Effects of Phytophthora ramorum on volatile organic compound emissions of Rhododendron using gas chromatography-mass spectrometry

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. As part of a program to develop new detection methods for cryptic infections in nursery stock, we compared volatile emissions of P. ramorum-inoculated and noninoculated Rhododendron plants using three gas chromatography-mass spectrometry methods. The first used a branch enclosure combined with headspace sorptive extraction to measure plant volatiles in situ. Seventy-eight compounds were found in the general Rhododendron profile. The volatile profile of inoculated but asymptomatic plants (121 days post-inoculation) was distinguishable from the profile of the noninoculated controls. Three compounds were less abundant in inoculated Rhododendron plants relative to noninoculated and mock-inoculated control plants. A second method employed stir bar sorptive extraction to measure volatiles in vitro from leaf extractions in methanol; 114 volatiles were found in the overall profile with 30 compounds less abundant and one compound more abundant in inoculated Rhododendron plants relative to mock-inoculated plants. At 128 days post-inoculation, plants were asymptomatic and similar in appearance to the noninoculated controls, but their chemical profiles were different. In a third technique, volatiles from water runoff from the soil of potted healthy and inoculated Rhododendron plants were compared. Runoff from the inoculated plants contained four unique volatile compounds that never appeared in the runoff from mock-inoculated plants. These three volatile detection techniques could lead to innovative approaches that augment detection and diagnosis of P. ramorum and oomycete pathogens in nurseries and other settings. Graphical abstract Detection of volatile signatures may aid in discriminating healthy vs. infected but asymptomatic plants in nursery and greenhouse facilities.