Seedlings less than a year old without a good root system may die in hot weather and have a greater need for irrigation than established plants. Mowing height during the summer should be at 2.5 to 3 inches. This may result in more disease than a lower mowing height but there will be better potential for recovery in the fall. Decrease shade and increase air circulation by pruning trees and shrubs which can help limit the duration of leaf wetness. Water early in the day, which actually washes the dew off of the grass and facilitates drying.
Some references also suggest irrigating between sunset and sunrise.
Avoid watering early in the evening prior to sunset. This provides an adequate duration of free moisture on the grass for infection of Rhizoctonia and other pathogenic fungi.