Histopathological Investigations of the Infection Process and Propagule Development of <em>Phytophthora ramorum</em> on Rhododendron Leaves

Abstract

In Europe, cultivated rhododendron is one of the most important hosts for Phytophthora ramorum. To better understand leaf infection and leaf capacity for sporulation, infection studies were carried out. Detached leaves of Rhododendron ‘Catawbiense Grandiflorum ́ and `Brigitte ́ were inoculated with zoospore suspensions of P. ramorum isolates of mating type A1 and A2. ‘Catawbiense Grandiflorum’ developed much more leaf necrosis than ‘Brigitte’ (average necrotic leaf area after 55 days 106.5 mm² versus 0.12 mm²). The trichomes on the ‘Brigitte’ leaves seemed to prevent the germ tubes from detecting the stomata. P. ramorum germ tubes from encysted zoospores invaded the leaf tissue via the stomata. Appressoria-like structures were observed. On the infected leaves new hyphae grew out of the stomata. Sporangia and chlamydospores developed on the mycelium from germinated zoospore cysts growing on the leaf surface as well as on the hyphae growing out of the stomata after infection. They could be observed mainly on the necrotic leaf areas. Single healthy-looking oospores developed within 12 days on the ‘Brigitte’ leaves. Observation of the infection process on leaves of Rhododendron ‘Cunningham’s White’ with the scanning electron microscope confirmed the various stages of the infection process.