Phytophthora species differ in response to phosphorous acid and mefenoxam for the management of phytophthora root rot in rhododendron

Authors: Weiland, Gerald - Jerry; Scagel, Carolyn; Grunwald, Niklaus - Nik; DAVIS, ELIZABETH; Beck, Bryan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2020
Publication Date: 12/18/2020
Citation: Weiland, G.E., Scagel, C.F., Grunwald, N.J., Davis, E.A., Beck, B.R. 2020. Phytophthora species differ in response to phosphorous acid and mefenoxam for the management of phytophthora root rot in rhododendron. Plant Disease. 105:1505-1514. https://doi.org/10.1094/PDIS-09-20-1960-RE.
DOI: https://doi.org/10.1094/PDIS-09-20-1960-RE

Interpretive Summary: Phytophthora root rot, caused by multiple Phytophthora species, causes an estimated $8 million in losses to the $42 million rhododendron nursery industry. This disease is usually managed with two fungicides, phosphorous acid and mefenoxam, that are applied as a foliar spray or soil drench. However, almost all previous research targeted only one (P. cinnamomi) of the many species that cause root rot and growers are increasingly reporting that fungicides fail to manage the disease. It is unknown how well these two fungicides manage the other Phytophthora species that cause Phytophthora root rot. Experiments were conducted to determine how well foliar sprays versus soil drenches of phosphorous acid and mefenoxam control the two most common Phytophthora species causing root rot, P. cinnamomi and P. plurivora and to determine how sensitive individuals of ten different Phytophthora species were to both fungicides. Our results demonstrated that soil drenches of phosphorous acid and mefenoxam were more effective than foliar sprays in controlling Phytophthora root rot caused by P. cinnamomi, but were ineffective against P. plurivora. We also discovered that Phytophthora species differed in sensitivity to both fungicides and that there were fungicide resistant individuals, especially among P. plurivora. Our results are important because they show three potential reasons for why fungicide control of Phytophthora root rot control fails in rhododendron nurseries: 1) the fungicide can be applied to the wrong portion of the plant for optimal control, 2) there are differences in fungicide sensitivity among Phytophthora species, and 3) there are fungicide resistant isolates in the rhododendron nursery industry.

Technical Abstract: Phytophthora root rot is a significant disease affecting production of the $42 million rhododendron nursery industry. Rhododendron growers have increasingly reported failure by two common fungicides, phosphorous acid and mefenoxam, to adequately control Phytophthora root rot. Both fungicides are systemic and may be applied either as a foliar spray or soil drench. Many Phytophthora species affect the industry, but it is unknown how pathogen diversity affects disease control by fungicides. Therefore, two experiments were conducted. First, a fungicide efficacy experiment was implemented determine whether differences in application method or fungicide chemistry affected control of root rot caused by P. cinnamomi and P. plurivora. Second, a fungicide sensitivity experiment was conducted to evaluate growth responses to PA and ME among ten Phytophthora species and isolates from the PNW rhododendron industry. Results demonstrated that soil drenches were more effective than foliar sprays for control of root rot caused by P. cinnamomi, but were ineffective for the P. plurivora isolate used in the experiment. Fungicide sensitivity results showed that there were differences among species and isolates to phosphorous acid and mefenoxam and that there were isolates that were not sensitive to either fungicide, especially among P. plurivora. Differences in fungicide sensitivity were also observed among nurseries and production systems, with some nurseries having less sensitive isolates compared to other nurseries and with container production systems having less sensitive isolates than field production systems. Our results provide three potential reasons for why fungicide control of Phytophthora root rot might fail in rhododendron nurseries: 1) the fungicide can be applied to the wrong portion of the plant for optimal control, 2) there are differences in fungicide sensitivity among soilborne Phytophthora species infecting rhododendron, and 3) there are fungicide insensitive isolates present in the rhododendron nursery industry.