Just like clockwork this time of year, diseases show up in some lawns, particularly those that are stressed.
There are numerous turf diseases, but two main ones cause problems: take-all patch (Gaeumannomyces graminis) and large patch (Rhizoctonia solani). These diseases can look quite similar, so it is very hard to tell the difference. Some people think that if they have a disease, no matter which one, they can put a fungicide on it and presto, the disease is gone and the grass spreads back in. But fungicides will not do any good if you treat at the wrong time, especially for take-all patch.
With both diseases, you get round to irregular patches of dead grass from 6 inches to 3 feet in diameter. As the disease progresses, patches can come together, eventually killing large areas of the lawn. With large patch disease this time of year, the grass almost gets a bronze halo in the infected areas.
The best way to distinguish between take-all and large patch is by pulling on a yellow or brown blade of grass. In most cases, the blade infected with large patch will give some resistance, whereas the turf infected with take-all patch will easily pull from the ground. Also, the stolon infected with large patch will not be brown/black as with take-all patch.
Take-all patch causes affected stolons to be easily pulled from the roots because of the rot infection that takes place. The roots of the grass will be blackened, shortened and rotted.
Good lawn management is the best approach to prevent both diseases. Take-all patch problems are closely related to soil pH. Keep the pH level below 6.5. Fertilizers and natural rainfall are acidifiers. Manganese deficiency increases the severity of take-all so supplements can be added. A premium fertilizer with micronutrients contains manganese, but it is too late to fertilize this fall.
To help prevent both diseases, maintain good surface drainage on the lawn. Core aerate the lawn to allow better drainage and movement of air and water to the roots. This is especially needed on turfgrass growing in heavy clay soil.
Water your grass only when it needs it and avoid frequent, light irrigation to reduce the humidity. The less often you wet the grass the better. Water deeply, running the sprinkler sometime between midnight and before 10 a.m. You should already be watering during this time if you are properly following the water restrictions.
Follow the recommended fertilization practices next spring, particularly nitrogen. A soil test will let you know the fertility levels and give nitrogen recommendations. Excessive nitrogen can make large patch worse, which is one of the main reasons for not fertilizing lawns past Sept. 15. Conversely, fertilizing too early next spring, before April 15, can also make the grass more susceptible to disease or make it worse.
Be careful when applying herbicides to damaged areas of the lawn. St. Augustine does not have a high tolerance for herbicides. This is especially true when the grass might have been damaged by chinch bugs.
Don’t let your grass build up a big thatch layer. It should be no more than 1 inch thick. A thick thatch layer is a haven for diseases to develop.
With large patch, it helps to increase the air circulation around the lawn. Shrub and tree barriers contribute to shade and lack of air circulation. Some pruning of limbs and foliage can help.
Fungicide applications can be helpful. With large patch, you can apply them anytime you see the fungus. But this is not true with take-all patch. Research has shown that fungicides do little good during the heat of the summer. The best time to apply them for take-all patch is during the last week of September or early October. A good signal is when the night temperature drops to 55 degrees. Then reapply 28 days later. Watch it next spring after green-up and apply again if needed.
One of the best fungicides for these two diseases that are readily available in most garden centers are ones with myclobutanil (Immunox, Fertilome F-Stop, Green Light Fungaway). Two others for large patch is thiophanate methyl (Scotts Lawn Fungus Control) and maneb (Hi-Yield Maneb Lawn & Garden).
During this time of year after a fungicide treatment, you won’t see the grass spread back into damaged areas because there is basically no growing season left. Your goal is to just stop the disease where it is and not have it get worse.