Dog Spots are caused by an overdose of ammonia nitrogen in by the urine of the dog. The ammonia in the dog urine causes a nitrogen burn in the grass. The center of the dog spot is exposed to too much nitrogen, therefore, burning the grass. The edges are exposed to large quantities of nitrogen, turning the grass dark green.
There is no recommended product to prevent dog spots from occurring, but some homeowners said they saw effective results from changing dog food manufacturers, supplements of herbal vitamins, and training the dog to go to the bathroom in a different location.
The most effective way to stop dog spots from occurring are to promote very deep digging grass roots. The deeper and more established the grass roots are in the soil the better the grass can fight off a nitrogen burn. The best method in promoting deep digging roots is to water the lawn deeply and infrequently, aerate the lawn spring and fall, mow the grass tall, and use organic fertilizers instead of chemical fertilizers.
Many Organo-Lawn customers in Boulder and Fort Collins areas have had great success by following the 1,2,3,2,1 lawn watering recommendations, which was invented by Organo-Lawn to promote deep digging grass roots.
Read about what Dr. Steve Thompson, DVM -Director of Purdue University Veterinary has to say about dog spots in his article "DOG-ON-IT" LAWN PROBLEMS
If you don't want to read the article it basically says this:
Dog Urine Spots are small dead circles in the grass surrounded by dark green grass.