Abstract
The microscopic examination of naturally infected plant material for the presence of Phytophthora cinnamomi can be problematic as structures such as hyphae, hyphal swellings, chlamydospores, and oospores are often indistinguishable from those of other oomycetes or fungi. Frequently, it would be useful to be able to clearly differentiate P. cinnamomi from other microorganisms, especially when trying to determine how the pathogen is surviving in plant material particularly in harsh environments. Consequently, the lack of stains that can clearly and definitively localise hyphae and reproductive structures of P. cinnamomi within plant material is a limitation in increasing our understanding of the biology of the pathogen in susceptible and tolerant plant species in different ecosystems. This study demonstrates that a P. cinnamomi specific, fluorescently labelled DNA probe can be used to specifically detect and visualise P. cinnamomi in plant material using fluorescent in situ hybridization (FISH) without damage to plant or pathogen cell integrity. The method will allow us to more accurately study plant‐P. cinnamomi interactions in plants, and to be particularly useful in naturally infected material.