Flooding by California Rivers Results in Walnut Scion Infections by Species of Phytophthora

Abstract

Many walnut orchards were inundated by flooding from the Feather and Stanislaus Rivers in winter and spring 2017 and developed bleeding cankers in trunk, root, and crown tissues exposed to the water. Orchard surveys and diagnostic isolations associated Phytophthora pini, P. chlamydospora, and P. gonapodyides with the cankers in 2017. Pathogenicity of P. pini was confirmed in seedlings and excised shoots of Juglans regia, but the other species caused negligible amounts of disease. Feather River and associated flood waters were assayed using culture-independent sequencing of rRNA gene amplicons and pear baiting methods; 14 species of Phytophthora were detected, including P. chlamydospora and P. gonapodyides, but not P. pini. Severe and prolonged walnut orchard flooding from rivers, such as occurred in 2017, places diverse mixtures of Phytophthora species from multiple sources into close, infective proximity with susceptible walnut tree scions. Systemic chemical or genetic protection strategies may be valuable for orchards subject to such flooding.