Histological studies of Phytophthora ramorum in Rhododendron twigs

Freshly cut Rhododendron twigs were inoculated with Phytophthora ramorum S. Werres, A.W.A.W. de Cock & W.A. Man in't Veld. Disease development (discoloration of the twigs) was observed. Tissue samples were taken for reisolation and for histological studies from four different zones of the inoculated twigs. Thin sections were stained with toluidine blue O. Reisolation of the pathogen was successful from all samples. The histological studies showed that P. ramorum was not only present in the brown zones of the Rhododendron twigs, but also in the healthy-looking green zones. Hyphae were found in different twig zones and in different tissues, but chlamydospores occurred only in brown twig zones where they developed mainly in the cortical parenchyma. Results from the histological studies provide a basis for how P. ramorum colonizes host tissue in Rhododendron twigs.