This fungal needle disease of spruce is a particular problem on Colorado blue spruce, but it may also infect other spruce species as well as some pines. If you have had a problem with this disease in the past, it’s time to take action now. How do you know you have had problems? For one thing, the disease causes first-year needles to turn brown to purple in the fall. (Keep in mind, however, that many environmental stress factors cause these symptoms.) On trees with Rhizosphaera needle cast, the affected needles may stay attached until the next summer or fall. The newest growth (buds now opening) will be green and healthy, but the rest of the needles will be brown on the affected branches. So how do you know this fungus is the problem rather than environmental stress? When infected needles are moist, the fungal pathogen will form pinhead-sized fruiting structures (pycnidia) in neat rows on the needles. To test for pycnidia, place affected needles in a plastic bag with a wet paper towel. If the fungus is present, pycnidia should develop in one or two days. These pycnidia actually protrude from the needle surface and are readily visible, especially with a hand lens.
The pattern on the tree is the appearance of holes in the canopy. The holes are not in the trunk; they are in areas of the tree canopy from which needles have dropped. The needles will be cast from the tree—thus, the name “needle cast.” Because spruce trees are not able to form new needles where the old ones dropped, the holes in the canopy will stay with the tree for its entire life. The disease will not kill the tree, but it will harm its beauty. For this reason, preventive fungicides are recommended for Rhizosphaera needle cast. As long as entire branches are not killed, the new growth at the branch tips can be protected from infection. As tips continue to grow, new growth may mask the defoliation that has occurred on older needles near the center of the tree.
Chemical options are listed in the usual commercial and homeowner pest management guides. Fungicides are used to protect healthy new growth, so two sprays are recommended—one when new needles are half grown and another when new needles are fully expanded. The critical factor here is knowing when to apply the chemical. The spruce buds in central Illinois are open now, and needles are beginning to expand. Watch your trees carefully for needle growth in your specific site to know when to spray. It may be any day now.
We do not have a Report on Plant Disease that discusses Rhizosphaera needle cast. Refer to Diseases of Trees and Shrubs by Sinclair, Lyon, and Johnson.