Wickerhamomyces anomalus is a predator of the Castanea spp. ink disease-causing oomycetes Phytophthora cinnamomi and P. xcambivora. Based on morphological evidence, a model illustrating a specific mode of action is provided

Phytophthora cinnamomi and P. xcambivora are serious phytopathogens, namely causing root rot/ink in chestnut trees, which severely threatens this historical crop. Oomycetes produce mycelium but are hard to control with traditional fungicides or BCAs. Recent research revealed Wickerhamomyces anomalus, a yeast endophyte, as a strong antagonist of key pre- and post-harvest fungal pathogens in laboratory settings. This study shows it also inhibits Phytophthora growth. Microscopy revealed that W. anomalus cells adhere to and are found inside Phytophthora hyphae, accumulating within collapsed areas, possibly nourishing on hyphal contents. These interactions occur without the intervention of volatile compounds, siderophores, or hydrolytic enzymes, leaving hyphal walls intact. SEM and TEM of hyphae from P. cinnamomi when co-cultured with W. anomalus showed numerous intrahyphal structures formed in response to the yeast-imposed stress. Otherwise, the yeast shows polarised cells, nuclei fusion, irregularly numbered spores and intercellular bridges, indicating a disrupted reproductive cycle. This is consistent with P. cinnamomi and W. anomalus recognising each other's pheromones, triggering a mating-like response leading to the yeast's attachment and internalisation without damaging the hyphal cell wall. This W. anomalus specific mode-of-action, different from the ones previously reported, suggests potential as a BCA for the pre-harvest management of Phytophthora.