That lawn looks awesome for this time of year! After looking at the pic's, it looks more like drought type stress from the high, intense heat - even though there's an irrigation system present. However, it's tough to tell from a picture. Be aware that Dollar Spot is raging right now in many lawns up here in the Northeast.
Red Thread will flare up on Ryegrass lawns pretty easily in the spring and carry over into the summer. Obviously, watering correctly reduces this but will not corral it completely. As for watering, it's ALWAYS going to change and any grass that stays wet for extended periods of time will certainly be affected by this fungus (as well as by other funguses). I try to tell people that fungus growth in a lawn can be compared to fungus growth in a tiled bathroom, where mold grows in the grout between the tiles (usually in the shower). Unfortunately, you cannot use Bleach to clear your Red Thread like you can with mold!
I also try to tell my customers to water deeply but infrequently! How deep? How infrequently? That will always change during the season depending upon what Mother Nature gives us in the form of rain, heat, hot sunshine, evaporation rate, etc.... You need to be able to monitor your irrigation system and make changes if necessary. Learn about matched precipitation rates and make sure your system is up to par in that department. Also look into learning how to program your timer and how to override your rain sensor if needed. Also learn to look at the lawn daily and educate yourself on the signs of drought stress vs. funguses.
For my own lawn, I'll run you through my schedule so far this year, both for fertilizer and watering and fungal growth has been kept to a minimum.
March 15th - 28-0-12 + 3% Iron
April 15th - 19-0-6 Dimension
May 27th - 19-0-6 Dimension
July 8th - 18-0-8 Allectus
My next one will be a "Bridge" product called Sustane on August 19th and it's an organic/conventional fertilizer with Nutralene. It has an 18-1-8 analysis on the bag.
As for my watering schedule, my town has restrictions of "odd/even" days to water, thus I can only water on even numbered days. Knowing this, I programmed my Hunter PC300I so that each zone would come on during those even numbered days and I began the season in mid April at 15 minutes per zone. On some of those days, I didn't even have to water and manually shut the system off so that it wouldn't come on. As things began to heat up, I adjusted the times to every other day for 30 minutes per zone and I didn't shut it off at all. About 6 weeks ago, I had to adjust it to 45 minutes per zone and then 3 weeks ago, I went to 60 minutes per zone. That's where I am right now and my lawn looks fantastic. I've had a little fungal activity but nothing to worry about.
I will say a few things though. Last week I noticed the typical "Black haze/brown haze" beginning in a few areas of the lawn - even at 1 hour per zone every other day. Thus, I manually turned the system on later in the afternoon (on the days that I can water) for 30 minutes per zone - in addition to the 1 hour per zone each morning. That extra "spritz" helped to eliminate those spots and return it to "Green" status. Also, we were supposed to get thunderstorms one night last week and we got a 10 minute shower in the middle of the night. That morning, my rain sensor was wet enough to NOT turn on the irrigation system. With temperatures of 96 degrees and a very high evaporation rate, I manually overrode the system and turned it on myself.
For those people who have more than 5 zones on a system, I recommend breaking up the watering into multiple days to allow the lawn more time to dry out in between wettings. In other words, if you have an 8 zone system, you may want to break it up into two separate days of watering (4 zones each day). That way, you can start your system later in the morning (to reduce the amount of time the lawn is wet) and still have it done by 9:00am each day. So, that would mean starting at 5:00am and be finished by 9:00am (assuming that you'll be watering 1 hour per zone). The next day, do the other 4 zones. Of course, this would only work if you have a well and can water whenever you want. If you have town water, you'll have to get a little more creative.
Basically, I'm trying to convey that the lawn doesn't care when it gets the water as long as it's enough! Be creative with your watering (there's no need to be locked into doing every zone on the same day), try to water later in the morning (4:30am on - NOT 3:00am or so), stagger your days for different zones, and try to allow longer times in between wettings to allow your lawn to dry out. This will work to reduce Fungal growth.
Again, I think you're doing a fine job with the lawn. Compared to the majority of them out there today, it looks like a Golf Course!