The other option for you is to spray glyphosate product such as Gly Pho-Sel Pro or Ranger Pro on the bermuda when it is green in early August, wait three weeks to see if any bermuda survives and spray again by early September if needed and then re-seed the spots with fescue in mid-September.
Bermuda would be considered a grassy weed, so you need a product that is geared towards grassy weeds and not broadleaf weeds. We want to be clear that bermuda can be suppressed, but it is VERY difficult to kill completely. It will take multiple treatments to finally kill it. But Fusilade II is usually the go to product for this issue.
This is directly off of the product label:
Apply 0.07-0.14 oz/1000 sq. ft. (3-6 oz./A) along with 0.25% v/v (1/2 pt./25 gals.) of a nonionic surfactant. Application should be made every 28 days when the grass weeds are actively growing. The higher rates may result in temporary discolorization of the desirable turf with recovery in 10-14 days. Do not apply to desirable turf which is under stress. For best results, make applications in spring and fall and avoid treatments during July and August.
Complete control of undesirable grass may take 1-2 growing seasons.
We also recommend calling your local county extension office as they usually know the optimum times for spraying lawns in your area.
The Fusilade II instruction label does not list St. Augustine grass, so we are not sure of the answer to your question. We would like you to direct your question to Syngenta, the manufacturer of Fusilade, as they are the true experts on their own product and can better answer your question. You can reach them at 1-866-SYNGENTA.
Updated Jan 2025:Fusilade is not labeled to be used on St Augustine for bermuda grass control when tank mixed with Recognition Herbicide. Following these instructions Recognition Herbicide 2(ee) Supplemental Label-St. Augustine
This is directly off of the product label:
Apply 0.07-0.14 oz/1000 sq. ft. (3-6 oz./A) along with 0.25% v/v (1/2 pt./25 gals.) of a nonionic surfactant. Application should be made every 28 days when the grass weeds are actively growing. The higher rates may result in temporary discolorization of the desirable turf with recovery in 10-14 days. Do not apply to desirable turf which is under stress. For best results, make applications in spring and fall and avoid treatments during July and August.
Complete control of undesirable grass may take 1-2 growing seasons.
We also recommend calling your local county extension office as they usually know the optimum times for spraying lawns in your area.