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.

really hard to tell for sure but typically fungus in my area is reddish/yellow. You may have multiple problems. The problem with newbies is that they want to solve problems with chemicals (I was one of the worst offenders). The worst time to add fertilizer is when a grass is stressed and excessive nitrogen will show up as patterns matching the application method. If you just applied the nitrogen within the last few days you can reduce the effect by watering heavily for one day but if you applied it more than a few days ago you are probably past the danger period. Again, check the soil moisture at about 2-6" deep at several areas in your lawn each week. It should be slightly wet the day of watering and then let it go to slightly damp before watering again. Wet all the time just rots the roots and causes yellowing due to lack of nutrients. With most soils a twice weekly 1" watering in the summer is all you need. When the grass comes back on it's own (and I'm almost certain it will) then evaluate the soil like I said and take any corrective action required. Guessing and throwing chemicals at it will probably make things worse.

Are you cutting at the same height as the old mower, or trying to go lower? Any other differences between the old mower & new? Maybe the old one had a higher blade count or something? I'd check the Fiskars to make sure everything is aligned properly, but it's only 5 blades, so some tufting, pulling & ridges are expected. I'd also like a close up of the bad spots when you get a chance. Did you buy the bagger for the Fiskars?

I might well move the edging on the house side to make a 4'-5' path that's mulched with wood-based mulch or pavers, depending on how often this area is used for walking, and then plant the entire area between the timbers and the house with groundcovers such as spreading ferns. You may find that there are other shade-loving natives or non-natives that work as well, though I won't make suggestions since my area is so different. I would go to a good nursery or a nursery that specializes in local native plants (or both) to get some ideas for low maintenance plants for shade. Use mulch between the plants until they have filled in.