With the rise of global commerce and transportation, we have seen a tremendous rise in the number of foreign diseases and insects for which our native and widely used ornamental trees have limited defenses. In addition, these trees have to fend off the problems that they have been dealing with in an increasingly more difficult environment. Pollution, poor soil conditions, improper care are some of the conditions or abiotic (not living) stressors that decrease a tree’s vigor and make it susceptible to the other bugs and diseases in the environment.
Outward symptoms of tree distress can be categorized into three types: insects, diseases and abiotic. By observing patterns of symptoms on the tree, one can determine what type of problem the tree is having and how bad it is for the general health of the tree. It is also important to keep an eye out for foreign invaders that are not yet in our area. Tree pathology is very much like detective work in that one must be very observant and put together clues from many areas to make a determination of the problem.
Insects damage trees in a variety of ways. They are usually the most visible symptoms because they crawl, fly and otherwise move around attracting our attention (cause we used to eat them!). They attack the tree in a variety of ways. There are defoliators, borers, bark eaters and rotten wood eaters. Trees exhibit symptoms and signs that give a clue as to the type of insect that is attacking it.
|Chewed leaves or other parts||Catepillars, leaf miners, beetles or sawflies|
|Bleached, yellowed or stippled leaves||Leaf hoppers, aphids, psyllids, thrips or mites|
|Distortion of plant parts (twisting, cupping, swelling, etc.)||Thrips, aphids, blister mites, gallmakers, psyllids|
|Disback of twigs or branches||Boarers, scales|
|Frass (bug poop), sooty mold, wooly appearance, etc.||Aphids, soft scales, whiteflies, adelgids, thrips, lacebugs,|
Some of these signs are also indicators of infectious diseases or abiotic stresses, so don’t go out and buy an insecticide to cure your tree’s ills. Broad-spectrum insecticides have a nasty habit of killing not only the bugs that are eating your tree but the ones that are eating them. This can lead to a downward spiral and reliance on Dupont to solve problems that a ladybug or two could do for free. In addition, insect outbreaks often point to a deeper problem.
Diseases need to have more going for them than insects to successfully attack and damage a tree. Unfortunately, once established, they can turn a once healthy tree into a dead tree. And in some cases, rather quickly. Tree diseases can be spread from tree to tree by contact through root grafts or infected soil, transmission from insects, transmission from the air and in many other ways. They can be treated by cultural or “medical” methods.
Diseases need a susceptible host, the right conditions and the right timing . If one of these conditions is absent, the disease will be able to infect the host. Often this is the best way to keep trees free of disease. Disease resistant species or cultivars will be able to more easily fight off a particular ailment. If it’s relatively warm and moist, like spring time in western Oregon, scale will spread through apples and pears –if it’s present.
Often keeping a tree vigorous is the best way to help it fight off a disease. Adding compost to the root zone or fertilizing with a balanced, organic fertilizer (fish emulsion, compost tea, etc.) will give the tree the extra vigor that it needs to deal with the disease, while not juicing it up with excess nitrogen, which may cause it to put on a flush of new tender growth, ready to be attacked by something else. Mature trees rarely need large amounts of nitrogen fertilizer.
A sick tree may not be diseased or infested by bugs. It may be suffering from other stresses somewhere else in the system. Too little water or too much can cause discoloration and curling of the leaves. Herbicide drift can cause dieback in small, localized areas or it can cause intermittent distortion of leaves. Soil compaction, poor fertility or upset pH can cause general tree vigor problems, evident by poor yearly growth of shoots, unusually thin crown, and presence of insects and diseases. Decompaction can be attained through soil fracturing, vertical mulching, or total soil replacement.