Why are there red patches in my lawn?

As temperatures rise and rain falls, lawn diseases become more aggressive and noticeable. One of the more common diseases that we see in home lawns in late spring is called Red Thread.

This lawn disease appears as patches of straw-colored, blighted turf that can be as small as a few inches across to many feet across, depending upon the severity of the infection. Closer inspection will reveal that the tips of the individual blades of grass appear ragged, almost torn. Threadlike is a better way of putting it. If the disease is observed early in the morning when dew is present, or at other times when ample moisture is available, the infection site may appear red; a disease that has symptoms that look like threads that can also appear red in certain situations – oh, those clever plant pathologists and their wacky disease names!

Red Thread is known as a disease of slow-growing turfgrass. The reasons for the slow growth are many, including lack of sufficient nitrogen and poor environmental conditions for growth. Indeed, we notice that when the sun goes in and the rain comes on, an outbreak of Red Thread is likely to follow, as it is necessary for the plant to remain wet for an extended period in order for infection to occur.

All three of the major turfgrass species that we grow in the north for home lawns, bluegrass, ryegrass, and fescue, are susceptible to Red Thread. Moreover, lawns planted around new houses are especially susceptible, likely due to the fact that since the grass has not grown in the soil surrounding the house it does not contain antagonists, organisms that would feed upon the Red Thread fungi lessening its severity. As time goes by and the amount of organic matter in the new lawn increases, the severity of the disease decreases.

Fortunately, Red Thread is, in almost all circumstances, a cosmetic disease. Once the environmental factors necessary for its infection change, the turfgrass can easily recover without intervention. It is a rare case where the lawn is so overrun with the disease than we have to resort to applying a fungicide.

What can you do to lessen the severity of Red Thread?

We can’t do anything about the environment – You can, however, make sure that when you irrigate the lawn that you do not leave the turfgrass canopy wetter than it would otherwise be if you did not irrigate. For instance, if the morning dew naturally dries by 9:00 AM, then your irrigation should not cause the lawn to be wet later than that. Irrigation should not commence in the evening until the time when dew would naturally form and should conclude in time for the lawn to dry as it naturally would anyway. Having your irrigation system finish by 6:00 or 7:00 in the morning, all things being equal, should be fine.

You may have heard that Red Thread is a disease that travels on lawnmowers from place-to-place. While it is true that if your neighbor has Red Thread and the landscaper stops there to cut before he comes to your house he will bring inoculums to your lawn, Mother Nature has already beaten him to it. The threadlike strands that you see growing out of the tips of the turfgrass plant will eventually dry and are blown around in the wind.

We really don’t want to apply a fungicide, what can we expect?

In this case, we are patient, we cause the turfgrass plant to grow at a rate greater than the rate of infection. Essentially, we push the infection up past the height of cut and remove it with the mower. Sometimes this is quite easy, we can put a little fertilizer on the lawn, the lawn will grow a little faster and problem solved. Other times, we get into one of those miserable weather patterns when all it does is rain. We forget what the sun looks like. It gets cold. The lawn never dries. We get terrible infections of Red Thread. That’s where the patience comes in. The benefit of having rainfall is always greater than it not having fallen, but it requires that the sun come out to realize the benefit. Usually, just a day or so of sunshine is all that is required to get the lawn growing again to push out the infection.