Gardening Q&A: Battling red thread disease in a lawn

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Q. We have a problem with red thread disease in our Penn State Mix lawn every year. It doesn’t seem to affect our neighbors’ lawns, just ours. We have a company treat the lawn in spring and summer, but we can’t seem to get ahead of this problem. Do you have any recommendations?

A. Whenever we talk about plant diseases, we use the plant disease triangle. The triangle has three legs: the susceptible host (your lawn), the pathogen (Laetisaria fuciformis, the fungus that causes red thread), and the environment (cool, wet weather). Controlling diseases requires acting on one or more of those legs.

We have had spring and early summer weather for a number of years that is very favorable for red thread to develop This common fungal disease thrives in prolonged periods of wet weather and temperatures of 65-75 degrees.While there is not much you can do about the weather, you can keep the grass mowed regularly so that it dries quickly after rain. If your lawn is shady, you might have an arborist thin out the crowns of trees and limb them up a bit to allow more sun to reach the grass. It helps to keep your lawn mower blade sharp, too. The clean cut from a sharp blade heals quickly, while the ragged blades of grass left by a dull mower blade do not heal and provide an easy entrance for disease-causing organisms.

Red thread can be indicative of low nitrogen levels in a lawn. If you have not fertilized your lawn by mid-May, an application of 1 pound of nitrogen per thousand square feet can help reduce the severity of red thread. Do not go overboard with nitrogen – more is not better, and it can open your lawn to much more serious and destructive diseases when we get into hot, humid weather.

While your neighbors’ lawns have experienced the same weather as yours, the probable difference is the type of grass in your yard. If perennial ryegrass or fine fescues are the predominate species in your lawn, they are more severely injured by red thread than Kentucky bluegrass and turf-type tall fescues.

Penn State Mix is a combination of the three most common cool-season grasses: Kentucky bluegrass, perennial ryegrass and fine fescues. The percentage of each species in the mix varies by supplier, but it has a big impact on the species that predominate in the resulting lawn. Perennial ryegrass is very aggressive and can easily dominate Kentucky bluegrass if it accounts for more than 10 percent of the seed mix. If your lawn is shady, the fine fescues will predominate eventually. You may want to overseed your existing lawn with a blend of Kentucky bluegrass varieties to reduce the severity of red thread damage. Choose those with shade tolerance such as ‘‍Glade’‍ or ‘‍Bensun’‍ if your lawn is shady.

Finally, the main way to act on the causal organism is to apply a fungicide. Fungicide applications for red thread are rarely warranted in home lawns. You do not say what treatment was applied by your lawn service, so it is difficult for me to comment. The best course of action is to focus on cultural conditions – proper mowing, allowing more sun to reach the grass, proper fertility levels and perhaps adjusting the balance of grass species in the lawn. Fungicide applications should be the very last resort. This can get expensive because applications should be repeated at intervals recommended by fungicide labels as long as environmental conditions favor disease development. Also, it is best to rotate among fungicides with different modes of action to avoid developing resistant strains of the causal organism.

Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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