First report of Phytopythium helicoides causing root rot on peach rootstock in California

Authors: Browne, Greg; Ott, Natalia; FICHTNER, ELIZABETH - University Of California - Cooperative Extension Service

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2019
Publication Date: 8/20/2019
Citation: Browne, G.T., Ott, N.J., Fichtner, E. 2019. First report of Phytopythium helicoides causing root rot on peach rootstock in California. Plant Disease. 103(11):2968. https://doi.org/10.1094/PDIS-09-18-1697-PDN.
DOI: https://doi.org/10.1094/PDIS-09-18-1697-PDN

Interpretive Summary: We isolated Pythium and Phytopythium spp. from: (i) stunted peach seedlings grown in pots of almond orchard soils being assayed for potential replant pathogens and (ii) 2- to 10-year-old almond trees affected by root rot. The isolates were purified and extracted for total DNA. PCR primers ITS1 and ITS4 were used to amplify fragments including ITS1 and ITS2 regions of the rRNA gene. Sequencing of the amplified DNA fragments (i.e., “genetic fingerprinting”) identified Phytopythium helicoides (Phe) from two of the assayed soils (collected near Hanford and Wasco, CA) and from necrotic roots of two of the almond orchards (near Sanger and Shafter, CA); the identifications were based on 692- to 813-bp fragments, which matched NCBI vouchers for Phe at 98.2 to 98.9% sequence similarity. The isolates were tested for pathogenicity on Nemaguard peach in a greenhouse. Two-week-old Nemaguard seedlings were transplanted into 0.6-liter pots of pasteurized sand/peat potting mix that had been infested with single isolates of Phe grown on V8 juice-vermiculite-oat seed substrate added at 2.5% (v) to the potting soil; controls received sterile substrate. All four isolates of Phe suppressed final top and root fresh weights and caused significant levels of root cortex necrosis. Without flooding, top and root weights were reduced by Phe to 23 to 74% of the control weights (plants inoculated at 2 weeks of age) or 51 to 79% of controls (inoculated at 10 weeks). With soil flooding, Phe reduced top weights to 1 to 8% of controls (inoculated at 2-weeks) or 9 to 26% of controls (inoculated at 10 weeks). In both experiments, Phe was reisolated from roots of inoculated plants but was absent in isolations from control plants. This is the first report of Phe as a pathogen on almond trees and peach rootstock.

Technical Abstract: We isolated Pythium and Phytopythium spp. from: (i) stunted peach seedlings grown in pots of almond orchard soils being assayed for potential replant pathogens and (ii) 2- to 10-year-old almond trees affected by root rot. The isolates were hyphal tipped and extracted for total DNA. PCR primers ITS1 and ITS4 were used to amplify fragments including ITS1 and ITS2 regions of the rRNA gene. Sequencing of the amplicons identified Phytopythium helicoides (Phe) from two of the assayed soils (collected near Hanford and Wasco, CA) and from necrotic roots of two of the almond orchards (near Sanger and Shafter, CA); the identifications were based on 692- to 813-bp fragments, which matched BOLDSystems isolates for Phe at 98.2 to 98.9% sequence similarity. Our isolates were tested for pathogenicity on Nemaguard peach in a greenhouse. Two-week-old Nemaguard seedlings were transplanted into 0.6-liter pots of pasteurized sand/peat potting mix that had been infested with single isolates of Phe grown on V8 juice-vermiculite-oat seed substrate added at 2.5% (v) to the potting soil; controls received sterile substrate. All four isolates of Phe suppressed final top and root fresh weights and caused significant levels of root cortex necrosis. Without flooding, top and root weights were reduced by Phe to 23 to 74% of the control weights (plants inoculated at 2 weeks of age) or 51 to 79% of controls (inoculated at 10 weeks). With soil flooding, Phe reduced top weights to 1 to 8% of controls (inoculated at 2-weeks) or 9 to 26% of controls (inoculated at 10 weeks). In both experiments, Phe was reisolated from roots of inoculated plants but was absent in isolations from control plants. This is the first report of Phe as a pathogen on almond trees and peach rootstock.