What is this stuff in my yard?

I have some things on my yard that I am not sure what it is. I have gone to two garden shops and got two mutually exclusive responses. After rains the last couple of weeks, I have a cottony cover on my grass (I thought originally it was stuff from the cottonwood or some weed sticking to the wet grass (see picture). However, soon I realized that the grass in the area was turning brown in roughly the outline of the cottony cover. One guy suggested fungicide and another just thought I should fertilize and it would come back. These are seen only after a rain, it seems to be only in areas that are full or near full sun. And it has been high temperatures in the high