What are Lawn Diseases?

Lawn disease occurs only if all three causes line up—grass susceptibility, a present pathogen and the right weather. If the fungus and conditions for Summer Patch are in two yards—one in Bluegrass and one in the non-susceptible Perennial Ryegrass—only Bluegrass would have a bad summer.

The Host—Your Lawn

The type of turf grass in your yard can either be susceptible, tolerant or resistant to a lawn disease:

• Susceptible. The turf hasn’t built up an immunity, so anytime the pathogen is present and the weather is favorable, that disease can reoccur.
• Tolerant. The type of grass can be infected, but the disease will be less severe and recovery will be easier and faster.
• Resistant. The species of grass can be attacked, but it resists infections and won’t show any symptoms or signs.

The Pathogen—Lawn Fungus, Etc.

Pathogens use your lawn as fuel for survival and reproduction. They can be one of several microorganisms:

• Lawn fungus. The biggest cause of lawn disease, fungi can’t produce their own food, so they infect a host to eat. Most feed on living tissue such as leaves, crowns, stems and roots, but some fungi, like Fairy Ring, feed on decaying organic matter in the soil. Fungi enter through leaves, stems, roots, natural openings (stomates) or wounds (mowing). They’re spread by soil, rain or wind.
• Viruses. Viruses are so tiny that they live within turf grass cells. They’re transported by mowers or insects carrying contaminated sap from diseased grass to healthy grass. The only major viral lawn disease is St. Augustine Decline.
• Nematodes. These microscopic worms spread when soil or water moves, feeding on turf grass by using a piercing mouthpart to suck nutrients out of roots and stems. Nematodes are more common in warm-season grasses and sandy soils.

The Environment—Weather, Soil, Etc.

Managing lawn diseases boils down to understanding the environmental conditions that favor each disease, including any one or more of the following:

• Air temperature. Lawn fungus diseases require certain temperatures to activate—mostly, the nighttime lows have to rise above a minimum temperature. For instance, Dollar Spot activates only when nighttime temperatures are above 68°F for several consecutive days.
• Soil moisture amounts. This affects the humidity of your lawn’s surface and can either contribute to or prevent lawn disease development.
• Relative humidity. Free water or high humidity is required for infection to begin in most leaf diseases caused by lawn fungi.
• Dew on leaves. Dew accumulation amounts are dependent on temperature and relative humidity—how long it sits on the grass is a critical factor for foliar disease development. One key to lawn disease management is reducing dew drying times.

Because you can’t control the weather—or the type of grass you have—control your lawn’s environment through proper mowing and watering techniques to keep lawn disease at bay. The goal is to increase your lawn’s health and strength so you can get back to relaxing outside—lawn fungus-free.

Cultural Control for Lawn Diseases

While lawn fungus is a relatively minor problem, dead grass on your lawn is not. Every action you take has an impact on your lawn’s health, from fertilizing to choosing a disease-tolerant or disease-resistant species of turf grass. For cool-season grasses, plant mixtures of different species. For warm-season grasses, mixing species isn’t possible, so picking the right turf grass for your region is critical.

Another good example of cultural control can be found in lawn diseases requiring excess water to develop. Reduce free water by decreasing the amount and frequency of watering, improving drainage and air movement and reducing thatch and shade so excess water and dew are both absorbed by the soil and evaporated to the air quicker.

Lawn Mowing Tips to Reduce Lawn Fungus

Even when done properly, mowing stresses a lawn. When done improperly, it’s the most potentially damaging aspect of lawn maintenance. A lawn’s tolerance for disease is lowered—or even eliminated—by low-cutting or scalping your lawn. To help prevent lawn diseases like Brown Spot and Red Thread:

• Mow at the correct height and frequency with sharp blades
• Don’t mow when your grass is wet or under drought stress
• Always return grass clippings to the lawn (grasscycle)

Water Management Tips to Control Lawn Diseases

The two main types of lawn diseases are foliar and root. Foliar diseases attack grass leaves, and root diseases attack root systems—some diseases attack both. To reduce root diseases:

• Don’t allow your turf grass to become stressed with too much moisture
• Water thoroughly and deeply—but not too frequently—to encourage deep rooting
• Use a turf syringe during the day to cool the root zone in severe cases

Foliar diseases need extreme humidity and/or free water on the leaf to begin infections. Dew—and how long it stays on the leaf—is a critical factor. Dew forms based on temperature, air movement and humidity, so decrease shade to speed up your lawn’s morning drying time.

Yards with large trees on the eastern side of the property cause the western lawn to dry out more slowly, making them more susceptible to foliar lawn diseases. To reduce foliar diseases, water early in the morning, avoid overwatering and reduce shade by pruning tree limbs.