Visualization of fungal hyphae in the trichomes of sawtooth oak leaves

The trichomes of Quercus acutissima (sawtooth oak) leaves were investigated to ascertain a possible fungal habitat on the phyllosphere. Brown spots with yellow halos occurred on naturally-infected leaves of sawtooth oak in Suwon, Korea. Tubakia dryina was isolated from the brown spots and identified based on its morphological characteristics and DNA sequence data. Field emission scanning electron microscopy revealed hyphal branching and prevalence of septate hyphae on the adaxial leaf surface. Non-glandular trichomes were usually colonized by septate hyphae. Hyphae coiled around the trichome rays. Epidermal shrinkage of trichomes appeared rather frequently observed on the diseased leaves than on the control leaves. Transmission electron microscopy revealed the presence of fungal hyphae in the naturally-infected trichomes of this oak species. Hyphal growth appeared to dissolve the non-glandular trichomes on the abaxial leaf surface. Concentric bodies and intrahyphal hyphae were often found in the hyphal cytoplasm. These results suggest that the foliar trichomes of sawtooth oak could serve as fungal habitats or infection sites.