Pyracantha ‘Mohave’ fruit infection by Phytophthora ramorum and transmission of the pathogen from infected fruit to roots of Viburnum tinus

Authors: Tooley, Paul; Browning, Marsha; Shishkoff, Nina

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/14/2015
Publication Date: 2/25/2016
Publication URL: https://handle.nal.usda.gov/10113/5700693
Citation: Tooley, P.W., Browning, M.E., Shishkoff, N. 2016. Pyracantha ‘Mohave’ fruit infection by Phytophthora ramorum and transmission of the pathogen from infected fruit to roots of Viburnum tinus. Plant Disease. 100:555-560.

Interpretive Summary: The destructive plant pathogen that causes sudden oak death disease is named Phytophthora ramorum, and it can infect a wide variety of plant species in addition to oaks. The pathogen moves from the Western U.S., where it is established, to the Eastern U.S. with plants shipped in the nursery industry. One possible means of transmission of the pathogen is via infected seed or fruit of certain plant species. We explored the possibility that P. ramorum can infect fleshy fruit of certain plant species, and spread from such fruit to soil and roots of healthy plants using an experimental system developed in our laboratory. We found that fleshy fruit of the common landscape plant Pyracantha 'Mohave' could become heavily infected by P. ramorum and that infected fruit was able to transmit the disease to healthy roots of the test plant Viburnum tinus. Furthermore, many spores of P. ramorum were found to be washed from the root system of V. tinus, potentially providing the means to infect surrounding healthy plants in a nursery setting. The results are significant because it is the first demonstration of this type of disease transmission from infected fruit to healthy plant roots for this pathogen, and means that regulatory officials will need to consider this type of pathogen movement in formulating inspection, quarantine, and monitoring efforts to prevent the further spread of P. ramorum within the U.S.

Technical Abstract: We conducted a study to investigate the potential role of Pyracantha ‘Mohave’fruit as a source of inoculum for subsequent root infection of a susceptible host by Phytophthora ramorum. An examination of the fleshy pulp revealed an abundance of chlamydospores in fruit following inoculation with P. ramorum sporangia. Pyracantha ‘Mohave’ fruits that had been inoculated with a P. ramorum sporangia suspension were placed in pots containing rooted cuttings of Viburnum tinus set up in a misting tent or placed in water-filled trays in a climate controlled greenhouse. Run-off was collected for 24-30 days, and roots were plated after the final collection of run-off. Mean percent infected roots was not significantly different between bottom-watered treatments in trays and misted treatments, averaging 58 percent for bottom-watered and 54 percent for mist treatments. The number of colony-forming units (CFUs) collected in run-off from plants held in trays of water was consistently lower than from plants held under mist, likely due to desiccation of the fruit. The results show that root infection of a susceptible host can occur by P. ramorum via infected fruit of Pyracantha ‘Mohave’. This phenomenon represents a pathway of infection for P. ramorum not previously reported, which may play a role in disease epidemiology.