Several things could cause the yellowing.
St. Augustine Decline (SAD) is a virus causing a chlorotic mottling or stippling of St. Augustinegrass leaves. St. Augustinegrass and centipedegrass are the only turfgrasses that the virus is known to affect. The virus is widespread in Texas and has been reported in Louisiana and Arkansas.
Description. In the early stages of infection, St. Augustinegrass leaves show a chlorotic mottling or stippling. As the mottling progresses, leaves develop a chlorotic appearance.
The first symptoms of take-all root rot usually
appear in the spring and summer as yellowing
(chlorosis) of the leaves, which gives the affected
areas of the lawn a yellow-green cast
The chinch bug damages St. Augustine grass by feeding on the stems at the base of the leaf sheath. Populations of chinch bugs may reach several hundred per square foot with damage usually apparent at 20 to 30 chinch bugs per square foot. Initial injury symptoms from chinch bugs resembles drought stress -- stunted, chlorotic spots in open (full sun) areas of the lawn. As feeding continues, irregular areas of dead grass develop in the lawn.
St. Augustine grass grows satisfactorily at a pH range from 5.0 to 8.5, but develops a chlorotic appearance in highly alkaline soils (above pH 7.5)