April 16, 2012
Cat-faced tomatoes, lawns speckled with almond-colored spots, twisted or deformed rose buds, and bleached-out leaves on a lemon tree. What do all of these things have in common? They’re examples of plant problems that may be caused by environmental or man-made conditions rather than some pest or disease. A frosty blast of arctic weather in early spring or hot, desert-dry Indian summer winds, dull lawn mower blades, too much water or not enough iron are just a few things that can produce symptoms in plants that make them look sick. The good news is that plenty of them are preventable.
Many different things can cause plant health problems. Living (biotic) things like insects, mammal pests or disease-causing microbes are usually what come to mind first, while nonliving, or abiotic, agents are frequently the actual culprits. These abiotic agents produce disorders rather than diseases, and are the result of the environment (temperature, light or water extremes), physical or mechanical damage or conditions (breakage, abrasions, compaction), chemicals (nutritional deficiencies, toxicities or exposure to pesticides), and cultural practices. They may be caused by a single, extreme, environmental event like a weekend of blustery, frigid air in the midst of vigorous spring growth, or by ongoing, chronic conditions—hot sun pelting the trunk of a young unprotected tree for months. Abiotic disorders are often overlooked as the source of plant health problems because they can be difficult to identify.
Some of the more common abiotic problems include:
Air temperature extremes: High temperatures increase the plants’ rate of transpiration resulting in wilting or injury to plant tissue, while very low temperatures cause the moisture in plant cells to freeze, then burst, blacken and die as temperatures rise.
Light: Too much sunlight can “burn” portions of plants (bark, foliage, fruit or other plant parts) during periods of high temperatures, or result in faded foliage. Too little and plants may “stretch” for available light, producing elongated space between leaf nodes and sparse blooms.
Water: Foliage can wilt, discolor and drop prematurely without adequate water, and a prolonged water deficiency can lead to smaller leaves, slower growth, dieback and susceptibility to insect pests. Too much water—from over irrigation or inadequate drainage—deprives roots of oxygen and they suffocate. As they die, the plant may show signs of chlorosis or have small, thin or dying foliage.
Mechanical damage: Plants can be torn, cut, crushed, chewed, sliced or punctured from wind, animals, lawn mowers and weed whackers, the errant gardener or a myriad of other possibilities.
Nutrient deficiencies and excesses: The absence or unavailability of an essential nutrient to a plant may lead to yellowing of leaves; too much of a particular nutrient or mineral can damage plant tissue. For example, look at those tell-tale golden spots from dogs urinating on the grass.
Pesticide damage: Exposure to fungicides, insecticides and plant growth regulators, especially during temperatures above 90?F can result in leaves that are burned, discolored, distorted, spotted or drop prematurely. Herbicide damage can appear as cupped, curled, or chlorotic leaves, small leaves, or death of the entire plant.
Diagnosing plant problems, when there is no apparent pest or disease-causing agent, can be a real challenge and involves some detective work along with a bit of science. There are a few clues that help distinguish between abiotic and biotic problems—diseases caused by living things often show physical evidence (signs) of the villainous pest or pathogen where abiotic disorders do not. That mass of foamy froth on a rose bud would be a sign of spittlebugs (the little green insects that produce the foam), while dry, crispy-looking leaves on a maple tree in summer is a symptom without any obvious cause. Abiotic damage often occurs on different plant species at the same time—like frost damage to your succulents, citrus and other tender plants after freezing temperatures. Maladies resulting from an infectious agent are generally limited to an individual plant species, like fire blight on a pear tree. And abiotic damage doesn’t spread from plant to plant over time where a disease may move and infect neighboring plants of the same or related species.
There may also be multiple factors, abiotic and biotic, contributing to a plant health problem. A mature birch tree stressed by ongoing drought conditions (abiotic) may be susceptible to attack by the bronze birch borer (biotic) that will ultimately kill the tree. Or mushrooms at the base of the tree—the primary problem may have been damage to the trunk of the tree by a lawn mower (abiotic) that subsequently allowed entry of a disease-producing fungus into the tree (biotic).
Prevention is the key for most abiotic disorders. While you can’t control temperature or rainy weather, there are plenty of things you can do. Start with “right plant, right place, ” i.e., selecting plants appropriate to the planting location. Know how well your soil retains water and nutrients, and supplement with amendments to produce vibrant soil with good drainage. Provide plants with the water they need at the time they need it; check irrigation equipment regularly and maintain it in good working order. Protect plants, where possible, from temperature extremes and wind, and provide appropriate staking and support. Shield them from potential mechanical damage from lawn mowers, weed whackers and shovels, and keep tools in good shape.
About those cat-faced tomatoes? Mother Nature was responsible for the temperature swings from too-cool to really hot temperatures that likely caused them, but the rest? The gardener! Dull mower blades, thatch build up or compaction are possible culprits for the speckled lawn; a bit of herbicide drift produced the deformed rose and the lemon tree was growing in soil without enough iron.