Tree problems can be very sneaky. A lot of the time they creep in, and before you know it, the problem is staring you in the face, demanding attention. One of the most common diseases we see here in Nebraska with Colorado spruce are the tree’s needles turning purple and eventually falling from the tree. In a word, the trees look horrible. Sometimes the problem is localized; sometimes it seems to be entire trees.
The problem appears to be a fungal disease called Rhizosphaera needle cast, caused by the fungus Rhizosphaera kalkhoffii. It is a disease that affects spruce trees, and Colorado blue spruce happens to be the spruce considered to be most susceptible to this disease.
Infection typically begins in the lower branches of the tree, because this is the part of the tree most likely to have longer periods of wet conditions which favor fungal growth. The wet springs we have had were a wonderful opportunity for Rhizosphaera to thrive. It typically infects the needles of spruce trees in May and June, but it takes at least until late the following fall or even the next spring for there to be noticeable symptoms.
Infected needles first appear purple, then brown and drop from the tree. Inspection of the infected needles with a magnifying glass usually reveals black spots in rows down the length of the needle. These are called the fruiting bodies of the Rhizosphaera fungus—the structures of the fungus that produce spores.
Rain splashes, sprinklers, or even when hand watering, the water will splash the spores onto healthy needles and helps the fungus spread to more parts of the tree. It’s very easy to overlook a lower branch or two that is affected by Rhizosphaera, but after a year or two, when the disease has spread to multiple branches, it’s harder to miss. This is why everyone is taking notice of their trees now. The disease has simply had enough time to spread and make its presence known.
In severely infected trees, branches will have new green needles on the tips of branches, and purple, brown, or even no needles further back on the branch. Pruning out the affected branches may seem logical, but it’s not always the best option, as Colorado blue spruce doesn’t typically regrow new branches in the affected area.
While you can’t reverse the damage from this disease, you can control its spread. Recommended management of Rhizosphaera needle cast is applying a fungicide containing chlorothalonil when the new needles are half elongated, and again when they reach full size. Also, rake up and remove as many infected needles as possible, to remove sources of the fungus.