Have you tried to pull the dead stuff to see if it easily pulls up and the roots are gone? If so, it's Take-All-Root-Rot (TARR). If so, see Neil Sperry's solution in the links below:
Most Common St. Augustine ProblemsHere is the sequence in which the problems will usually show up:
April/May: take all root rot (TARR, also known as take all patch). Grass is lethargic about greening up, may even be dead in patches. Application of 1 inch of s****num peat moss is best control. Fungus is repressed by the acidic interface. Conventional fungicides do not usually do much to help. Sun or shade.
Note: The s****num peat moss application is only effective in controlling TARR. I have so many people telling me that they have applied it in summer or fall and for things I know are not TARR it's not inexpensive. Double-check before you go to that expense.This is TARR as it appeared in April.
Battling Take-All Root RotTARR Update!