What can cause yellowing St. Augustine lawns? My own lawn suffers every year from yellowing that can be caused by excessive growth and loss of nutrients from heavy summer rainfall. Every year at the beginning of September I watch as my lawn turns an undesirable yellow color. The soil here in Old Marco is basically shell. Digging a garden requires a pick ax and lots of brawn. This type of soil does not hold its nutrients and needs its fall fertilizer sooner than a lawn with more desirable soil. Another great example of how fertilizer black out dates can be harmful to some plants, but I digress.
Also with the summer rains comes the disease Take-all Patch. Lawns on Marco Island are being attacked by this disease that causes them to slowly die. After treatments with the usual fungicides from spray companies failed to stop the slow death of many lawns I sent samples of three lawns to the University of Florida. All three came back with the same diagnosis, Take-all patch (Gaeumannomyces graminis var. graminis). Take-all patch looks very similar to brown patch but does not respond to the same treatment and so here are the details of this stubborn lawn disease.
During extended drought periods a lot of lawns can develop dry, brown patches which are the result of improper coverage by irrigation systems due to broken or misdirected sprinkler heads or low water pressure. These dry areas are weakened and therefore more susceptible to insect and disease problems.
Take-all patch on St. Augustine is a disease which was diagnosed in 1991 at Texas A & M University. They received many samples of St. Augustine with symptoms of a patch disease which proved not to be the common Rhizoctonia brown patch. Specialists and homeowners reported that the patches did not respond to fungicides usually effective against brown patch.
The symptoms of the disease are very similar to those of Rhizoctonia brown patch. The leaf blades become yellowish or bronze just before they become brown and necrotic. In the winter, patches appear tan to straw-colored. The spots are somewhat circular and a foot to several yards in diameter. These spots may become irregular as they merge or as the disease is spread. Weeds will begin to invade these damaged areas.
The nodes (joints on the grass runner where roots and leaves develop) may become infected and develop brownish discoloration. The roots may be dry and decayed near the node. Plants in infected areas are easily pulled or lifted from the ground. During heat or drought stress the diseased turf declines in comparison with the surrounding healthy turf and the patches become irregular.
Take-all Patch is most active when abundant moisture and cool temperatures are present. When conditions are favorable the fungus grows on the surface of roots, stolons, rhizomes, crowns and blades of the grass and then penetrates and infects the tissues. During this stage of the disease the distinct patches are formed in contrast to the healthy turf. As the weather becomes warmer and dryer in spring the weakened turf may become easily stressed and begin to decline further forming irregular patches on thinned, weakened turf. The disease begins to spread with heavy summer rains.
Nitrogen fertilizer seems to have an effect on take-all patch. Nitrogen in ammonia form seems to be less favorable for disease development than nitrogen in nitrate form. Soil-acidifying fertilizers, such as ammonia sulfate, tend to suppress the disease. For those of you who cannot fertilize in September with nitrogen local professionals have used a combination of iron, manganese, magnesium and potassium. Look for a product labeled 0-0-14 plus the iron(Fe), manganese (Mn) and magnesium (Mg) in sulfate or chelate forms.
Disease pathogens are commonly present in turf and Florida’s climate frequently favors disease development. Fortunately, using good cultural practices including mowing, irrigation and fertilizing will help protect your lawn from succumbing to disease.
Improper mowing height and frequency can weaken the grass and lower its tolerance to disease. Dull mower blades can tear rather than cut the blades. This torn tissue provides an avenue for the pathogens to infect the grass.
Prolonged periods of high humidity, rain, fog, or heavy dew mean moisture on the plant surface which is also necessary for most pathogens to infect turf. Turf entering the night period moist from irrigation or rain is more likely to become diseased. While we cannot modify nature, irrigation can be controlled to reduce the incidence of disease by assuring leaf drying before nightfall. Always irrigate early in the morning. Since the pathogen can survive on infected thatch, efforts to de-thatch and to prevent thatch accumulation may help.
The susceptibility of turf to disease is also affected by nutrition. Properly fertilized grass is more resistant to disease and recovers faster if it should become diseased. Moderate fertilization using slow release fertilizer is best. I see a lot of over spraying of chemicals as well. Mixing insecticide and herbicide in hot summer weather almost always stresses the lawn. Desperately spraying chemical cocktails trying to save lawns doesn’t usually end well.
Areas infected with Take-all Patch can be replanted with healthy grass. Surrounding areas should be treated with a preventative systemic fungicide. Fungicides which have been recommended by Dr. Phil Harmon for preventing this disease include Prostar, Heritage, Compass, granular “Q o I”, Armada and Headway.
After treatment with on of the above fungicides I have seen the decline stop and these areas begin to slowly regenerate as the stolons begin to fill in the damaged areas. This is always an option if you don’t want to re-sod. Just remember to practice proper horticultural practices to prevent the disease from taking hold again.