This fungal disease of spruce is aptly named “cast,” because it causes needles one year and older to turn purple-brown and fall from the branches. Although this needle disease will not kill a tree, it will cause some serious aesthetic injury to the most majestic old spruce trees. Evergreens will not replace fallen needles, so infected trees often have holes of bare branches in their canopies.
If you have had trouble with this disease in the past, now is the time to take action to prevent its spread. If you think your spruce may be infected, read on to see what you can do now. Fungicides will stop this disease, but they must be applied when needles are half grown and again when fully grown.
The newest growth on infected spruce trees will be green and healthy. Last year’s needles, however, will be brown or purple-brown and may still be attached to the tree branch. They will be cast soon. Other factors such as drought stress, root injury, flooding, fertilizer burn, and so forth could also cause similar symptoms. If the Rhizosphaera fungus is the cause of decline, it will be present in the purple-brown needles. When infected needles are moist, the fungal pathogen will form pinhead-sized fruiting structures (pycnidia) in rows on the needles. These stick up out of the needle like black pinheads. A simple test you can do is to place suspect needles in a plastic bag with a wet paper towel. Close the bag and seal with a twist tie. Wait for 24 hours and then use a hand lens to look for the diagnostic fungal structures.
Fungicides are used to protect healthy new growth, and that takes two sprays to accomplish. The critical factor is knowing when to apply the fungicide. Purchase the fungicide and have it ready to apply. You must watch your spruce to see when buds open and new needles start to expand. Chemical options for control of Rhizosphaera of spruce need to be applied when the needles are half grown, so compare new growth to typical growth of the past year. There are many chemical options listed in the llinois Commercial Landscape & Turfgrass Pest Management Handbook 2000 and the llinois Homeowner’s Guide to Pest Management.