Rhizosphaera Needle Cast
By José Fernández
Rhizosphaera needle cast (RNC) – quite a mouthful, especially when one is an arborist in training, still green in the green industry. My supervisor had just handed me a photocopy of a fact sheet on the disease since I had asked him about spruce trees I had been seeing with defoliating branches.
This took place early in my career, 14 years ago or so, here in central Ohio. Fast forward to the present when the disease has become so prevalent that news stories on local television and in local papers have covered the issue. For several years my colleagues at Russell Tree Experts and I have discussed the need for a short article about this since we have this conversation so often with clients, but we never have made a point to write one! So much to do in so little time. I think I run into declining spruce trees with this disease 3-5 times per week so I know there are many readers who will benefit from this topic, or know someone who will.
Colorado spruce (Picea pungens) is a species that is probably overused due to its attractive evergreen foliage. In particular, the “Blue” varieties (‘Glauca’, ‘Hoopsii’, among others) seem to be the most popular, used as accent plants where they really stand out against a larger backdrop of more traditional green foliage. Interestingly, a quick check of the literature does not show that the species is prone to very serious disease, even though needle cast disease is mentioned as a possibility.
Young Colorado spruce rapidly defoliating
This is the reason why I seem to have suddenly shifted from one topic (RNC) to another (Colorado spruce). The tree is widely used (and therefore widely available) yet there is not a lot of information about how the species is performing in central Ohio. Please keep in mind that as an arborist I am trained to consider long term performance of a tree as opposed to short term. This means that I view trees as potentially permanent part of the landscape that we can choose to work around when subsequent changes in site use or design are necessary. This view is not shared by all, and my purpose here is not to defend my position against others. I mention this because Colorado spruce may still be a great choice of tree if its purpose is short-lived by design. I find that most individuals who plant trees do so because they treasure the feeling of starting a living process that will continue long after they are gone from this life. I am in this category, and I derive great benefit from seeing a tree remain healthy as it grows, changing and maturing. I help the tree along and it becomes a living, contributing part of my local environment.
I have come to believe that Colorado spruce is not a good option for a long term investment of resources (time, space, money). I would guess that about 90% of these trees eventually develop RNC to an extent that makes removal necessary as symptoms progress, causing the tree to lose its needles prematurely.
Fungal fruiting bodies
RNC is caused by a fungal pathogen. The fungus reproduces by forming spores in fruiting bodies (see photo above) that grow in the leaf stomata (openings in the leaf or needle, usually on the underside, that allow for gas exchange with the environment). Because of this, one diagnostic tool is to look for small black fruiting bodies lined up nicely on the underside of a spruce needle. Usually the stomata are white, which makes fungal fruiting bodies stand out when they are present. The spores spread with wind or rain, moving into the tree. In the case of RNC, disease symptoms soon follow, usually marked by needles that first turn a purplish color, then brown (see photo below), then dropping.
Purplish brown needles
The defoliation pattern will be from the inside of the tree moving outward, and generally from the bottom of the tree moving upward, although sometimes it can move downward as well. Cool, moist conditions will favor the development of most fungal diseases, including RNC. Hosts of this pathogen include most spruce, several pine, and some fir and hemlock species. I have found several Norway spruce with RNC, but generally I consider this tree to be resistant consistent with the literature. To my knowledge I have not seen this disease on any pine or hemlock, although the early disease symptoms present differently and I may have missed it. For example, in Norway spruce the browning pattern is preceded by a mottled yellowing of the needles rather than the purplish color seen in Colorado spruce. The “fall color” (see photo below) of Norway Spruce can be confused with needle cast symptoms.
Normal needle cycling “fall color” of Norway Spruce
To differentiate between the two, the diagnostician should look for the fungal fruiting bodies and evaluate the pattern and extent of needle loss in the tree. I still heartily recommend Norway spruce as a good option for us in central Ohio, but for several years now have discouraged people from planting Colorado spruce simply because I am either removing them, waiting to remove them, or spraying them with fungicides.
Fungicide sprays for this disease are mostly protectant rather than curative. For this reason multiple applications are recommended each season, with the goal of providing a chemical barrier over newly emerging needles as growth occurs each spring. Once past the cool, moist spring conditions the needles harden off, climate gets warmer and drier, and infection is less likely, but still a possibility. One of my colleagues visited Colorado a few years ago. When he returned he said two things to me: “There are still streets with ash trees in Colorado, and the spruces don’t have needle cast”. Why do our Colorado spruce get needle cast? Our climate is much warmer, much more humid. Our soil tends to be poorly drained, and alkaline. Bring a tree genetically adapted to a specific environment into a different environment and it will likely become stressed, making it vulnerable to disease. In central Ohio, I have seen a trend with Colorado spruce: The trees look great when they are newly planted. If the site is very poor, they quickly develop RNC or decline for other reasons. If the site is reasonable, the trees grow very well and remain beautiful. Then we get to that 90% mark I mentioned earlier, which seems to be with trees that have done reasonably well for 15-25 years, but now have entered into rapid, noticeable decline resulting in tree death within 10 years. Most people choose to have the trees removed well before the trees are completely dead since they lose their beauty and no longer serve as screening plants. This is why I consider this species to be useful if the intended design is only for the next 15 years or so.
If you have a Colorado spruce you want to preserve here are some things that will help:
I recommend repeat applications of fungicides on a seasonal basis to protect newly emerging needles. Note that this does not guarantee that your tree(s) will remain disease free, but it will greatly reduce the infection rate.
Change any existing irrigation so that water is not being sprayed onto the tree foliage. Water the tree with drip irrigation at the base.
Weeds and neighboring plants should be kept away from the lower foliage to increase sunlight and air penetration to try to reduce moisture and humidity levels.
You may consider enjoying your Colorado spruce (whether a green or blue variety) for as long as it looks good, but plant some replacement trees nearby to get them started now. Concolor fir is an evergreen tree that has a bluish cast to it and will perform well given well drained, biologically active soil. Norway Spruce is a good option if the space requirements are met, as this tree will grow large. Western redcedar is proving to be a top performing tree for us, and is also a good screening evergreen plant.
In closing, it is worth mentioning that the dwarf forms of Colorado spruce, whether blue or not, seem highly resistant to RNC. I rarely see this disease on the dwarf varieties.
Thank you for reading! My sincere hope is that this information is useful to you.
Your friendly neighborhood arborist,
José Fernández | Regional Manager, Russell Tree Experts
José became an ISA Certified Arborist® in 2004, and a Board-Certified Master Arborist® in 2015. Currently he is enrolled at The Ohio State University pursuing a Master’s Degree in Plant Health Management. José likes working around trees because he is still filled with wonder every time he walks in the woods. José has worked at Russell Tree Experts since 2012.