When it comes to tree care, spray and pray doesn’t cut it. Predetermined, automatic, one-size-fits-all applications are not only unnecessary, they’re bad for the industry. The once-a-month method needs to be replaced by accurate triage techniques and inspections.
What you’ll need
At least three skills are needed to become proficient at triage. First, a good diagnostician has worked long enough in the tree care industry to be able to identify any woody plant in front of them, as well as the typical causes of tree injury and best management practices to create an environment that supports healthy tree growth. It’s important to keep a wide spectrum of possible causal agents in mind instead of quickly leaning toward an insect or disease. Causes for tree decline may include nonliving factors, such as adverse environmental conditions, mechanical injury from construction or human activity, nutrient deficiency or planting errors. Living organisms, such as mites, insects and fungi, can also be responsible.
Second, well-developed observational ability is necessary. The ability to differentiate between normal plant appearance and that which could be caused by a damaging factor is a skill that can be both taught and learned through on-the-job experience. If you know what you’re looking for, it’s not difficult to see, but if you are unaware of the normal pattern of growth, you are not likely to be successful at the diagnosis.
Lastly, the personal trait of inquisitiveness is necessary. Like the trait of extroversion, everyone has the ability to be curious about what is causing a particular tree to be ailing; it just comes more natural to some than others. And, like becoming more extroverted when the situation calls for it, this trait can be developed to assist with triage.
Don’t jump to conclusions
As best you can, fight the natural tendency to decide on a certain diagnosis too soon. Keeping your mind open as long as possible greatly increases your chances for accuracy. For example, a tree suffering from an unknown malady produces what appears to be generalized nondescript symptoms of brown leaves. There are at least 40 or 50 possible causes of such a malady. As you systematically work through the symptoms, finding clear, definitive signs that are germane only to a specific disease, insect or abiotic causal agent, the chances for finding the actual cause of the decline increase.
Keep in mind that two or three causal agents can be responsible for the decline of a tree. In fact, after an initial diagnosis has been made via triage, ask yourself, “What else could be responsible for the way that the tree looks?” Start triage by observing symptoms and signs, arriving at an initial decision, then consider other major groups; if the diagnosis is anthracnose, look for evidence that causal agents in the insects and abiotic groups could have contributed to the demise of the specimen.
By looking closely at the damage caused to tree leaves, insect damage can be classified as being caused by chewing insects, sucking insects and leaf changing insects. The remnants of insect feeding will be good clues; of course the presence of actual insects in large numbers is also quite useful, but that should be considered a bonus during triage.
Chewing insects leave the telltale signs of irregularly shaped holes in leaves and round holes in bark and wood. Some chewers, such as pine sawflies, eat entire leaves, which can lead to confusion. Frass, or granular fecal material, may also be present on leaves or stems, which is also a sign of chewing insects. Cankerworms cause classic chewing damage to broadleaf trees, such as birch and oak, but at first glance the damage may be confused with a bacterial shot hole disease, which may look similar. Some leaf miners use chewing mouthparts to enter and consume the tissues inside the leaf, known as the mesophyll. When present, a meandering or serpentine pattern of damage is evident. The beetles, caterpillars and sawflies are the most common within the chewing insect group.
Sucking insects create a stippled or lackluster appearance of the leaves that they feed upon. Insects in this group possess a piercing stylet that allows entry into plant tissues during feeding. Joining with the maxillae and other internal mouthpart structures, a two-channel extraction arrangement is set up. One channel injects saliva to liquefy plant tissues, while the other is used in conjunction with a small hydraulic pump in the head to suck the partially digested liquid cell sap into the digestive track of the insect. Trees that have been fed upon by sucking insects often have sticky leaves, caused by the deposition of honeydew as a waste product of the insect feeding. Aphids and lacebugs are classic sucking insects. While not true insects, various mite species have similar feeding habits.
Leaf and stem changers cause damage to trees by distorting, cupping, rippling and creating oddly shaped structures on the stems and leaves. This is somewhat of a catch-all group, with insects that use a variety of feeding and adaptive methods to create damage. Most notable in this group are the galls, which are frequently caused by cynipid wasps. They distort plant tissues by disrupting the normal growth process of leaves and stems. For the most part, they do not pose a significant threat to tree health.
Diseases, either foliar or systemic, can be a major contributing factor in the demise or success of a tree. While foliar diseases, such as tar spot, are not usually serious in the long run, systemic pathogens, such as Dutch elm disease or oak wilt, can move a tree from healthy to hazardous in a couple of years.
Diseases that cause spotting on tree leaves are fairly conspicuous in terms of the signs they produce. If a pattern of round or oblong spots are evident on tree leaves, working to confirm a foliar disease as the initial diagnosis is a reasonable step. However, injury from certain insects can mimic this, especially if the insects are not present and the disease has progressed far enough for infected tissues to fall away from the leaf. Apple scab, dothistroma needle blight and cedar apple rust are diseases representative of this group.
Another category is characterized by pathogens that induce a softening of tissues. Armillaria root rot and heartwood decay of hardwoods are good examples. As the tree is inspected, pay close attention to the integrity of the wood, stems and leaves, as this may be a good indicator that pathogens from this group are responsible. The development of soft and punky heartwood is usually due to the invasion of wood-decomposing fungi after a limb has been removed or a crack in the bark occurs. The most important consideration with decay is to evaluate the relative amount of sound to weak wood present. Quite a bit of inner decay may be present, yet if a solid mass of wood surrounds it, it may not be a primary concern.
Pathogens that don’t fit either of the other two groups are maladies less obvious in terms of signs and symptoms, and include the blights, where various parts of the tree turn brown in random or nondescript patterns. Phomopsis blight and Dutch elm disease are good examples. Further investigation is necessary when faced with these types of scenarios.
In addition to the maladies caused by living organisms, there are a significant number of potential responsible agents that are nonliving. Many factors fit in this group.
The various components of soil (drainage, nutrition, pH, organic matter content, compaction, residues from construction) create the rooting and water/nutrient extraction capacity for tree roots. If any of these factors are not optimal, such as pH values that limit the uptake of iron or manganese or inadequate soil porosity that prevents adequate gas exchange, tree vigor can be compromised. Recent soil activities, such as application of additional soil or trenching, are also important considerations. Any factor that reduces infiltration of water and nutrients is a concern. Measure the degree of slope and consider it in conjunction with compaction and other soil factors.
Other plants in the landscape can limit the amount of water and nutrients available to trees. When placed too closely, shrubs, grasses, ground covers and even nearby trees can compete for necessary inputs, causing stress, inadequate growth and overall unhealthy specimens.
Largely a phenomenon of proper placement in the landscape, weakness can occur when a tree that is favored by full sunlight becomes shaded by a new building or growth of nearby trees. This is common when the crowns of adjacent trees become larger. In certain cases, the effects of phototropism become evident, where young shoots begin to orient themselves in the direction of the available sunlight.
If evidence of new hardscape (benches, concrete slabs, tree surrounds, patios, brickwork, etc.) is present, it’s likely that some roots were damaged in the process. In some cases, damage is minimal and trees can recover easily, but more often than not, construction activity leads to a degradation of tree health, especially if combined with other stress factors.
Although an important concern, root maladies can be quite difficult to observe. In some cases, surface rooting symptoms are obvious and can be noted during triage. Root girdling, damage from construction, compaction or hardpan are the most common concerns.
Editor’s note: This article was originally published in June 2010 and has been updated.