Steamed About Lawn Diseases?
By Jane Morse, UF/IFAS Extension Agent
Chinch bugs may be the best known lawn pest, but there are diseases that are pests too. Humid, sticky, muggy and wet - welcome to the dog days of summer in Florida. High temperatures and lots of rain and moisture are the perfect combination for fungus to thrive and infect our lawns.
St. Augustinegrass is susceptible to three major fungal attacks during this time of year, namely Grey Leaf Spot disease, Phythium Root Rot and Take-all Root Rot.
Gray Leaf Spot can slow growth, thin established lawns and kill large areas of the lawn during hot, humid and wet weather usually from May through September. The easiest way to spot the disease is to look for an oblong leaf blemish that has a dark edge with a center that is a gray color. Disease occurrence can be reduced by only watering in the early morning hours just before sunrise. The longer the leaf blades stay wet the more likely they are to become infected. If we are getting rain every few days then there is probably no need to apply more water.
It is important to minimize stress and avoid excessive flushes of lush rapid growth during the rainy season. Applying Atrazine to the grass is stressful, so timing is important. Once temperatures get above 85 degrees F avoid applying this herbicide. Consider spot-treating problem areas and make sure the grass is being mowed at the proper height.
Phythium Root Rot can appear any time of year, but it is always associated with wet soil conditions. Excessive watering, abundant rainfall or poor drainage conditions can all promote this disease. Since the roots are affected, the symptoms on the leaves are due to what is happening to the root system. There is a general decline in lawn quality. Small or large areas become yellow, light green, or brown in color and become thin. The lawn will seldom die from this root rot. Roots will appear thin with few root hairs and will be discolored, but not black and rotted.
To prevent the disease, improve drainage and reduce watering the lawn, especially before periods of high rainfall. Avoid watering schedules that keep the soil wet. During periods of high rainfall mow grass at the proper height and mow frequently enough to only remove one-third of the leaf blade per mowing. Apply a fertilizer that has an equal amount of nitrogen (N) and potassium (K).
Take-all Root Rot is naturally present on the roots of warm-season grasses. The trigger for disease development is high rainfall and stressed grass. The disease is normally seen during the summer and early fall months, especially during periods of prolonged rainfall. Since this is a root disease, the early symptoms can only be seen on the roots. If the grass is only slightly stressed you may never see the above ground symptoms of this disease. Irregular, yellow or light green patches ranging in diameter from a few inches to a few feet will be the first above ground symptoms. Once you do see above ground symptoms on the grass, the roots have been under attack for 2-3 weeks or longer and they will be thin and off-white in color with isolated black lesions. As the disease progresses the roots will become short, black and rotted. The grass stolons or rhizomes (runners) may have black lesions and may even begin to rot. Entire plants will die and can lead to large dead patches. This disease can easily be mistaken for chinch bug damage, so the first step in any pest management program is to identify the pest (weed, insect or disease).
Once above ground symptoms are seen this disease is very hard to control, so prevention is the key. The grass must be mowed at the correct height and mowed frequently enough so that only one-third of the leaf blade is removed each time it is mowed. Scalping is very stressful because it damages the growing point. Apply equal amounts of slow-release nitrogen and potassium. Avoid nitrate-nitrogen products. Micronutrients should be applied as a foliar (leaf) feeding and they should all be in the sulfate form. If the root system is badly damaged the roots will not be able to get their nutrients from the soil so frequent foliar feeding of all nutrients (N, P, K and micronutrients) in small amounts will be necessary (remember to follow local and county ordinances). Avoid using herbicides on St. Augustinegrass as this is a stressor. Learn how to manage the turfgrass to limit weeds so herbicide usage can be reduced or eliminated.