Fungal disease of some kind. How often do you water and for how long? How much rain have you gotten? Are you down in a low area or has that area been covered with brush or something? It looks like it was smothered or otherwise protected from sun and wind for a time. I take an organic approach to lawn fungal diseases. I use ordinary corn meal at a rate of 20 pounds per 1,000 square feet. Corn meal attracts another fungus which is predatory on the fungus causing your disease. That fungus is called trichoderma (try ko DER mah). It takes 3 full weeks before you see any improvement. That can be the longest 3 weeks of your life, but after that you should see less yellow and much more deep green. Corn meal is also an organic fertilizer so the dark green color will be very noticeable at the 3 week point. If you have used a chemical fungicide recently, then the corn meal will not work against the disease but it will definitely green up the grass. And yes this is the same corn meal you cook with. You can get it in 50-pound bags at the feed store. And no, it will not attract varmints. Seems like it would but it does not. It does seem to attract birds but they don't eat it. They just come and visit. You can apply corn meal any day of the year, rain or shine, or every day of the year, without fear of hurting anything. If you wan to take a chemical approach, I cannot help you. Our temperatures, humidity, and rainfall are usually outside the recommendations on most chemical fungicides. I gave up on them years ago when I found out how well corn meal works. On another issue you did not ask about: You are mowing your St Augustine much too low. Raise your mower all the way to the highest setting. It will take a month or so before all the grass gets tall enough to be mowed but it eventually will. Mowing it low favors the bermuda which is mixed in with it. Mowing it high can have the effect of choking out the bermuda. Tall St Augustine looks very lush.

Thanks for the input so far. A few other pieces of info. 1) I increased the amount of water (sometimes sequential days, sometimes longer duration) starting about 3 weeks ago when I first noticed the problem (thinking it was not ENOUGH water that was the problem - I'm sure that's a common response). 2) On the edges of the lawn where there is treated lumber edging I've been dowsing the lawn w/ a hose since that seemed to be more yellow a few weeks ago and/or I thought perhaps my Melnor fan sprinkler wasn't hitting the edges enough. Those edges seem very lush and green. 3) On some spots near the Yucca palm I have where I drove in palm food spikes, the lawn is also noticeably greener (circles 8"-12" right where the spikes went in). So I'll experiment a little w/ the watering volume & frequency (less of each, though maybe in a small patch I'll do more.) Will keep apprised.

If it isn't mushy I wouldn't worry about it by the looks of things the stalk is just starting to harden and turn woody.. same as a young tree would as it gets older. The top of the cactus is a compleatly different cactus to the base that it is on ... if you are worried about the life of the yellow part of the cactus remove it and graft it to a desert cactus instead. It will help it live longer and gives it much better living conditions. The green part of your cactus is a compleatly different cactus all together and is actually a tropical cactus with different watering needs. It's like attaching a cat to a dog and feeding it fish food... neither will survive long unless you seperate them and give them the individual care that they need. both parts look compleatly healthy so I wouldn't panic, the stem is just coming into adulthood :)

Here are some extra photos of the less-affected plants.