Although I've been diagnosing plant problems for a few years, I'm sometimes surprised at the number of plant samples we receive in the Clinic that have no infectious disease or insect problem (about half our samples). The plants have problems, and may have shown symptoms or declined for several years, but the symptoms are not caused by an infectious disease or an insect. Instead, some "abiotic" stress is causing the plant to wilt, turn brown, or decline or, in many cases, a variety of stresses are accumulating to cause serious problems.
So what are the most common stresses to plants? It's a well-kept secret that when we diagnosticians can't figure out a plant problem, we like to blame the weather. In reality, the weather can be one of the major stresses causing plant problems. This year's weather has been especially tough on plants. In central Iowa we had a warm spell early in the spring followed by a long, cold freeze in April, followed by widely fluctuating moisture and temperature levels with lots of wind. Now, it's hot and dry. Those wild fluctuations wreak havoc on plants, causing browning of evergreen needles, brown spots on maple leaves, and other symptoms. The huge ice storm that covered much of Iowa last winter is another example of a weather-induced stress. A well-placed lightning strike can kill a tree single-handedly or can be one of many stresses that leads to its decline.
Probably the biggest cause of plant stress is humans. We choose improper plants for a given site, we transplant trees when they're big and plant them too deep, often with coiled or J-shaped roots, we plant grass underneath trees and then whack off tree bark with lawn mowers and weed-eaters, we dump herbicides on our lawns and fields, we fail to water recently planted trees and shrubs or we water and fertilize them to death, we pave over tree roots or compact their soil by driving heavy construction equipment over it, we remove most of the topsoil in new housing developments and expect grass to grow on compacted subsoil, and we overwater our houseplants, etc., etc., etc. The list of abuses could go on and on.
How can we give our plants a little stress relief? We can't control the weather, but we can control where we decide to put plants and how we treat them. Choose an appropriate plant for a site, plant it properly, and protect it from unnecessary injury afterwards. Watering trees and shrubs deeply (throughout the rootzone) but infrequently (not more than once a week or so) during very dry periods can help to minimize stress. A three-inch-deep layer of organic mulch over the rootzone can also help to buffer the soil moisture and temperature levels, along with keeping away mowers. If we do our best to minimize the human-caused stress, our plants will be better able to handle the weather-related stresses that we can't control.
Damage at Base of Tree: The base of this tree has been injured by a lawn mower. This is one of the most common stresses we see on urban trees.