Evaluation of mancozeb and propiconazole to control Keithia leaf blight of container-grown western red cedar

Abstract

Two fungicides, propiconazole (Topas®) and mancozeb (Dithane®), were evaluated for their efficacy to control the foliar disease, Keithia leaf blight (Didymascella thujina) of western redcedar (Thuja plicata), in three container-seedling nurseries in British Columbia, in 1995. At each nursery, five fungicide and a control treatment were applied. Each treatment was applied to one-year-old container-grown seedlings at two nurseries, and to two-year-old container-grown seedlings at the third nursery. The fungicide propiconazole was applied at four rates (0.05, 0.075, 0.125 and 0.250 kg ai 1000L−1 ha−1) every four weeks, and mancozeb at one rate (2.06 kg ai 1000L−1 ha−1) every two weeks, and a water control applied every two weeks. Applications were made from July 10, 1995 until the end of November, 1995. Efficacy and phytotoxicity evaluation of the fungicides was done by subsampling 160 seedlings from each treatment at each nursery in early December, 1995. The results established that blight severity on the one-year-old seedlings was significantly less (p≤0.05) than the controls, for all four propiconazole treatments and the one mancozeb treatment. Blight severity on the two-year-old seedlings was significantly less (p≤0.05) than the controls, for the treatments with the two higher rates of propiconazole (0.125 and 0.250 kg ai 1000L−1 ha−1), as well as the one mancozeb treatment. In general, seedling growth of both the one- and two-year-old seedlings was no different than, nor better than, the control, suggesting that the fungicides were not phytotoxic. Both fungicides, propiconazole and mancozeb, are effective in controlling Keithia leaf blight, however, mancozeb must be applied every 10 to 14 days through the entire growing season (11 applications), whereas propiconazole requires fewer applications (6) through the growing season, for an equivalent efficacy. Key words: Didymascella thujina (Durand) Maire, Thuja plicata (Donn.), cedar leaf blight, nursery seedlings, reforestation