Q: My son has a neighborhood lawn mowing service. Could he be spreading the brown patch fungus to our lawn on his mower blade? Should we be wiping the blade with alcohol each night?

A: Your question makes good sense to a logical brain but it turns out to have a less-than-obvious answer. My friend Taft Eaker runs the Plant Disease Lab for the University of Georgia Extension Service. When I asked him if he thought mower disinfection was a good idea, he drew me a picture of a three-legged stool.

“The seat of the stool is the disease, brown patch,” he began. “The seat has to have three legs to support it. One leg is the host plant, the lawn grass. Another leg is the fungus spores; they’re present all of the time in the soil. The third leg is the environment around the lawn. If any of the three legs is not present, the stool falls and there is no disease.

“Two legs: the grass and the fungus, are permanently present. They can’t be changed. The third leg, the environment, can be changed in order to keep brown patch at bay in a given lawn. One good thing to do is to water deeply only once per week, in the morning. Another is to stop fertilizing fescue in May each year. The third, but most expensive, way to change the environment is to spray a fungicide on the grass so the brown patch spores can’t infect it.”

To sum it up, your son’s mower might be able to spread fungus in a lawn that had a favorable brown patch environment – but have no effect on the lawn next door that has a different environment.

You don’t have to disinfect the mower. I would advise your son to take a three-legged stool with him to explain these concepts to his customers.