Probably the most popular ornamental plant species arriving at the Plant Clinic the last few weeks has been spruce. From a pathological standpoint, the only two concerns are Rhizosphaera Needle Cast (discussed in issue no. 2, 1998) and Cytospora canker.
Colorado blue and Norway spruces, especially 10- to 20-year-old trees, are very susceptible to Cytospora canker. Look for dead or dying branches, usually starting at the base of the tree and moving upward, at a rate of one or two branches per year. Occasionally the affected branches are scattered throughout the tree. The needles may drop early from affected branches or they could hang on for several months, leaving dry, brittle twigs. Cytospora canker can continue to spread until all branches or even the entire tree is dead. A white, sappy resin is usually associated with cankered branches. Look for this resin at the base of the branch or at axils of smaller branchlets. When the bark is cut away, the wood beneath will be brown, indicating infection by the fungus. Black pinhead-sized fruiting bodies (pycnidia) of the fungus form in the inner bark, often embedded in the resin, but these can be very difficult to find without the aid of a dissecting microscope.
Cytospora, like most canker diseases, is considered a stress pathogen. This means that the fungus only infects trees under stress. Technically, a stressed tree is one that is not growing under ideal conditions, so the stress could include many factors: limited root zone, moisture extremes, root compaction, chemical injury, nearby construction, insect or mite problems, and more. There are no magical fungicide cures for this disease. As logic implies, disease management should focus on determining the source of stress and then trying to alleviate the stress. At the very least, remove dead wood, water the tree in periods of drought stress, and fertilize in the fall.
So how do you determine whether a tree is infected with Rhizosphaera needle cast or with Cytospora canker? Look at the overall damage pattern. Rhizosphaera occurs on scattered branches on the tree, while Cytospora tends to occur on lower branches and move up the tree. Also look at the pattern on a single branch. Rhizosphaera affects newest needles last; Cytospora girdles the stem and therefore causes newest needles to turn brown first. Finally, look for resin, which is present with Cytospora but not with Rhizosphaera. For more information on Cytospora canker of spruce, consult Report on Plant Diseases No. 604.