Should I apply fungicide to my lawn preventatively or wait for signs of disease?

Does my lawn need preventative fungicide applications?

Fungal lawn diseases can be frustrating because they seem to show up overnight. One week the lawn looks green and healthy, and the next week there are brown patches, thinning areas, or spotted blades. It is tempting to think the safest approach is to apply fungicides preventatively on a lawn with no symptoms, especially since many fungicides are much better at protecting healthy grass than curing a severe infection. However, for most home lawns, the best approach is to invest in building strong, healthy turf that is less inviting to disease. Routine preventative fungicide applications are best reserved for specific scenarios, for example when the lawn has a known recurring problem or the weather is setting up a high-risk situation. Read on to learn more about which route might be best for you.

Is your lawn fighting a mild or severe disease?

Some fungal infections can be mild and temporary, causing discoloration as a cosmetic issue without severe underlying damage. To give some examples, small patches of rusts, red thread, dollar spot (in its early stages), snow mold, and minor leaf spot issues can often improve with careful attention to lawncare practices. Proper mowing, watering, regular fertilizer, and some time for the weather to change might be all the lawn needs to recover—no fungicides required!

On the other hand, severe cases or more deeply damaging diseases will need extra support and can warrant preventative measures for repeated infections. These fungi pathogens move quickly, attack large areas of turf, and/or destroy grass roots and crowns (not just the leaves). Pythium blight, necrotic ring spot, and large/summer patch diseases can cause more serious damage. If your lawn has a history of these diseases, for example an outbreak every year in the humid summer months, a preventative program may be more effective than reacting after the fact.

If needed, how often should preventative fungicides be applied?

So your lawn has a predictable disease problem, and the humid summer months are kicking into high gear. Here are some guidelines for preventative fungicide use:

• Use products labeled for the type of grass and disease you’re concerned about.
• The best schedule depends on region, grass type, weather, and product label. Typically applications would be around every 3 weeks, only during the disease-risk window (not the entire growing season).
• Rotate! Rotation refers to using at least 2 different products rather than applying the same one multiple times in a row. Using repeated, preventative applications of one product can promote resistance and essentially train the disease to be immune to the fungicide. Instead, use 2 or even 3 fungicides with different modes of action indicated by their FRAC codes, e.g. Group 3 and Group 11 (not just different brand names, which could be the same active ingredient under different labels).
• The most important rule: read the label! Follow the label exactly for rate, watering instructions, interval, maximum applications, and safety precautions.

Will fungicide solve my lawn disease issues?

Fungicide can reduce disease pressure, but it cannot make up for poor lawn care. A lawn that’s been requiring fungicide applications year after year is a sign to look deeper. The real issue may be too much nighttime moisture, poor drainage, excessive thatch, compacted soil, mowing too short, too much nitrogen at the wrong time, the wrong grass for the site, or a variety that is especially disease-prone. Click here to read a previous ASK the Expert article from SoilKit about preventing the spread of lawn diseases.

To sum up, for an average home lawn with no history of severe disease, the best number of preventative applications is often zero. At the same time, for a predictable pattern of disease emerging again and again, prevention can be worthwhile during the infection-risk window. Using the right product, at the right time, for the right disease—without slacking on basic cultural practices—will give you the best foundation for a healthy lawn, not just this year but for seasons to come.

Want to dig deeper?

Click here to learn more about turfgrass disease control from Missouri State Extension.

Click here to learn more about turfgrass disease management from University of Florida IFAS Extension.