Evaluation of chemical and biological agents for control of Phytophthora species on intact plants or detached leaves of rhododendron and lilac

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 that target Oomycete pathogens for their capacity to inhibit infection of rhododendron or lilac leaves by P. ramorum (both NA strain 2027 mating type A2 and European strain D12A mating type A1) compared to other species, including P. cactorum, P. citricola, P. nicotianae, and P. citrophthora. We inoculated needle-wounded leaves from plants previously treated with various chemicals that were (a) removed and inoculated, or (b) inoculated on intact plants maintained in high humidity. 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 drench application. Some chemicals had varied efficacy depending on the species of Phytophthora. All chemicals were fungistatic, not fungicidal. Dipping leaves in the chemicals 24 hr prior to inoculation resulted in the same activity profile as applying chemicals to intact plants or detached leaves. Bacterial antagonists (Bacillus brevis or isolates of Paenibacillus polymyxa) significantly inhibited all Phytophthora species in in vitro challenges, but were ineffective when applied to leaves 24 hr prior to inoculation with P. ramorum or other species.