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United States Department of Agriculture

Agricultural Research Service

Title: Evaluation of Chemical and Biological Agents for Control of Phytophthora Species on Intact Plants Or Detached Leaves of Rhododendron and Lilac

Authors
item Linderman, Robert
item Davis, E Anne

Submitted to: Sudden Oak Death Science Symposium
Publication Type: Proceedings
Publication Acceptance Date: December 6, 2005
Publication Date: N/A

Interpretive Summary: The Sudden Oak Death pathogen, Phytophthora ramorum, has occurred in numerous nurseries during the last year, causing Ramorum blight. Nurseries need information on the most effective fungicides or biological control agents to use to suppress the development of this pathogen, compared to other species of Phytophthora. This study compared a range of commercially-available Oomycete-suppressing chemicals for efficacy in suppressing Phytophthora species, including P. ramorum. A number of materials effectively suppressed P. ramorum, but not always other species of Phytophthora. All were fungistatic, not fungicidal. Bacterial biocontrol agents that were very effective in vitro were applied to leaves prior to inoculation, but none were effective in these tests. These results indicate that some systemic or contact chemicals would be useful in efforts to protect nursery plants from infection by species of Phytophthora, including P. ramorum. More work is needed to develop effective biological control strategies against these pathogens.

Technical Abstract: The recent incidence of Ramorum blight, caused by Phytophthora ramorum, on many nursery crops has focused attention on improving management strategies against Phytophthora diseases in nurseries. We evaluated several chemical agents for their capacity to inhibit infection of rhododendron (cv Nova Zembla) leaves by P. ramorum compared to P. cactorum, P. citricola, P. nicotianae, and P. citrophthora. We also inoculated wounded rhododendron or lilac leaves from plants previously treated with various chemicals that were (a) removed and inoculated, or (b) inoculated and left on intact plants. Inoculation of leaves on chemically-treated intact plants with P. ramorum or other Phytophthora species yielded similar results to those from inoculation of leaves detached from the same treated plants. Most of the chemicals tested had some efficacy on some species of Phytophthora, but Subdue Maxx (drench or foliar) had the greatest disease-suppressive activity against all species of Phytophthora except P. citrophthora, and was effective for at least 6 weeks after application. Some chemicals had varied efficacy depending on the species of Phytophthora. Bacterial antagonists (Bacillus brevis or isolates of Paenibacillus polymyxa) significantly inhibited all Phytophthora species in in vitro challenges, but were ineffective when leaves were dipped in a cell suspension prior to inoculation with P. ramorum or other species. These tests indicate that inoculating detached leaves was comparable to inoculating intact plants to evaluate chemical and biological agents against Phytophthora species, and that several systemic or translaminar chemicals were effective in suppressing infections but without eradication of the pathogens.

Last Modified: 9/10/2014
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