The pictures supplied off line of this program indicate a definite fungal problem. Once the damage has progressed as the pictures show, recovering those areas will include a hard raking of the dead grass, topdressing to bring the soil up level with the surrounding grass, reseeding using a high quality blend of hybrid Kentucky bluegrass (3-5 different cultivars or varieties), lightly raking the seed in and keeping the soil moist while germination and early growth occur. A light covering of straw will slow the drying of the soil. Causes of any fungal infection can and will have several factors. A susceptible grass, too much thatch, environmental conditions favoring the fungal organism, fertilizer rates, irrigation are most of them. It is not uncommon for a lawn over time to have spots colonized by an individual species of grass that can be prone to disease over the surrounding grasses. These grasses may be thatch generators as well. if you want to make any treatments, fungicides are only preventative, so make a note on the 2018 gardening calendar and start treatments at least two weeks before you noticed damage this year.
