Phytophthora plurivora: A Serious Challenge for English Walnut (Juglans regia) Cultivation in Europe

English walnut (<i>Juglans regia</i>) is a species that is highly valued for the quality of its wood and the nutritional and nutraceutical properties of its fruit. A severe dieback of <i>J. regia</i> trees was observed recently in orchards located in three geographically distinct areas of Tuscany, central Italy. Symptoms included root and collar rot, necrosis of the under-bark tissue, bleeding cankers, stunted growth, and crown dieback. Four <i>Phytophthora</i> species were obtained from 239 isolates found on symptomatic <i>J. regia</i> individuals. They were identified, on the basis of macro-morphological (colony shape and texture), micro-morphometric (shape and size of oogonia, antheridia, oospores, sporangia, and chlamydospores) and molecular (ITS sequencing) characters, as <i>P. gonapodyides</i>, <i>P. cactorum</i>, <i>P. citricola</i>, and <i>P. plurivora</i>. Among these species, <i>P. plurivora</i> was the species isolated with overwhelming frequency from symptomatic tissue and rhizosphere soil, suggesting it to be the putative etiological agent. Pathogenicity assays were conducted on 20 cm long detached <i>J. regia</i> branches for a definitive establishment of disease causation. Severe symptoms (extended necroses) were exhibited by branches infected with <i>P. plurivora</i>, proving its pathogenicity and high virulence on this host. The other <i>Phytophtora</i> species produced negligible necroses around the inoculation site. <i>P. plurivora</i> was recovered from all the investigated orchards, providing evidence that it is quite widespread. This study highlights the growing threat posed by the polyphagous <i>P. plurivora</i> to walnut cultivation and the sustainable business it fuels in Europe, underscoring the need for integrated management strategies to mitigate its economic and ecological impacts.