The past few weeks have brought the Plant Clinic several cases of this fungal disease of Douglas fir. We donít see it often at the clinic, but it is fairly common in the Midwest. The disease causes yellowing and leaf drop, symptoms also caused by many site and environmental stress factors. Read on to learn how to identify this disease so that you can provide better long-term management.
This disease is caused by the fungus Phaeocryptopus gaeumannii. Douglas fir is the only known host. Look for yellowing of foliage now. Affected trees hold only the 1- or 2-year-old needles. Older needles drop. Healthy Douglas firs hold 4 or 5 years of needles. Affected trees grow slowly in both height and girth because of the defoliation.
In the lab, we look for characteristic fungal fruiting bodies on the needles to make a positive identification of Swiss needle cast. These fruiting bodies are black, pinhead-sized, and appear in rows on the underside of needles. They look much like Rhizosphaera needle cast on spruce (see issue no. 2 of this newsletter). Place affected needles in a plastic bag with some moist (not dripping-wet) paper toweling overnight. If the disease is present, the fruiting bodies will be evident the next day.
Management suggestions include spacing trees for good air circulation, controlling weeds around the base of the tree, and avoiding susceptible seed sources. If nurseries cannot provide varietal resistance information, stick with a variety developed in your planting zone. In cases where the disease has been identified as a problem, fungicides are used to protect new growth. Applications are made at bud break and repeated according to label directions until the new growth is fully elongated. For homeowners, chlorothalonil products such as Dragon Daconil and Ortho Garden are effective, as is mancozeb. Commercial applicators can also use some systemic products, including Heritage (azoxystrobin), Manhandle (mancozeb + myclobutanil), Spectro (chlorothalonil + thiophanate-methyl), or TwoSome (chlorothalonil + fenarimol). Registered products are listed in the Home, Yard, and Garden Pest Guide and the 2005 Commercial Landscape and Turfgrass Pest Management Handbook. Always read and follow the label of the selected product.
A good Web site discussing Swiss needle cast can be found at Oregon State University, http://www.cof.orst.edu/coops/sncc/hansen.htm.