An Ultrastructural Study of the Asexual Spores of the Plant Pathogenic Fungus Exobasidium vaccinii

The abaxial surface of a leaf of Vaccinium angustifolium infected with Exobasidium vaccinii eventually became covered with a thin white layer of material consisting of basidia, basidiospores, asexual spores (secondary spores formed from basidiospores and conidia formed from secondary spores), and some hyphae. Detached basidiospores trapped in this layer routinely germinated to form short germ tubes that gave rise to slender spores, termed "secondary spores." Ultrastructural examination revealed that the secondary spore was a holoblastic conidium because it lacked the hilar appendage and droplet present on the secondary sporidia of some basidiomycetes. Except for size differences, fine structure was similar in secondary spores and in conidia produced either from secondary spores on the leaf surface or from sparingly branched hyphae growing in culture on agar medium.