Aspen sap could be indication of problem

We have a problem with our aspen trees. Nearly all of them are sapping quite a bit and are also splitting down the sides. We live at 6,100 feet in Paonia. Last summer the leaves were mottled. Any ideas what might be wrong?

— Connie

I’m a little torn on what to tell you. It’s normal for aspens to bleed sap just before or as they break bud for the season. The sap originates usually from where a branch was cut off, an old wound, or something of that nature. As spring progresses, the sap dries up and the tree motors along as if nothing’s happened. The important thing is that this bleeding isn’t harmful to the tree — they’re not going to bleed to death, it’s just a normal wound response of the plant. However, there are times that this sap indicates a bigger problem.

The first possibility is poplar borer. Take a close look at where the sap is leaking out. If the tree has poplar borer there will be some stringy sawdust material extruding from the hole. Poplar borer is not that uncommon, and I don’t consider it to be an emergency. This borer does most of its feeding in the heartwood of the tree. That tissue is dead, so there isn’t any immediate threat to the tree usually. They can very occasionally tunnel through the cambium layer of the tree, which is a bigger but pretty rare problem and worth doing something about. Their damage can structurally weaken the tree where a gust of wind can snap it off and their holes can provide an opening for cytospora canker, which is a much bigger concern.

Treating these guys will take some persistence. The treatment itself is a spray of the trunk with 38 percent permethrin. Usually one spray a year about the first of July will do it. You can apply a second spray a month later if you really want to be sure. They have a two- to three-year life cycle, so you’ll need to continue doing it for three or four years at least to get rid of them.

The second possibility is a fungal disease called cytospora canker. Identifying this can be a bit trickier. There’s usually a sunken, discolored (grayish-black to orange) section of the bark on the trunk or main branches. In smaller twigs, it usually just kills them. This guy can occur on a wide range of different host plants, but we see it overwhelmingly on aspen trees. It’s a stress-related disease, attacking trees that are weakened for one reason or another. We see it most often on newly planted trees since they can often be under some stress in the digging and transplanting, on older trees (15 to 25 years old) since that seems to be about the lifespan of aspen down here in the valley, or on trees that have been neglected.

The disease travels through the vascular tissues of the plant, killing them and choking the plant to death. Branches with small, sparse, pale green or yellow leaves are usually the first symptom. That’s followed by death of smaller twigs and then larger branches. If your tree does have cytospora, you’re eventually going to lose the tree. We don’t have any fungicides that will do the job for you. About all you can do is to cut the infected tissue out by completely removing the branch that’s involved. That’s usually not an option because by the time we figure out what’s going on, the disease is in the trunk of the tree and you end up cutting the tree down anyway.

Now that I have you sufficiently panicked (really, I don’t mean to), what I’d do is to just sit tight for now and see what the tree does in the next month or so. If it leafs out well, then it’s probably normal sap leakage and nothing to worry about.

However, if you see branches (especially larger ones) that don’t leaf out or leaf out with stunted, distorted foliage, you might be dealing with cytospora. If you do have it, you’ll want to remove that tree as soon as it’s practical before the disease has a chance to spread.

Dennis Hill is the nursery manager at Bookcliff Gardens, bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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