Rhizosphaera needle cast of spruce is seen frequently in Illinois. This fungal disease causes one-year-old needles to turn purplish brown before they drop. The newest growth remains green and healthy. The pattern on the tree is usually scattered hot spots, giving areas of thinned foliage scattered in the tree. The disease is often associated with site stress, so don’t be too quick to blame only this disease. We have seen cases where spider mites, root rot, and even weed trimmer injury of the trunk were the major problem, but Rhizosphaera was present and was the only problem addressed. There may be more than one problem on a plant. Rhizosphaera often accompanies other problems.
The disease is Rhizosphaera needle cast; and the cause is a fungus, Rhizosphaera kalkhoffii. Blue spruce is most often infected, while Norway spruces are resistant. Infection is favored by wet weather, but symptoms do not often show for 12 to 18 months after initial infection. Rhizosphaera needle cast may cause severe defoliation of spruce, a species that does not refoliate readily. A few years of infection may cause a very unsightly and weakened spruce. Fungicides are often recommended for control, but such fungicides are preventives, meaning that you apply the fungicide before new infection occurs in a given year. The usual series of events is the positive identification of Rhizosphaera in year 1, followed by use of a fungicide in the spring of year 2. It may be necessary to use cultural controls, as well as fungicide sprays, for several successive years until the disease is under control. Two sprays are recommended for control of this fungus–one when the bud caps have fallen off and another about 2 or 3 weeks later. Chemical options for commercial growers include Camelot, Chlorostar, Daconil, Echo, Kocide, Manicure, Protect T/O, Spectro, and TwoSome. Home growers can choose a labeled chlorothalonil product, such as Dragon Daconil or Ortho Daconil. Tables at the end of disease chapters in the pest management handbooks list the active ingredient and mobility of chemicals mentioned. Company names are also listed. Pick a product that works best for you, but read and follow label directions carefully. Refer to the Home, Yard, and Garden Pest Guide or the 2005 Commercial Landscape and Turfgrass Pest Management Handbook.
Remember, there are many other noninfectious problems that can mimic this disease. A few possibilities include an imbalance in soil pH, poor fertility, fertilizer or chemical burn, root injury, root rot, drought stress, and spider mite infestations. To confirm the presence of the pathogen, look for fruiting bodies (pycnidia) on the discolored needles. It might be necessary to place some affected needles in a moisture chamber (plastic bag with moist toweling) overnight to encourage growth of fruiting bodies. Look for pinhead-sized black structures poking out of the needle through stomates. A hand lens is usually required to observe these structures, which occur in rows. They do not easily rub off because they are embedded in the tissue. They are about the size of the head of a pin. A good image of these fruiting bodies can be seen at http://www.extension.iastate.edu/Pages/plantpath/clinic/Rhizosphaera%20close.jpg. If your are uncertain of your diagnosis, consult a university Extension office or the Plant Clinic.