Plants get sick. They succumb to infections. Infectious plant diseases are caused by pathogens, including bacteria, viruses, nematodes and fungi. The king of plant pathogens is the fungus, accounting for over 80 percent of all infectious plant diseases.
Some fungi are very familiar and helpful. The yeast we put in bread dough, the truffles and mushrooms we eat and the blue mold that gives us penicillin are all fungi. Fungi play important roles in breaking down organic matter. Lacking chlorophyll, most fungi get their nutrients from decaying matter. However, others attack living plants. The results can be devastating.
In the early part of the 20th century, the streets of many European and American cities were lined with beautiful elm trees. New Haven, Connecticut was known as Elm City. Huge elm forests were national treasures. Then the elms started to die. In 1921 Dutch scientists identified the disease agent as a fungus, and ever since it has been known as Dutch elm disease. Of the roughly 80 million elm trees in the U.S. in 1930, more than 80 percent had been wiped out by 1990.
Many familiar plant diseases gardeners contend with are caused by fungi. These include late blight, club root, sooty mold, anthracnose, scab, rust, botrytis, bulb rot and powdery mildew. Some pathogenic fungi are host specific. Others will attack a variety of plants. Fungi produce spores that are carried by wind, water, soil or animals to plant surfaces. If environmental conditions are right, those spores will germinate and infect the plant.
Typical early symptoms of fungal infection include leaf curl, leaf or stem rust, white or grayish powdery mold, leaf spots, and chlorosis (yellowing of leaves). Fungal infections typically develop more slowly than bacterial or virus infections. However, this does not mean that you can ignore early stages of a fungal infection. The more established it becomes, the more difficult it is to control.
Control starts with prevention. Good garden practices dramatically reduce the likelihood of fungal infections. Ensuring good air circulation is critical. Overcrowding plants invites fungal infections. Thin out your garden vegetables and flower beds. Keep annuals and perennials away from hedges, fences and house walls. As your trees and other plants build foliage in late spring, keep that foliage from becoming dense by pruning judiciously. As you prune, sterilize your garden tools to avoid transporting fungal spores from one plant to another. Wiping a tool with a cloth dipped in alcohol is sufficient.
Damping off, a fungal infection which kills many seedlings, is caused by soil borne fungi. If you are growing seedlings indoors for later transplant, use sterile potting soil or pasteurize your own soil in the oven. Rotate crops in vegetable beds to prevent re-infecting new plants with spores that can live in the soil for a year or more.
Fungal infections often get their start through wet leaves. Avoid overhead sprinklers by using drip lines, or use your overhead sprinklers early in the morning so leaves dry completely before evening moisture sets in. When using a hose, avoid splashing mud onto leaves.
Some organic fungicides (such as those using extracts of neem oil) are both safe and effective at preventing many fungal infections. Established infections may require a stronger fungicide. Before using any fungicide, make sure your diagnosis is correct. If you aren’t sure, contact the County Extension Office for help with diagnosis. Sulfur and copper based fungicides have been shown to be effective in dealing with many established infections. It is imperative, however, that the fungicide used is specifically recommended for the host plant and the pathogenic fungus you are trying to control.
When using a fungicide, follow label instructions carefully. When not in use, keep fungicide containers in a secure place away from children. Pay attention to special warnings on the fungicide container. Some fungicides should not be used if the temperature is expected to be over 85 degrees. Some are toxic to pets. Some are toxic to pollinators and should not be used when pollinators are active. If spraying fruit trees or vegetables, check to see how close to harvest date you can safely apply the fungicide. Wear goggles and long sleeve pants and shirt when using. Use a mask to avoid breathing fungicidal mist or dust. Spray when wind is minimal (usually early morning). Spray to cover all surfaces of the plant but avoid excessive runoff.
One final point. Like other organisms, fungi can mutate. Misuse or overuse of fungicides can lead to fungicide-resistant strains that will be even more difficult to control. Your best strategy is to anticipate and avoid fungal problems through sound garden practices.