EVALUATION OF MANCOZEB AND PROPICONAZOLE TO CONTROL KEITHIA LEAF-BLIGHT OF CONTAINER-GROWN WESTERN RED CEDAR

Two fungicides, propiconazole (Topas(R)) and mancozeb (Dithane(R)), we re 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 1000 L-1 ha(-1)) every four weeks, and mancozeb at one rate (2.06 kg ai 1000L(-1) ha(-1)) every two week s, and a water control applied every two weeks. Applications were made from July 10, 1995 until the end of November, 1995. Efficacy and phyt otoxicity evaluation of the fungicides was done by subsampling 160 see dlings from each treatment at each nursery in early December, 1995. Th e results established that blight severity on the one-year-old seedlin gs was significantly less (p less than or equal to 0.05) than the cont rols, for all four propiconazole treatments and the one mancozeb treat ment. Blight severity on the two-year-old seedlings was significantly less (p less than or equal to 0.05) than the controls, for the treatme nts 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 dif ferent than, nor better than, the control, suggesting that the fungici des were not phytotoxic. Both fungicides, propiconazole and mancozeb, are effective in controlling Keithia leaf blight, however, mancozeb mu st be applied every 10 to 14 days through the entire growing season (1 1 applications), whereas propiconazole requires fewer applications (6) through the growing season, for an equivalent efficacy.