves. Light and transmission electron microscopic observations revealed the presence of intercellular hyphae of E. camelliae var. gracilis in the apical meristems of shoots that gave rise to enlarged infected leaves. Very slender intercellular hyphae were observed between the tightly packed cells of the apical meristem as well as between the cells of the tunica that covered the meristem. Hyphae also filled many of the intercellular spaces in the stem immediately below the apical meristem. Some hyphae formed distinctive haustoria that protruded for short distances into the invaded host cells. Hyphae also were present in the leaf primordia and proliferated in the intercellular spaces while the primordium differentiated into a leaf. Eventually a distinct pseudoparenchymatous layer of intercellular hyphae developed some four to six cell layers above the lower epidermis. This layer then gave rise to the basidia that made up the hymenium of E. camelliae var. gracilis. As basidia developed, the underlying cell layers of the leaf separated from the overlying hymenium and eventually sloughed away to expose the basidia. Localized infections of random leaves also were observed. In many cases only a small part of a leaf was infected. Localized infections also culminated in successful hymenium development.