A fungal evergreen disease, Rhizosphaera Needle Cast can cause unsightly damage. It primarily affects the short-needled evergreen varieties – spruce, fir, pine and hemlock species. Most commonly, this fungus affects Colorado Blue spruce and is caused by one of several bacteria in the Rhizosphaera family.
You can identify this type of Needle Cast by looking at the base of the tree. If you notice discoloration, browning, and death of needles on the lower branches of a tree, this is probably what you’re seeing. Get a magnifying glass and check the needles you believe to be infected for small, black, spherical growths. If you find them, you’re certainly looking at Needle Cast.
Needle damage propagates upwards through the tree as the disease spread progresses, leaving the tree appearing that it is dying from the inside out. Unfortunately, once these needles have browned and fallen off, they will not regrow, leaving those sections of the branches bare. While fully dead branches do not further contribute to the spread of the fungus, many tree owners choose to have dead branches cut at the trunk to leave the tree with as much active growth as possible. Typically once a tree exhibits 75% or more of its branches affected, the tree will not recover and must be taken down.
Consider treating affected trees with fungicides containing the active ingredient copper (e.g., Bordeaux mixture) or chlorothalonil. Treatments will not cure existing infections, but can prevent additional infections. Apply treatments every three to four weeks starting as new needles emerge in the spring. Continue applications through periods of wet weather. For fungicide treatments to be effective, thoroughly cover all needles. This may be extremely difficult with large trees. Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the fungicide(s) in the safest and most effective manner possible.
The easiest way to avoid Rhizosphaera needle cast is to avoid planting Colorado blue spruce. If you do plant blue spruce, consider using dwarf varieties and allow adequate spacing between trees so that branches will not overlap when trees are full size. Dwarf varieties and properly spaced larger spruce varieties will have better air penetration and needles will dry more quickly. Dry needles are less likely to be infected. Also check existing spruce trees for the disease. Remove and destroy any diseased branches and needles by burning (where allowed), burying or hot composting.