How crazy is this change?

Long time lurker, first post. I renovated my back yard and put new sod down in 2016. Leveled it last year. Over seeded both winters. It was looking pretty good up to two weeks a go. I was hand watering brown spots, as I have some shallow rocks and roots under part of the lawn. I fertilized and cut as normal (2x week, 5/8). It seemed I developed a fungus all at once and it spread rapidly. Last week I sprayed some Bayer bioadvanced Fungus Control. It seems to have stopped the the damage, but wow! This is Tifway 419 in Phoenix.