From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156714-180873
Question: My fescue lawn has numerous dead spots throughout. What
might be causing this problem, and how can I remedy the situation?
Answer: The first thing to rule out is if the problem is being caused
by neighborhood dogs who visit your lawn. Dog urine can cause brown
spots. Some tall fescues are susceptible to brown patch. Conditions
favoring the disease include hot, humid weather; overly wet,
overfertilized lawns; and a thick layer of thatch. You can reduce the
chance of future outbreaks by watering deeply and infrequently, and
allowing grass to dry between waterings. Also, you might want to
aerate the lawn to help break down the thatch. Be sure you are not
overfeeding with a nitrogen-rich fertilizer. You may also want to test
your soil to check levels of calcium, phosphorus, and potassium, since
improper levels of these nutrients can encourage the disease.