Phytophthora diversity in river water in California

Many walnut orchards were inundated by flooding and seepage originating from the Feather and Stanislaus Rivers in winter and spring 2017 and developed bleeding cankers in trunk and root crown tissues exposed to the water. Orchard surveys and diagnostic isolations associated Phytophthora pini, P. chlamydospora, and P. gonapadyides with the cankers in 2017. Pathogenicity of the P. pini was confirmed in seedlings and excised shoots of Juglans regia, but the other species caused small to negligible amounts of disease. Additional isolations in 2018 detected P. pini and P. citrophthora, suggesting that they had persisted from infections facilitated by flooding in 2017, whereas P. chlamydospora and P. gonapodyides were not detected in 2018. Pear-bait-based and assays of water in the Feather River in 2018 coupled with ITS 1 and 2 sequencing of pear isolates, detected Phytophthora riparia, P. irrigata, P. pseudocryptogea, and P. gonapodyides, suggesting that water in the river may facilitate walnut tree infection by dispersing inoculum into orchards as well as by facilitating infection of susceptible scion tissues by pathogenic species of Phytophthora that reside in the orchard soils.