I fear that some of those companies will try to scare you into work you don't need done. I see it with my neighbors every so often. Brown patch is a fungal disease that occurs on lawns that may be too wet. Here is a short article from the Library of Organic Information:
Brown Patch DiseaseQuote:
Cool-weather fungal disease of St. Augustine. Brown leaves pull loose easily from the runners. Small spots in the lawn grow into large circles or free forms that look bad and weaken th turf but rarley kill the grass. Go organic to build the beneficial microbes in the soil. Spread a thin layer (1/2") of compost or for faster results corn meal at 20 lbs./1,000 square feet. Spray the problem area with Garrett Juice plus garlic tea. Potassium bicarbonate at 1 rounded tablespoon per gallon can be added for additional control.
You don't need to do anything more than these organic treatments, especially of you are going to dig up the area to put in a garden. You may need to check your watering setup if you have an automatic system. Put it on manual and water only when needed. Water deeply less frequently.
Careful not to plant the trees too deep (check out all of the
Dirt Doctor planting and care guides for more information) and don't over water them.
We have a fellow who specializes in turf questions so you'll probably hear from him with all sorts of things you can do yourself. But the tips in the quote should be enough to remove the brown patch problem from areas where you don't plan to put a garden or plant trees.
Good luck!