Aw, geez! Well, I need more information.

Where do you live? Are you in downtown SD? Are you in La Jolla (where Marathon should work fine)? Or are you in El Cajon (where Marathon should not be used in a drought)? San Diego isn't all that big but the people who claim to be living there cover a wide area of climate and soils.

Fescue is generally thought to be a 'cool season' grass. That means it's grown in the upper midwest and in the east but north of Georgia. It is more successful the farther north you get. Coastal California should be an okay place for fescue. But it is totally a luxury grass for interior Cali and to the east. I've seen it growing in Palm Desert and Indio, but what a mistake! (I've grew up in the Inland Empire but also lived in Pomona, Hawthorne, Downey, Huntington Beach, Palm Desert, Indio, Temecula, Hemet, Winchester, and Stanton. We had friends who lived in several places in the SD area, so I took some mental notes).

You're not going to like my watering advice, but it will work almost no matter where you live. First you need more info about your own system. How much water does your system put out in 10 minutes? You can test that by putting cat food or tuna cans in the yard and measuring the amount you collect. Ultimately what you want to know is how long your system takes to fill those cans. My oscillator sprinkler takes 8 full hours to fill the cans. My neighbor's high flow system fills them in 20 minutes. You can't just guess at this. Okay so the watering mantra is 'deep and infrequent.' Deep means 1 inch (1 can) of water all at one time. It's the 'infrequent' part you're not going to like. Infrequent means once per week when the temps are in the 90s (La Mesa and to the east). When the temps are in the 80s (most of the SD metropolitan area), water once every 2 weeks. When temps are in the 70s (La Jolla and within the beach influence), water once every 3 weeks. I realize you have millions of Californians telling you that you need to water every day, but that is incorrect. And when I say incorrect, I'm talking about after reading thousands of messages in lawn care forums over the past 13 years. A friend of mine in Phoenix waters his St Augustine lawn once per week with temps of 115. Surely you can water less than 2x per week in SD. But before you jump into deep and infrequent watering, let's get some answers first. So, where are you?

Also what are your specific watering restrictions? In San Antonio during normal times we're allowed to water 8 hours per day, but only once per week. During harder drought it's 7 hours per day and only once every 2 weeks. Every community is different, so what are your restrictions?

I live in La Mesa. The water restrictions limit watering to twice per week and 10 minutes maximum for spray type nozzles, which is the type that I have installed (Rain Bird HE-VAN). I was watering for 7 minutes per session for most of the summer, but I have now increased that to 10 minutes. I have previously measured the water output using the tuna can method. The output was around 2 inches per hour. Just for reference, last summer I was watering for 6-7mins every other day and the lawn looked pretty good.

I am assuming that the fescue will start to come back and fill in once the temperatures start going down in the fall. Is this a good assumption? I am also wondering what to do about the bermuda grass, which has taken over some of the dryer areas. I was thinking that I'd use Bermuda grass control from bayer, as I have used that before with good success. Thoughts?

How married are you to Marathon? La Mesa is on the wrong side of the cusp for Marathon. San Diego has
all those canyons that trap heat and block the sea breezes. If you were
west of the 805 I'd feel better about your success.

Bermuda is going to be the bane of your existence if you care about this lawn. Fescue is the type of grass that grows in clumps. It does not spread out like bermuda does. When the fescue is challenged by drought, heat, disease, or wear and tear, it will thin out and not come back without new seed. Those thin spots are where the bermuda will encroach. The best time to reseed Marathon is in the fall to avoid the spring weeds. So if you have an unusually hot June and lose some Marathon, then that gives the bermuda all summer to fill in the thin spots. It sounds like that's where you are now. If the Marathon has thinned out, the only recourse is to kill the bermuda as best you can and reseed those thin spots every fall.

What I was going to suggest was to go to St Augustine instead of Marathon. St Augustine is a coarse bladed grass that will choke out bermuda when mowed high. St Aug also regrows to fill back in if it thins out. The bermuda might refill faster, but the St Aug will eventually retake the battle field without herbicides. St Aug would be much less hassle for you.

You have to make the best of your water restrictions. With your watering system it sounds like you are making the best. When the temps cool back to the 80s (is that now?), you should be able to water once (twice a week) but only every other week.