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

R. G. Linderman and E. A. Davis, USDA-ARS Horticultural Crops Research Laboratory, 3420 NW Orchard Ave, Corvallis, OR 97330; (541) 738-4062; lindermr@science.oregonstate.edu

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 (cv Nova Zembla) leaves by P. ramorum (both NA strain 2027 mating type A2 and European strain D12A mating type A1) compared to P. cactorum, P. citricola, P. nicotianae, and P. citrophthora. We inoculated needle-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, 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 application. Some chemicals had varied efficacy depending on the species of Phytophthora. 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, and in some cases efficacy was increased. Bacterial antagonists (Bacillus brevis or isolates of Paenibacillus polymyxa) significantly inhibited all Phytophthora species in in vitro challenges, but were ineffective in inoculation assays of leaves dipped in a cell suspension of each antagonist 24 hr 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.

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