Putnam M L
Oregon State University, Botany and Plant Pathology, Corvallis 97331-2903.
Plant Dis. 1999 Feb;83(2):200. doi: 10.1094/PDIS.1999.83.2.200A.
The Oregon State University Plant Clinic received samples in 1996 and 1997 of Douglas-fir (Pseudotsuga menziesii) with web blight from Christmas tree plantations in Douglas, Lane, and Benton counties. Affected needles were 1 or more years old, uniformly brown, with no chlorosis or banding, and had a tendency to hang down from the branches and cling together, bound by mycelial webbing visible at ×10 magnification. In 1998, the disease was found on grand (Abies grandis) and noble (A. procera) firs in Christmas tree plantations as well as in native stands of Douglas-fir. Twigs of grand fir were killed, but not those of Douglas-fir. Brown needles may encompass half the side of a tree, destroying its value. This disease has been severe in Christmas tree plantations. A binucleate Rhizoctonia-like species has been consistently isolated from affected needles from all three tree species. The fungus has hyphae averaging 6.0 μm in diameter, with dolipore septa, slight constriction at the branch points, and acute to right angle branching. No sclerotia are formed, but monilioid cells develop after 2 weeks. Cell nucleation was determined with cotton blue or ethidium bromide. Koch's postulates were fulfilled by inoculating with an isolate obtained from grand fir onto 2-year-old Douglas-fir trees. Plugs (7 mm in diameter) of mycelium growing on potato dextrose agar plates were removed and placed onto the foliage of three trees. Plugs from uninoculated plates were used as a control on three additional trees. Inoculated trees were placed under mist on a greenhouse bench. Symptomatic needles were observed 5 weeks after inoculation. A fungus identical to the inoculum was obtained from symptomatic needles. No Rhizoctonia-like fungi were obtained from needles of the control plants. Further characterization of the fungus is underway. This new disease of Douglas-fir and true firs is potentially devastating, since Douglas-fir is a major component of managed timber forests in western Oregon, and the disease has been found at numerous sites in timber trees in the region.
