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.

众多核桃林在2017年冬季和春季遭受了由费瑟河和斯坦尼斯劳斯河引发的洪水和渗漏的侵袭，导致暴露于水中的树干和根颈组织出现了流胶病。2017年的果园调查和病原分离实验将松树疫霉（Phytophthora pini）、P. chlamydospora 和 P. gonapadyides 与树瘤联系起来。在胡桃（Juglans regia）幼苗和切离枝条上确认了 P. pini 的致病性，但其他物种仅引起少量或可忽略不计的病害。2018年的额外分离实验检测到了 P. pini 和 P. citrophthora，表明它们可能因2017年洪水造成的感染而持续存在，而 P. chlamydospora 和 P. gonapodyides 在2018年并未检测到。2018年，利用梨子诱饵和费瑟河中的水进行检测，结合梨分离株的ITS 1和2序列分析，检测到了桃疫霉（Phytophthora riparia）、P. irrigata、P. pseudocryptogea 和 P. gonapodyides，这表明河水可能通过将接种体分散到果园以及通过促进病原性桃疫霉种类的感染，从而有利于核桃树的感染，这些病原性种类存在于果园土壤中。