Control of Swiss Needle Cast on Douglas-Fir Christmas Trees with Aerial Applications of Chlorothalonil. G. A. Chastagner, Assistant Plant Pathologist, Western Washington Research and Extension Center, Washington State University, Puyallup 98371. R. S. Byther, Extension Plant Pathologist, Western Washington Research and Extension Center, Washington State University, Puyallup 98371. Plant Dis. 67:790-792. Accepted for publication 20 December 1982. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/PD-67-790.

The effectiveness of helicopter applications of chlorothalonil in controlling Swiss needle cast on sheared Douglas-fir Christmas trees was compared with backpack mist-blower applications in four plantations in western Washington. Chlorothalonil (Bravo 500F, 500 g a.i./L) was applied at 2.35, 4.7, and 9.4 L/ha by mist blower and 4.7 and 9.4 L/ha by helicopter. Aerial and mist-blower applications were applied in the equivalent of 93.5 and 374 L of water per hectare, respectively. Applications were made on 4 or 5 June and/or 30 June. A single early mist-blower application at all rates tested provided effective disease control. Single early aerial applications of chlorothalonil at 4.7 or 9.4 L/ha also provided effective disease control. Aerial applications of chlorothalonil at 9.4 L/ha, however, provided significantly better disease control than the 4.7-L/ha rate on needles near the middle and bottom portions of larger trees. There was no difference in disease control on needles from smaller trees or needles from the top portions of larger trees. Aerial or mist-blower applications on 30 June did not provide significant disease control. Timely aerial application of chlorothalonil can provide an effective, convenient, and rapid means of controlling Swiss needle cast.