I didn't see on this thread anything about your general lawn fertilizer schedule, types of fert the lawn gets, or even what specific specie(s) of grass you have... But I CAN tell you that turf that is generally bombarded with 'quicker-release' nitrogen sources, such as urea, will often tend to fall into disease cycles such as what you're experiencing, particularly if they're over-applied by the applicator, or applied at points in time that doesn't make sense for the turf to respond to it...or even be able to store any of it readily. Essentially, hyper-blasts of 'N' serves to make individual grass' cell walls thinner and thinner...to the point where they easily burst at even the slightest provocation, or often at NO provocation at all. Over application of urea 46-0-0, or even the "standard" 1#/ 1000 rate/ round, has been directly attributable to higher incidences of brown patch and $$$ spot in many turf trails over the years here in the Midwest. I can walk down any street here in S. Ohio in late summer, and just about pinpoint every yard that gets regularly abused by high-octane N! Because, often, I see some of the trucks that pull up to them, and can discern the stuff they're using. Compared to others nearby who use a more balanced slow-release N, or organic program...those lawns (generally) look like hell by now!