DALLAS – All over the South, St. Augustine grass lawns are turning yellow and thinning. While the problem has been occurring for several years, it seems to be worse this year.
“I believe the problem or problems we are seeing in the St. Augustine grass lawns is being caused by a combination of factors,” said Dr. Jim McAfee, Texas Cooperative Extension turfgrass specialist at Texas A&M Dallas. “These include environmental stress, disease activity and, in some cases, excess levels of phosphorus.”
According to McAfee, for the last several years, summers have been above average in temperature and below average in rainfall. While this combination places a huge amount of stress on all the turfgrasses used in home lawns, St. Augustine grass has suffered the most. And stressed summer lawns are more subject to fall disease problems and to winter freezes.
“The winter of 2001-2002 was very dry until late spring,” he said. “This, coupled with some freezing temperatures in late February and early March, added additional stress to the lawns.”
“These environmental conditions have resulted in loss of turfgrass in our lawns, particularly in St. Augustine grass lawns. Also, these conditions have made the lawns more susceptible to diseases such as Take-All Root Rot, Brown patch and Gray Leaf Spot.”
According to Texas A&M Dallas plant pathologists, the main disease affecting St. Augustine grass is Take-All Root Rot. This disease is active in the fall and spring when the soil temperatures are in the 60- to 65-degree range. At this time of the year, the fungi attacks the plant’s root system, weakening it as it goes into the winter and summer stress periods. This disease appears to be particularly active on plants that have been become stressed for one reason or the other.
“A lot of the yellow discoloration in St. Augustine grass lawns is due to the early stages of Take-All Root Rot activity,” said Dr. Phil Colbaugh, Experiment Station plant pathologist at Texas A&M Dallas. “Roots of these affected plants are usually short, blackened and rotted, and the stolons (runners) can easily be lifted from the soil due to the poor root system. As the disease progresses, the yellow leaf blades will eventually turn brown.”
Colbaugh said Take-All Root Rot can often be mistaken for Brown patch. These two diseases can be distinguished by pulling on the leaf blades of the yellow to brown leaves. Leaves of Take-All Root Rot plants are still firmly attached to the stolons, while leaves with Brown patch can easily be pulled away from the stolons. Also, Brown patch rarely causes the roots to turn black.
“The best time to treat for Take-All Root Rot is in the fall and spring when the soil temperatures are in the 60- to 65-degree temperature range,” said Colbaugh. “While control has been difficult, fungicides such as Heritage, Rubigan and BannerMaxx have shown some control for this particular problem.”
Colbaugh has been researching the use of a composted cow manure product on this disease with some success. For more information on this work, check the Texas A&M Dallas Web site at http://dallas.tamu.edu/ and then click under faculty and then under Dr. Phil Colbaugh.
Other disease problems that appear to be enhanced by stressed conditions include Brown patch and Gray Leaf Spot. The experts believe Brown patch will be a problem in the fall, especially on St. Augustine grass and Centipede grass. St. Augustine grass lawns already stressed from summer conditions are going to be more susceptible to Brown patch activity in the fall, they said.
“Proper irrigation during the summer months and particularly fall months will aid in preventing Brown patch activity,” said McAfee. “Also, avoid excess applications of nitrogen fertilizers during the summer months and especially in the fall months when night temperatures drop below 70.”
Plants that have been damaged by Brown patch in the fall are more susceptible to injury from desiccation and/or freezing temperatures.
“Gray Leaf Spot is most active in late spring to early summer months and again in the early fall,” said McAfee. “Unlike Brown patch, Gray Leaf Spot can cause loss of St. Augustine grass lawns. Again, St. Augustine grass lawns that are under stress are going to be more susceptible to Gray Leaf Spot activity. The key to controlling this disease is to avoid nitrogen applications when the disease becomes active in the lawn.”
According to McAfee, while environmental stresses and some diseases appear to be the main reason for the yellow, thin St. Augustine grass lawns, excess soil levels of phosphorus can also contribute to yellow patches in lawns. In alkaline or acid soils with high levels of phosphorus, the phosphorus can react with micronutrients, such as iron, zinc and manganese, to form an insoluble compound that prevents the plant’s root system from utilizing them.
For home gardeners who want to know what to do for their yellow, thin lawns, McAfee’s first recommendation is patience.
“It is still too early to determine how much of the grass is going to recover and fill in the lawn. Next, avoid excess applications of nitrogen fertilizer. In these damaged lawns, the plants have very little root system, and applying too much nitrogen fertilizer will force top growth at the expense of the root system. This will only make the situation worse.”
“Also, avoid application of herbicide to these stressed areas of the lawn. St. Augustine grass does not have a high tolerance to herbicides, and adding a herbicide to these already stressed lawns will only make the situation worse. You may want to try the composted cow manure product on these affected areas to aid in recovery. The recommended rate for this product is 200 to 250 pounds per 1,000 square foot.”
To lessen these problems in the future, McAfee said, use best management practices for St. Augustine grass lawns, including proper fertilization, proper mowing height and frequency and proper irrigation. And use herbicides with caution, especially on St. Augustine grass growing in moderate to heavily shaded areas of the lawn.
St. Augustine grass growing in full sun areas only needs three to four pounds of actual nitrogen per 1,000 square feet per year for good growth and development, he advised, while grass in shaded areas only needs actual nitrogen at the rate of one to two pounds. Grass in full sun should be mowed at two to two-and-a-half inches while grass in shade should be mowed at three to three-and-a-half inches.
When watering, Mcafee said, generally one inch of water per week during the spring and fall is sufficient, while one and a half to one and three-quarters inches of water per week is required in the hot, dry summer months.
Remember, if adequate rainfall occurs during the week it is not necessary to irrigate the lawn.
Also, water the lawn during the winter even though the lawn is dormant. Generally, one inch of water every four to five weeks during the winter is sufficient.