First report of Phytophthora chlamydospora causing crown and root rot on almond in California

Authors: Browne, Greg; Ott, Natalia; Forbes, Holly; YAGHMOUR, MOHAMMAD - University Of California - Cooperative Extension Service; MILLIRON, LUKE - University Of California - Cooperative Extension Service

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2020
Publication Date: 5/5/2020
Citation: Browne, G.T., Ott, N.J., Forbes, H., Yaghmour, M.A., Milliron, L.K. 2020. First report of Phytophthora chlamydospora causing crown and root rot on almond in California. Plant Disease. 104(7):2033. https://doi.org/10.1094/PDIS-10-19-2072-PDN.
DOI: https://doi.org/10.1094/PDIS-10-19-2072-PDN

Interpretive Summary: More than 10 species of Phytophthora are known to attack almond trees in California, causing, depending on the pathogen, crown rot, root rot, and trunk and scaffold cankers. It is important to monitor and assess these populations so that evaluations of rootstock resistance adequately reflect pathogens encountered in California almond orchards. Phytophthora chlamydospora (Pchl), not previously known as a pathogen of fruit or nut trees in North America, was isolated from dying almond trees in two orchards, one in the Sacramento and the other in the San Joaquin Valley of California; incidence of the disease was < 1% among the trees. Semi-selective PARP medium was used to culture isolates from the necrotic crown and root tissues and from pears used to bait the soil. Two isolates were identified as Pchl according to: spacer sequences in rRNA gene (ITS 1 and 2; amplified with primers ITS 6 and ITS 4) sequences in cytochrome oxidase subunit 1 gene, and morphological characteristics. In order to confirm whether Pchl was the cause of the symtpoms, both isolates from almond were tested for pathogenicity on 5-mo-old potted rootstocks of Hansen 536 (Prunus dulcis × P. persica ‘UC 1-8-2’) and Nemaguard seedling in a greenhouse with air temperature ranging from 17 to 28 °C. Five-mm-diameter mycelial disks from V8 juice agar cultures of the Pcl isolates were wound inoculated into stems at 3 to 6 cm above the soil line, and sterile V8 juice agar disks were wound inoculated as a control. Pathogenicity was assessed 3 weeks after inoculation. Both isolates of Pchl induced cankers; mean necrosis lengths were: 31 mm (95% confidence interval 26 to 35 mm) for Pchl from orchard 1, 24 mm (20 to 28 mm) for Pchl from orchard 2, and 4 mm (0 to 8 mm) for the control. Isolation on PARP medium confirmed presence of Pcl in the necrotic bark of all plants inoculated with the pathogen and the absence of Phytophthora from all control plants. Our report implicates Pchl as a cause of crown and root rot of almond in California.

Technical Abstract: Phytophthora chlamydospora (Pchl), not previously known as a pathogen of fruit or nut trees in North America, was isolated from dying almond trees in two orchards, one in the Sacramento and the other in the San Joaquin Valley of California; incidence of the disease was < 1% among the trees. Semi-selective PARP medium was used to culture isolates from the necrotic crown and root tissues and from pears used to bait the soil. Two isolates were identified as Pchl according to: spacer sequences in rRNA gene (ITS 1 and 2; amplified with primers ITS 6 and ITS 4), sequences in cytochrome oxidase subunit 1 gene, and morphological characteristics. Both isolates from almond were tested for pathogenicity on 5-mo-old potted rootstocks of Hansen 536 (Prunus dulcis × P. persica ‘UC 1-8-2’) and Nemaguard seedling in a greenhouse with air temperature ranging from 17 to 28 °C. Five-mm-diameter mycelial disks from V8 juice agar cultures of the Pcl isolates were wound inoculated into stems at 3 to 6 cm above the soil line, and sterile V8 juice agar disks were wound inoculated as a control. Pathogenicity was assessed 3 weeks after inoculation. Both isolates of Pchl induced cankers; mean necrosis lengths were: 31 mm (95% CI 26 to 35 mm) for Pchl from orchard 1, 24 mm (20 to 28 mm) for Pchl from orchard 2, and 4 mm (0 to 8 mm) for the control. Isolation on PARP medium confirmed the presence of Pcl in the necrotic bark of all plants inoculated with the pathogen and the absence of Phytophthora from all control plants. Our report implicates Pchl as a cause of crown and root rot of almond in California.