Some evergreen needle diseases are called “casts,” and some are called “blights.” The needle casts are fungal diseases that cause one-year and older needles to turn off-color or brown and fall from the branches. The blights also kill needles, but the needles remain on the branches longer and give the tree a blighted appearance.
Now is the time to start looking for Rhizosphaera needle cast on spruce. Although this needle disease does not kill a tree, it causes some serious aesthetic injury to the most majestic old spruce trees. Evergreens do not replace fallen needles, so infected trees often have holes of bare branches in their canopies. If you had trouble with this disease in the past, now is the time to take action to prevent its spread. Fungicides stop this disease, but they must be applied when needles are half grown and again when fully grown.
If the Rhizosphaera fungus is the cause of decline, it is present on last year’s purple–brown needles. When infected needles are moist, the fungal pathogen forms 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 bread twist-tie. Wait 24 hours and then use a hand lens to look for the diagnostic fungal structures. Compare these to pictures in texts or on this North Dakota Web site, http://www.ext.nodak.edu/extpubs/plantsci/trees/pp789-3.htm.
Fungicides are used to protect healthy new growth; and that takes two sprays to accomplish. The critical factor is to know 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 must 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 2001 Illinois Commercial Landscape and Turfgrass Pest Management Handbook and the Home, Yard and Garden Pest Guide.