Dimorphism and microcycle conidiation of Pseudocercospora pseudostigmina-platani conidia from American sycamore

In this study, the micromorphology and surface details of Pseudocercospora pseudostigmina-platani conidia were investigated on American sycamore (Platanus occidentalis) leaves. Sooty films were collected from diseased leaves for fungal DNA extraction and observed using light and field-emission scanning electron microscopy. Two types of conidia were present on the leaves: (i) hyaline and curved Cercostigmina-like conidia and (ii) brown and straight Stigmina-like conidia. The fungal pathogen was identified as P. pseudostigmina-platani based on morphological characteristics and internal transcribed spacer region sequences. Some non-glandular trichomes were found near the centre of the conidial masses on the abaxial surface of the naturally infected leaves. Leaf ultrasonication to dislodge conidia revealed sporodochia erupting through the stomata. Hyphae from sporodochia grew on the leaf surface and entered host tissues through the stomata. Conidiogenous cells had terminal rings and tapered towards the flat region where a conidium was attached. The conidia produced secondary conidia directly without hyphae. After dislodging the conidia by ultrasonication, scars were observed. These results indicate that the fungus has dimorphic conidia and microcycle conidiation for simple and rapid asexual reproduction.