Jennifer Schultz Nelson
Extension Educator, Horticulture
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. I've had several calls in the last two weeks from owners of Colorado blue spruce trees, all describing the same thing: the needles on their beloved trees are 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 worst situation I've encountered this year is a windbreak of 35 Colorado blue spruce trees affected. Of course, everyone wants to know: what is the problem, and can it be fixed?
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.
I would consider this disease to be one of those sneaky tree problems that creeps up slowly on most trees. 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 in 2009 and 2010 were a wonderful opportunity for Rhizosphaera to thrive. But why are we seeing it now?
The answer is in the life cycle of the Rhizosphaera fungus. 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 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.
One way to avoid the problem altogether is to not plant Colorado blue spruce in the first place. One local nursery owner I spoke with recently told me he actually discourages his customers from planting the Colorado blue spruce because it is so prone to Rhizosphaera, among other problems.
Colorado blue spruce is really not well suited to life in central Illinois. It's much happier in places with cooler, wetter summers and moist, well-drained soils. The hot and often dry summer periods combined with heavy clay soil in central Illinois are a source of significant stress on these trees. Many sources have observed that spruce trees under environmental stresses tend to develop more serious infections of Rhizosphaera.
Unfortunately, the recent reports of Rhizosphaera needle cast are a demonstration of the "right plant, right place" philosophy. Choosing a plant that prefers the conditions we have in our area will usually save us a lot of headaches in the long run. Plants well suited to our climate will be healthier to start with, and generally speaking that translates to less money and time we have to spend on various treatments and remediations. If you have questions on what is the "right plant" for your landscape, please don't hesitate to call your local Extension office. The U of I Extension Macon County Master Gardener Help Desk can be reached at (217) 877-6872.