2010 has been a tough year for post oaks in Houston. Some have died, while others have lost all or most of their leaves. Once a post oak has defoliated in the middle of the summer, it's really difficult to stabilize the plant and keep its systems going until the following spring, when the new foliage offers an opportunity for recovery. Post oaks are notoriously sensitive trees, and how they respond to site alterations, such as home construction, turf grass installation, irrigation installation, paving and landscaping, is a flip of the coin. Let's face it, if you have a post oak on your property or under your care, you should be prepared for it to die just to spite ya. If you have a post oak, and you're worried about its health, then don't panic. There may be a few simple corrections that can help keep it stable and (more or less) healthy.
What I've been looking at this summer have been three main sets of symptom expression. One is the flash, in which the entire tree flashes paper-bag brown. Another symptom is premature defoliation, wherein the leaves begin raining down from the tree. The leaves aren't necessarily brown in this case, but they may appear scorched. The third is leaf scorch; here, the leaves appear scorched, particularly at the ends, and leaves don't necessarily rain down wholesale from the crown. I happened to visit a property with all three of the above-mentioned symptoms in the same front yard, which you may look at in the linked YouTube Video. There are numerous other symptoms that can be identified on post oak, including sticky-cottony yuck on the leaves, nipple galls, stippling, powdery mildew, and others. I do not address them here, but I'll be happy to do so--just ask!
Flashing brown has over the years been called post oak decline. Whenever you hear the word "decline", you should understand that this is a generally amorphous term used to describe a set of symptoms or pattern of mortality, the causes of which have not been determined. I use the term in a different way; generally I use it to describe a tree that is slowly dying from the tip down, and that has multiple stress agents associated with the symptoms. Whenever I go onto a site to inspect a post oak that's flashed brown, The prognosis is always the same: the tree is dead and needs to come down. Determining a cause of death is difficult, because there are usually so many factors that can be pointed to. Every conceivable way the site was altered in the last few years is a possible causal factor. Is the home less than ten years old? Was an irrigation system recently installed? Was sod put down on the root zone? Was the grade changed? Is the soil compacted? What is the internal drainage like? What is the soil's organic content? These questions are important if you have surviving post oaks that you want to preserve, but they don't really change the prognosis about the one that's flashed.
Premature defoliation can frighten a homeowner or property manager just as much as flashing brown. It usually happens some time after midsummer, and well before normal fall leaf drop. Premature defoliation has many causes, including nutritional deficiency, root loss, root stress, drought, leaf spot disease and bacterial leaf scorch. The best ways to prevent premature defoliation are to promote healthy soils with good internal drainage, aeration, soil biology and high organic content, to eliminate turf grass from the entire root zone of the tree, to take the tree off the irrigation schedule, and to apply a layer of mulch, taking care not to bury the root collar. Topical leaf spot diseases can, as mentioned, cause premature defoliation. One of the best ways reduce spore counts on a property is to rake, bag and haul off (or compost) the leaves after they have fallen. While I will apply fungicide in the springtime to reduce the incidence of leaf spot disease, I usually only prescribe this course of action when the tree is either in an emergency state, or the homeowner or property manager is unable or unwilling to improve the growing conditions.
The third symptom is leaf scorch. Here, the leaves appear damaged, but don't necessarily fall from the tree. Note that there is some overlap here, between what I described above as defoliation and leaf scorch. There's this nifty little bacterial pathogen called Xylella fastidiosa, which is transmitted to the plant by some types of sucking insect. This has only recently been found to be the cause of decline and mortality of a great number of trees and landscape plants, the cause of whose problems was previously lumped into the "decline" category. While it's great to be able to pin the cause of one symptom, it's important to remember that when we're talking about the urban/suburban landscape setting, there are many other stress factors that need to be addressed. Bottom line, if your tree's leaves look scorched, and if you notice a little yellow halo between the brown, scorched area and the green part of the leaf, then it could have bacterial leaf scorch.
There is no cure for bacterial leaf scorch, and the disease usually causes slow decline and death. The symptoms can be suppressed, however, and this is a great way to get continued enjoyment out of one's trees. Symptom suppression can also be argued to prolong the life of the tree. This is only done in the spring--usually the spring following diagnosis. The only known treatment for suppressing symptoms is application of the antibiotic oxytetracycline by trunk injection. Please note that this is one of a very few instances in which I will advocate the injection of trees. I point this out, because there are so darn many arborists out there leaping on the trunk injection bandwagon for all manner of ailments--diagnosed and undiagnosed--from micronutrient deficiency, to fungal disease to borers. Before you ever sign on to a treatment regime that includes capsules or injections, please get a second opinion from a grown-up arborist.
While I'm on the topic of post oaks and diagnosis, let me address some common diagnoses bandied about by some of the other arborists out there. The most common are anthracnose, Asian ambrosia beetle and borers (gotta get them borers!).
Anthracnose is a general term used to describe a number of different symptoms on a number of different host plants, caused by a number of different pathogens, most of them fungal. Now, with a definition like that, it's little wonder that a homeowner can be confused by his or her arborist! Anthracnose isn't really a term I associate with post oaks, but there are some symptoms--usually fungal leaf diseases, and arguably even bacterial leaf scorch--that could, I guess, be described as anthracnose. In any case, I'd be wary of anyone who wants to spray fungicide at the time of diagnosis. In keeping with what I stated above, most fungal leaf diseases infect the foliage as they emerge in early spring, although the symptoms don't show up until later in the growing season. This does not mean that I never apply fungicide any time other than in the spring: quite often, in fact, I'll see newly emergent foliage on a tree--especially live oak--and apply fungicide to protect it. But we're talking about post oaks, and for me to observe newly emergent foliage on a post oak that has any of the above symptoms would be a surprise indeed.
Asian ambrosia beetle mainly attacks fruit trees, and post oak is not currently considered to be a host plant for this pest. There is a native ambrosia beetle, by the way, but it's not interested in oaks, either.
Borers are blamed for decline, decay, cavities, etc. more than just about anything. It's so easy to look at a distressed tree and find a hole or two on the trunk. See? Look here, there's borers on the trunk. There's your problem, right there! Fact is, most wood-boring insects are simply capitalizing on exposed dead wood. It's best to think of them as a friend: their presence can be evidence of decay that you might otherwise not have known about. So, while I may treat a tree to help suppress borers, it's usually a newly transplanted tree.
In summary, the recommendations I make for post oaks are the basically the same as the recommendations I make for all shade trees, only with ten times the urgency: promote healthy soil in the entire root zone of the tree, and it will have few problems. This means, in a nutshell, good soil organic matter. High organic content in the soil means good soil biology. Good soil biology means good plant nutrition, decreased susceptibility to diseases, pests, moisture extremes and abiotic stress factors. Put a protective force field around your tree, starting at the dripline, and don't allow the soil to become compacted. You just might get years of enjoyment out of your post oaks.
Please feel free to leave a comment or send an email. If you're looking for a TCIA Accredited Houston Tree Service or Certified Arborist, call the company I've worked for since 2002, Bartlett Tree Experts, at 713-692-6371. This is my personal blog--it is not affiliated with Bartlett Tree Experts.
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