P-449 - Phytopythium vexans, a new pathogen causing root rot of sweet cherry in California, and its sensitivities to several Oomycota fungicides

Monday, August 4, 2025

4:30 PM - 5:15 PM HST

Location: Exhibit Hall 3

Presenting Author(s)

CO

Chaydon O'Fallon, Graduate Student

PhD Candidate
University of California Riverside
Riverside, CA, USA

Author(s)

HF

Helga Forster, Project Scientist

Department of Microbiology and Plant Pathology, University of California, Riverside
Riverside, California, United States
JA

James E. Adaskaveg, Professor

Professor
University of California, Riverside
Riverside, California, United States

Abstract Text: Phytopythium vexans (Phv) has been reported on many woody and herbaceous plants, including several Prunus spp. In surveys of sweet cherry orchards in California with declining trees, we collected 33 Phv isolates from roots/soil and confirmed their identity by morphology and DNA sequencing. To fulfill Koch’s postulates, 4- to 8-week-old Mahaleb cherry seedlings were transplanted into Phv-infested soil. Developing symptoms included yellowing and wilting of leaves and necrotic brown lesions on roots, crowns, and stems. These symptoms were similar to those observed on surveyed trees where Phv was recovered from roots. Mycelial growth of the 33 isolates was evaluated in toxicity assays with the currently registered Oomycota fungicides mefenoxam (MEF; FRAC Code-FC 4) and potassium phosphite (PO₃; FC P07/33), as well as those pending registration: ethaboxam (ETH; FC 22), fluopicolide (FLU; FC 43), mandipropamid (MAN; FC 40), and oxathiapiprolin (OXP; FC 49). Except for MAN with an EC₅₀ value of >40 μg/ml, all fungicides were toxic at low concentrations. EC₅₀ values ranged from 0.009 to 0.021 μg/ml for MEF, 8.8 to 51.6 μg/ml for PO₃, 0.081 to 0.320 μg/ml for ETH, 0.157 to 0.362 μg/ml for FLU, and 0.0015 to 0.0026 μg/ml for OXP. In greenhouse studies, Mahaleb seedlings were inoculated with Phv, and PO₃, FLU, or OXP was applied to the soil. All fungicides significantly decreased root rot incidence, but OXP was the most effective. With the new fungicides in progress for registration on sweet cherry for managing Phytophthora root and crown rot, growers will also be able to manage Phv.