Stress Encourages Fungal Disease
Fungal disease spores are everywhere in your yard, but are normally in balance with the other organisms there. When they explode into a full-blown disease condition on a plant, do not panic, just observe. It may just be weird weather conditions and it is unlikely to reappear next year. If it appears several years in a row this is often an indication of some underlying environmental or cultural problem. Typical conditions that trigger fungal disease are compacted soil, a change in available light or shade because a tree has been removed, or even the application of a powerful pesticide. Young or newly planted plants are particularly at risk.
These conditions stress plants, weakening their natural defenses. They become more attractive to pest insects, which have an uncanny ability to zero in on plants that are struggling. The effective response to a landscape disease problem is twofold--address the immediate problem to control the obvious damage, then determine the cause of the plant stress that encouraged the disease in the first place.
Some Causes of Plant Stress
Ultimately, your most important remedial action is to find out why the plant became diseased in the first place. Remember that germs, bugs, and raccoons are not the only threats to plant health. One researcher estimates that about 85% of plant problems are caused not by diseases, but by adverse cultural conditions that increase plant stress and with it the risk of disease. Here are some common stress factors to consider:
Soil that is too wet or too dry.
Soil that is deficient in one or more nutrients.
Overly acid or alkaline soil for the plant’s needs.
Overdose of nitrogen-rich fertilizer.
Too much or too little light.
Herbicide or pesticide damage.
Crowding that limits air circulation around plant foliage.
Plant is inherently prone to fungal infection.
Plant is not suited to your climate zone.
How Plants Fight Back
Over the last several years researchers have discovered that some plants, have evolved sophisticated defenses against attacks by fungi and bacteria. They seem to have immune systems that work a lot like that of animals and humans. Their responses to attacks by disease organisms include:
A plant sacrifices some of its cells, allowing them to self-destruct and immerse invading spores and bacteria with acidic cell contents.
Production of antibiotic substances.
Formation of “callus” tissue to entrap and wall off invading organisms.
Production of digestive enzymes that destroy invading fungi.
The chemical signals left by one defensive reaction seem to stimulate other parts of a plant’s defenses, thereby initiating the next step in the plant’s counterattack. Most astonishing of all, it appears that the plant’s immune system has a “memory”—it can recognize when it is being attacked by the same sort of organism it fought off before, and can therefore resist it and any close relatives. Researchers are studying plant genetic material (DNA) to isolate specific genes for disease resistance. On another front, researchers are using tools from biochemistry, molecular biology and medicine to explore why some plant fungi can destroy one type of plant but entirely fail to disturb other plants.