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Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 1/23/2014 Publication Date: 3/18/2014 Citation: Shishkoff, N. 2014. Effect of growth-inhibiting compounds on recovery of Phytophthora ramorum from infected foliage over time, on root colonization and production of inoculum from plant roots. Plant Health Progress. 15:36. Interpretive Summary: Phytophthora ramorum is an invasive species that can girdle and kill trees. In the US, P. ramorum has spread into forests in coastal California and Oregon and has been found on nursery stock in a number of states and from bodies of water adjacent to infested nurseries. A Federal Order put into place in 2005 restricts the movement of nursery stock out of California, Oregon and Washington. However, the use of fungicides in nurseries complicates the quarantine, because many of the chemicals used to control pathogens like P. ramorum, do not kill the pathogens, and their use against related, common pathogens might mask the presence of P. ramorum. In this study we asked if there was a masking effect of growth-inhibiting chemicals on plants already infected with P. ramorum. We tested plants with infected foliage and plants with infected roots, measuring the spore production from roots using an assay developed here. Our results indicate that although the chemical we tested is clearly effective in controlling symptoms and reducing the production of spores that spread the disease, it does not kill P. ramorum, and its use could mask the presence of the pathogen. This could make the pathogen harder to find in a large block of nursery stock, which may have implications for movement of nursery stock out of restricted areas. Technical Abstract: Phytophthora ramorum is an invasive species that can girdle and kill trees. In the US, P. ramorum has spread into forests in coastal California and Oregon and has been found on nursery stock in a number of states and from bodies of water adjacent to infested nurseries. Growth-inhibiting chemicals are used routinely to control common Oomycete pathogens and could mask the presence of a regulated organism such as Phytophthora ramorum. In this study, we asked if there was a masking effect of growth-inhibiting chemicals on plants already infected with P. ramorum and tested Rhododendron with infected foliage or Viburnum with infected roots. The foliage of Rhododendron x ‘Cunningham’s White’ was infected with P. ramorum and treated 3 days later with growth-inhibiting chemicals labeled for use on Phytophthora (Subdue MAXX, Banol, and Aliette) at standard rates, and the foliage sampled over time to see if the chemical prevented successful culturing of the pathogen from infected leaf tissue or interfered with detection using real-time PCR or ELISA. Over 5-9 weeks, Subdue MAXX significantly reduced the ability to culture the pathogen from plant tissue, while recovery from plant tissue treated with other chemicals did not differ from untreated controls. Detection using PCR or ELISA was not affected by chemical application. The roots of Viburnum cuttings were inoculated with P. ramorum and then treated 4 days later with Subdue MAXX, Banol, or Aliette. The amount of inoculum in runoff samples taken weekly for 5 weeks was quantified and the percent root colonization determined at the end of the experiment. Banol had no effect on inoculum production or root infection, while viable inoculum production was significantly decreased in Aliette- or Subdue MAXX-treated plants over 5 weeks, and root colonization was significantly decreased. |
