Over the past 20 years, this spruce needle disease has become more common in Illinois. This statement is not based on a scientific survey but on personal observation and an increase in spruce samples positive for Rhizosphaera at the Plant Clinic. This needle cast causes a discoloration of the second-year and older needles, often resulting in defoliation of all but the newest needles. The pattern on the tree is usually scattered hot spots, sometimes more uniform damage.
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 spray for several successive springs until the disease is under control.
This article is printed now to remind people that this is the time to spray for Rhizosphaera control. Two sprays are recommended for control of this fungus—one when the bud cap has fallen off and another about 2 or 3 weeks later. Chemical options for commercial growers include Camelot, Chlorostar, Daconil, Echo, Kocide, Manicure, PathGuard, Protect T/O, Spectro, Thalonil, and TwoSome. Home growers can choose from Bonide Fungonil, Dragon Daconil, and 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.
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. The Plant Clinic is available to help diagnose spruce problems (see issue no. 1).
Many diseases, including this one, occur more readily on plants under stress. Do not stop looking for causes of poor growth just because you find Rhizosphaera. It is possible that site or environmental stress is the true problem and Rhizosphaera has followed. Investigate soil type, drainage, injuries to the trunk, compaction possibilities, root injury, etc., so stress factors can be identified and alleviated. Remove dead wood and prune surrounding plant material to allow better air movement in the area. Water trees in periods of extended drought.