The record breaking heat and humidity this summer has had an effect on “Lawn-Gevity” …….physiologically cool season grasses do well when air temperatures are in the 60 to 75 degree range…….a couple months of 80’s and 90’s can create real difficulties for grass. 
When soil temperatures heat up the grass starts to stress, yellow and wither. Grass has an ability to cool itself through something we call evapotranspiration…….it’s something like people perspiring. The roots draw up water from the soil and it evaporates through the leaves dissipating heat. The problem is roots aren’t as active in summer as they are in spring and fall…..they even shrink reducing their water absorbing ability. Contingent on how healthy your lawn was heading into summer the stress can exacerbate other issues like grub damage or diseases. Raise the deck on your mower to shade the roots and crown of the grass plants. 
You may want to “syringe” the lawn during the days of greatest heat stress. Syringing is a very light application of water to wet the leaves of the grass but not necessarily to get water down into the soil. Water in the morning hours to get sufficient supplemental water to the roots, mow less frequently raising the deck on the mower and consider syringing during the hot part of the day to cool the turf and reduce heat stress. Don’t syringe in the evening hours because wet foliage at night can encourage disease issues. We see problems like rust
or red thread when the lawn is under stress and growing slowly due to the season and environmental factors.
Fortunately in my opinion the period of August 15 through October 15 is the perfect time of the year to repair, re-seed, over-seed, take out weeds, core aerate and feed lawns to reverse the damage done by stressful summer weather.
