Flooding by California rivers results in walnut scion infections by species of Phytophthora

Authors: Browne, Greg; HASEY, JANINE - University Of California - Cooperative Extension Service; Ott, Natalia; Forbes, Holly; ARNOLD, KARI - University Of California - Cooperative Extension Service; MILLIRON, LUKE - University Of California - Cooperative Extension Service

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2020
Publication Date: 2/21/2021
Citation: Browne, G.T., Hasey, J.K., Ott, N.J., Forbes, H., Arnold, K., Milliron, L.K. 2021. Flooding by California rivers results in walnut scion infections by species of Phytophthora. Plant Health Progress. https://doi.org/10.1094/PHP-02-20-0011-RS.
DOI: https://doi.org/10.1094/PHP-02-20-0011-RS

Interpretive Summary: Many walnut orchards were inundated by flooding 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 isolations into culture media, followed by diagnostic morphological study and sequencing of DNA of the isolates associated Phytophthora pini, P. chlamydospora, and P. gonapadyides with the cankers in 2017. Pathogenicity of the P. pini was confirmed by test inoculations in seedlings and excised shoots of Juglans regia, but the other species caused small to negligible amounts of disease. Culture-independent DNA-based methods and culture-based isolations from pear fruit baits detected 14 species of Phytophthora, including P. chlamydospora and P. gonapodyides, but not P. pini in the Feather River and flood water near it. Severe and prolonged walnut orchard flooding, 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 (i.e., based on systemic fungicides or genetic resistance) may be valuable for orchards subject to such flooding.

Technical 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 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. Culture-independent DNA-based methods and culture-based isolations from pear fruit baits detected 14 species of Phytophthora, including P. chlamydospora and P. gonapodyides, but not P. pini in the Feather River and flood water near it. Severe and prolonged walnut orchard flooding, 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.