Submitted to: Sudden Oak Death Science Symposium
Publication Type: Proceedings
Publication Acceptance Date: 8/15/2007
Publication Date: 4/14/2008
Citation: Widmer, T.L. 2008. Investigating the potential of biological control against Phytophthora ramorum. Sudden Oak Death Science Symposium. Frankel, S.J., Kliejunns, J.T. and Palmieri, K. M. (eds). USDA Forest Service Gen. Tech. Rep. PSW-GTR-214, Albany, CA. p. 491 Interpretive Summary:
Technical Abstract: Phytophthora ramorum is a unique organism in many ways, having a broad host range and both soilborne and aerial infection stages. This makes implementing effective control measures very complex. The use of biological control has been demonstrated against various Phytophthora spp. in general, but not specifically against P. ramorum. One aspect of biological control considered for use in this study was the use of microorganisms. Over 100 fungi, bacteria, and actinomycetes were isolated from natural field soil, leaf surfaces, and plant parts (endophytes). Some soil organism and endophyte isolates demonstrated strong growth inhibition towards P. ramorum colonies on agar plates. Encysted zoospores were completely inhibited from germinating on agar plates when pipetted 2 mm from active colonies of Sar-endo3, Alleg-endo2, Act-2Db, Act-3E, Act-3B1, and Act-3B2. Isolate Ch-1F inhibited zoospore germination 41%, while isolate CW-endo2 increased germination 27% compared to the control. Further tests using detached leaves, treated with some of these inhibitory organisms prior to inoculation with P. ramorum zoospores, reduced infection and leaf necrosis. Control leaves were 100% infected with an average of 35% necrosis, compared to leaves pretreated with isolates CW-endo2, Sar-endo3, Act-3E, Act-3B1, and Act-3B2 were 60%, 60%, 40%, 40%, and 40% infected, respectively, and 21%, 19%, 33%, 17%, and 17% necrosis, respectively. Several Trichoderma spp. isolates were mycoparasitic, attacking the sporangia and chlamydospores. Another aspect of biological control considered in this study was the use of natural plant products. Caffeic acid amended to V8 agar medium at concentrations of 1 or 3 g/L inhibited the germination of P. ramorum zoospores 98% and 100%, respectively. Direct germination of sporangia was inhibited 25% and 100%, respectively. Rice bran extract, which contains caffeic acid derivatives, inhibited leaf necrosis caused by P. ramorum zoospores when first applied to Rhododendron “Cunningham’s White” leaves before inoculation. Although preliminary, these tests do demonstrate a potential for natural plant products and microorganisms to limit the infectivity and spread, if formulated properly.