Chemical Control of Rhododendron Dieback Caused by Phytophthora heveae. D. M. Benson, Associate Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Plant Dis. 64:684-686. Copyright 1980 American Phytopathological Society. DOI: 10.1094/PD-64-684.

Shoots of hybrid rhododendron cv. Roseum Elegans were protected from Phytophthora heveae when sprayed with captafol (0.6 g a.i./L), captan (1.2 g a.i./L), or mancozeb (0.96 g a.i./L) 1–5 days before zoospore inoculation. Chlorothalonil (1.35 g a.i./L), cupric hydroxide (0.53 g a.i./L), or dodine (0.4 g a.i./L) did not adequately control dieback. The systemic fungicides, CGA-48988 (Subdue, 0.96 g a.i./L) and LS 74-783 (Aliette, 0.96 g a.i./L), prevented shoot infection of rhododendron when applied as drenches to 2-yr-old plants, 5–8 days before inoculation. Captafol and mancozeb, at 3.84 g a.i./L, were not phytotoxic on cv. Catawbiense Album and Purple Splendour. Tenacity of the fungicides was evaluated by using a leaf disk assay from plants treated and placed under overhead irrigation (0.8 cm/day). Disks from mature leaves of Roseum Elegans were protected 7–28 days by captan, chlorothalonil, and mancozeb, but disks from young leaves were not protected at 7 days. Disks from young leaves treated with LS 74-783 were protected up to 28 days, but disks from mature leaves were not protected. Captafol and CGA-48988 protected plants from infection for 56 days after treatment. In vitro toxicity of captan, chlorothalonil, and mancozeb to linear extension of P. heveae was similar (ED₅₀ = 4.0 µg/ml). Captafol was more toxic (ED₅₀ = 0.17 µg/ml), although rate of inhibition was only half that for captan and mancozeb.

Keyword(s): hybrid rhododendron, Phytophthora cactorum, P. citricola, P. nicotianae var. parasitica.