Analysis of volatile profiles for tracking asymptomatic infections of Phytophthora ramorum and other pathogens in rhododendron

Authors: THOMPSON, CAI HUGO - University Of California, Davis; MCCARTNEY, MITCHELL - University Of California, Davis; ROUBTSOVA, TATIANA - University Of California, Davis; Kasuga, Takao; EBELER, SUSAN - University Of California, Davis; DAVIS, CRISTINA - University Of California, Davis; BOSTOCK, RICHARD - University Of California, Davis

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2021
Publication Date: 2/22/2021
Citation: Thompson, C., McCartney, M., Roubtsova, T., Kasuga, T., Ebeler, S., Davis, C., Bostock, R. 2021. Analysis of volatile profiles for tracking asymptomatic infections of Phytophthora ramorum and other pathogens in rhododendron. Phytopathology. https://doi.org/10.1094/PHYTO-10-20-0472-R.
DOI: https://doi.org/10.1094/PHYTO-10-20-0472-R

Interpretive Summary: Phytophthora ramorum is an invasive and devastating plant pathogen that causes sudden oak death in coastal forests in the western United States and ramorum blight in nursery ornamentals and native plants in various landscapes. As a broad host-range quarantine pest that can be asymptomatic in some hosts, P. ramorum presents significant challenges for regulatory efforts to detect and contain it, particularly in commercial nurseries. In this study, we continue development of a volatile organic compound (VOC)-based test for detecting asymptomatic infections of P. ramorum in Rhododendron sp., an important host and experimental model. We confirm detection of P. ramorum from volatiles collected from root-inoculated Rhododendron plants in a nursery setting, finding that the VOC profile of infected plants is detectably different from that of healthy plants. In a growth chamber, we contrasted volatile profiles of Rhododendron plants infected with one of three pathogens: P. ramorum, Phytophthora cactorum and Rhizoctonia solani. Each pathogen induced unique and measurable changes. Forty-five compounds had significant differences compared to mock-inoculated controls in at least one host-pathogen combination. Our findings will inform and guide development of noninvasive field detection tests for asymptomatic infections caused by Phytophthora spp. and possibly other plant pathogens.

Technical Abstract: Phytophthora ramorum is an invasive, broad host-range pathogen that causes ramorum blight and sudden oak death in forest landscapes of western North America. In commercial nurseries, asymptomatic infections of nursery stock by P. ramorum as well as other Phytophthora species create unacceptable risk, and complicate inspection and certification programs designed to prevent introduction and spread of these pathogens. In this study, our group continues development of a volatile organic compound (VOC)-based test for detecting asymptomatic infections of P. ramorum in Rhododendron sp., an important host and experimental model. We confirm detection of P. ramorum from volatiles collected from root-inoculated Rhododendron plants in a nursery setting, finding that the VOC profile of infected plants is detectably different from that of healthy plants, both when measured from ambient VOC emissions and from VOCs extracted from leaf material. Predicting infection status was successful from ambient volatiles, which had a mean area under the curve (AUC) value of 0.71 ± 0.17, derived from corresponding receiver operator characteristic curves from an extreme gradient boosting discriminant analysis (XGB-DA). This compares with extracted leaf volatiles, which resulted in a lower AUC value of 0.51 ± 0.21. In a growth chamber, we contrasted volatile profiles of Rhododendron plants infected with one of three pathogens: P. ramorum, Phytophthora cactorum and Rhizoctonia solani. Each pathogen induced unique and measurable changes, but generally the infections reduced volatile emissions until 17 weeks after inoculation, when emissions trended upwards relative to mock-inoculated controls. Forty-five compounds had significant differences compared to mock-inoculated controls in at least one host-pathogen combination. Our findings will inform and guide development of noninvasive field detection tests for asymptomatic infections caused by Phytophthora spp. and possibly other plant pathogens.