Brown patch or large patch is a fungus that appears this time of year. It is caused by the pathogen, Rhizoctonia solani. It lives in the thatch layer and becomes active when the soil temperature reaches between 50 and 75 degrees. Sometimes it won't begin to show until the first frost has come through. Because of this, it is often confused or mis-identified as frost damage and vice versa. The tell tale sign of the brown patch being a fungus and not frost damags is the yellow ring or halo surrounding the brown. The best thing to do if you notice brown patches or spots is to notify a company who is licensed in lawn and ornamental pest control.