Dwelling in your soil and upon your lawn are myriad thread-like forms of plant life called fungi. Some of them, such as Trichoderma harzianum, are important components of healthy soil. They may facilitate nutrient uptake by grasses and keep disease-causing fungi in check through competition for resources. Problems arise when the disease-causing group becomes dominant. The initiators of most lawn diseases, these fungi are spread by wind, rain, grass clippings, and even your lawn mower. They can overwinter and remain dormant in soil or thatch for long periods of time, awaiting just the right conditions before growing. Like a person, your lawn becomes a target for infection if stressed. Although you cannot keep pathogens off your grass, you can sidestep disease by manipulating the other two factors--host and environment.

To keep your lawn from becoming a host, choose the right grass for a given location and keep it healthy. Given the range of disease-resistant grasses available today, you have a good chance of avoiding certain diseases right from the start. Refer to "Choosing the Right Grass" for more information on this topic.

You might not be able to change the weather, but you can lessen its negative impact on your grass. Well-aerated lawns with good drainage and air circulation will experience fewer moisture problems. In areas prone to brown patch, some homeowners remove the morning dew (which contains nutrient sugars that contribute to brown patch formation) by lightly hosing down the lawn or pulling a hose across it. Good cultural practices do make a difference in the health of a lawn.

Ironically, often the very actions we take to improve our lawns aid in the establishment of diseases. For example, frequent, light waterings encourage shallow root growth, making the grass vulnerable to drought stress. Watering late in the day leaves a wet grass canopy that is conducive to fungal growth. Also, excessive use of high-nitrogen fertilizer promotes unnecessarily lush top growth that is more prone to disease. And using a dull mower blade shreds grass tips, providing a potential entry point for infection.