Fungi are the most common causes of plant diseases, according to the University of Missouri Extension. Fungi cause the rusts, molds, mildews, blights and leaf spot diseases that affect trees and shrubs. At best, these diseases are unsightly. At worst, they can kill your tree or bush, sometimes in only a few weeks.
Botanists generally discuss fungal diseases in terms of the fungus that causes the disease and the species of plant that it affects. Nearly all trees and shrubs are susceptible to one or more fungal diseases. According to the Ohio State University Extension, there are more than 8,000 pathogenic species of fungi known to affect plants.
The University of Missouri Extension, therefore, discusses fungal diseases in terms of the damage they cause. Some fungal diseases form spots on the leaves. Others cause the stems or base of the plant to rot. Cankers occur on the bark of a shrub or tree and appear as sunken or swollen lesions. Vascular wilts, such as the devastating Dutch elm disease, infect the tissue that carries water throughout the plant. Blights cause death to parts or all of the plant.
Before you identify a possible problem with your tree or shrub, be sure you know the precise identify of the plant, since many fungal diseases are limited to one or a few species. Learn what is normal for the tree or shrub and what diseases commonly affect it. Observe the plant closely for signs of the disease and match symptoms against typical diseases for your tree or shrub. If you can't make an accurate diagnosis, extension offices and nurseries can often offer assistance or put you in touch with a diagnostic laboratory.
Fungicides can treat some fungal diseases or prevent the disease from infecting nearby susceptible plants. Always use fungicides labeled for the plant you are treating, and follow all instructions and precautions on the label.
You can often prevent a fungal disease more easily than you can treat it. Good cultural practices promotes plant health. For example, do not overwater, as fungi prefer a moist environment. Extension specialist Cheryl A. Smith recommends watering in the morning and spacing your trees and shrubs so air circulation can dry the leaves. Clean up old plant debris such as fallen leaves or branches, which may harbor fungal spores that can infect healthy plants.