Q: I have small to large areas of my St. Augustine lawn which have yellowing blades. They can easily be pulled up and seem to be rotting. Some of the edges look like circles. The smaller areas are about a foot across while other areas are much larger – almost 10 feet wide. What could be the cause?
A: I believe you most likely have a fungal growth called large patch or brown patch, which is commonly found in St. Augustinegrass. The fungal agent is called Rhizoctonia solani. This disease is most likely to be observed from November through May when temperatures are below 80°F. It is normally not observed in the summer. Infection is triggered by rainfall, excessive irrigation, or extended periods of high humidity resulting in the leaves being continuously wet for 48 hours or more.
This disease usually begins as small patches (about 1 ft. in diameter) that turn yellow and then reddish brown, brown, or straw colored as the leaves start to die. Patches can expand to several feet in diameter. It is not uncommon to see rings of yellow or brown turf with apparently healthy turf in the center. Turf at the outer margin of a patch may appear dark and wilted.
Avoid excess nitrogen during potential disease development periods. Limit readily available forms of nitrogen, such as soluble liquids or quick-release nitrogen sources, just prior to or during these periods. Instead, use slow-release nitrogen sources. Apply a balanced fertilizer containing equivalent amounts of potassium and nitrogen, preferably a slow-release potassium form. Irrigate only when necessary and do so only in the early morning hours when dew is already present.
Since mowers can spread this disease, mow diseased areas last, and wash turf clippings off the mower before proceeding to the next site. Chemical controls are listed in the publication attached, be sure to follow the directions on the label and alternate the type of chemicals used. But nothing will change if the watering and fertilizing practices are not corrected. http://edis.ifas.ufl.edu/lh044