Lawn fungus diseases can be very destructive. Under the right conditions, fungus diseases may reduce an attractive lawn to a patchwork of dead, dry spots in just a few days. When this happens, gardeners always wonder where such diseases come from and how they can be controlled.

It is important to understand that many of the fungi that cause diseases actually live in the soil or lawn thatch. As long as the grass is not stressed, the fungi never become a problem. The list of factors that may cause stress in turfgrasses is fairly long, and includes adverse temperatures, improper irrigation, fertilization, mowing and pesticide application, thatch buildup and soil compaction. Any one of these factors, or often a combination of several, can weaken turfgrass just enough to allow fungus diseases to gain the advantage.

All things considered, water is the most important single factor influencing lawn diseases. Water affects the growth and development of the lawn and the fungus organisms that cause disease. Most fungus diseases occur during the warm summer months, when we’re irrigating lawns, so it follows that we should be able to manage diseases by carefully managing water.

The common disease Pythium blight, caused by various species of the fungus Pythium, thrives during warm, wet conditions. It spreads rapidly by spores, which are capable of swimming. The movement of water over the grass surface can rapidly move the spores over a wide area, where they infect new plants. The Pythium fungus is most destructive in lawns that are growing in compacted or waterlogged soils.

Water that you apply in the late evening or at night often remains on the foliage for long periods, giving certain diseases maximum opportunity for infection. Water applied in the early morning usually dries rapidly, helping to prevent diseases. In order to help prevent lawn diseases, it’s important to avoid frequent, shallow irrigations.

Too little water can also stress turfgrasses and create conditions ideal for disease infection. For example, Fusarium blight is a serious fungus disease that commonly occurs in lawn areas that have been stressed for moisture and areas in full sun. The best way to prevent Fusarium blight therefore, is to keep the soil evenly moist with deep, periodic irrigations. It may be necessary to aerate your soil in order to improve deep water penetration, especially if the soil has become compacted by heavy foot traffic.

Another cultural factor important in lawn disease development is fertilization. Some diseases are more damaging when the grass is growing under high nitrogen levels, while other diseases are more destructive under low nitrogen levels. For example, soft, lush growth due to excess nitrogen favors Fusarium blight and Rhizoctonia blight. On the other hand, fertilizing with nitrogen is a good way to outgrow rust, a common springtime lawn disease. In general, applying just enough nitrogen to keep your lawn growing moderately works well for cool season grasses such as tall fescue, Kentucky bluegrass and perennial ryegrass. In most cases it’s wise to avoid fertilizing with soluble nitrogen fertilizers during the hot summer months.

There are a number of chemical fungicides that you may apply to help stop fungus diseases from spreading rapidly. While fungicides are very useful, it’s best to use them in combination with improved cultural conditions.