Fungi as Opportunists
Lawns are essentially a crop, a monoculture. The grass is the desirable plant, and with reasonable care it can be expected to crowd out all the weedy plants, keeping them controlled. It’s the same with lawn diseases. While some bacteria and viruses do attack turf grasses, most lawn diseases are caused by fungi. Fungal spores normally reside on grass blade surfaces, the thatch layer and the topsoil and participate in the complex lawn ecosystem. Those that cause disease are usually kept in balance by other benign microorganisms. However, this balance may be upset when the lawn is under stress.
Stress makes turf grasses vulnerable to lawn diseases. While it may be caused by cutting grass too short, or failing to fertilize, often it comes from environmental conditions such as unusually wet or dry weather which trigger fungal growth. There is no way to control the weather, however, many disease problems can be curbed with simple preventive cultural practices. In many cases the best tactic is to do nothing; when the weather changes the disease will just go away.
Life Cycle of a Lawn Fungus
Most fungi reproduce by forming millions of microscopic, powdery spores. These are spread by wind, rain splash, insects, contaminated soil--and by people walking around on the grass in wet weather. When the spores land on plants and moisture levels and other conditions are favorable, the spores germinate and send out tiny, branched threads called hyphae. The hyphae penetrate plant tissues and spread internally by digesting their way through the plant’s cell walls.
Eventually the hyphae grow into a tangled mass called a mycelium, which is the actual tissue of the fungus. Depending on the species of fungus, this mycelium may appear as a fuzzy coating on grass blades (as in mildews), or as a patch of “mold,” or, most obviously, as a mushroom. Under the right conditions the mycelium develops new spores which are shed freely to complete the life cycle.