Should You Use Fungicide On Your Lawn? A Turfgrass Specialist Explains

As fall begins, many turfgrasses show signs of common diseases. “These fungi are always present, but disease occurs when the environmental conditions are ideal for its development,” says Clint Waltz, PhD, turfgrass specialist, Turfgrass and Education Center at the University of Georgia. “That means an extended period of humidity and temperatures in the 60s to 80s.”

One of the most prevalent diseases in warm season grasses such as centipede, zoysia, and St. Augustine is large patch (Rhizoctonia solani). Shorter days and cooling temperatures slow growth, which leaves the grass more susceptible to disease. Warm season grasses may not be able to recover before going into dormancy, which results in slower green-up next spring, says Waltz.

Cool season grasses such as tall fescue, bluegrass, and ryegrass may be affected by brown patch, also caused by Rhizoctonia. While these grasses may be able to grow out of disease because their active growth period begins with cooler weather in the fall, fungicide treatment may be in order because cool season grasses can be killed by disease, says Waltz.

How To Tell If Your Lawn Has A Disease

“Not all pathogens will exhibit the same symptoms on all grass species,” says Waltz. It’s best to get a positive ID from your local university coop extension service (find yours here), but you’ll typically see symptoms such as:

• Circular brownish or yellowish patches of thinning grass, ranging from a few inches to several feet in diameter
• Smaller circles may merge to make one big dead area
• Brown or grey lesions, or discolored spots, on the leaf blades
• Grass that pulls up easily by hand from inside the discolored area

When To Apply Fungicides

Fungicides are available at big box retailers in both liquid and granular forms. Most products available to homeowners will work for the majority of diseases, though you may get slightly quicker results from liquid applications, says Waltz.

How To Use Fungicide

Here’s how to use fungicides effectively and appropriately:

• Use fungicide only when necessary. For turf with a history of disease, preventive applications are advisable, says Waltz. If you’re experiencing hot and dry conditions, you don’t need fungicide. But when the long-term forecast calls for three to four days of high humidity and high temperatures (such as the approach of a tropical storm), consider applying fungicide as a preventive measure so the grass is protected before environmental conditions are favorable for disease to occur, says Waltz.
• Make repeat applications, if needed. “Applying fungicides is not a one-and-done approach,” says Waltz. After an application, the disease will abate for 21 to 28 days. But if environmental conditions are right, it can recur. Follow up with another application, according to label instructions, if conditions still are conducive for disease.
• Treat struggling grass. “If warm season turf goes into dormancy diseased, it will be slow to come out of dormancy in the spring,” says Waltz. “Basically, if it goes in sick, it comes out sick.” For affected cool season grasses, applying fungicide treats current infections, which can kill some types of turfgrasses, and prevents disease occurrence as weather conditions become favorable for infection.
• Don’t clean your mower with bleach. It’s a common question from homeowners, but it’s not useful. “Fungi spores are everywhere in the environment, so washing down your lawn equipment isn’t going to help prevent diseases from developing,” says Waltz.

How To Prevent Turf Diseases

There are no guarantees, but good cultural practices can help your grass survive disease, says Waltz.

• Don’t let the grass stay too wet for too long. Most pathogens need 12 hours of moisture to take hold. “We can’t control the temperature or the type of grass we have, but we can control when we irrigate,” says Waltz. Avoid irrigating between 4 pm and 4 am to give your grass a chance to dry out. Essentially, don’t put your grass to bed wet!  
• Mow to the proper height for your type of turfgrass. If you aren’t sure what you have or what height to mow, check with your local university coop extension service.
• Apply nitrogen at the correct rates and times. Avoid using nitrogen on warm season grasses in late summer and on cool season grasses in mid-summer.