Summer patch is the common name for patches of brown or yellow grass caused by the Magnaporthe poae fungus. Summer patch leaves 6- to 12-inch circle patches of dead-looking grass throughout a lawn. Lack of moisture in the soil or improper irrigation is the likely cause. Proper soil moisture throughout the lawn focusing on the brown or yellow patches improve summer patch on most lawns.
Brown patches appearing shortly after mowing are likely caused by mowing the grass too short. For an optimal, healthy-looking lawn, keep at least 2/3 of the grass blade in tact. If your lawn has high areas, you are removing more of the grass blade in those areas, causing the grass to die in patches. Either mow the entire lawn less frequently or level the high areas in your lawn for a more even cut with each mow.
Dog urine has a heavy concentration of salt, which kills grass in patches, particularly if the dog urinates in the same areas each day. If there are areas of the lawn that are green and thriving and only a select few areas of dead patches of grass, it is likely from dog urine. Water washes away the salt concentrate and encourages new, healthy growth.
If your lawn develops brown patches shortly after you fertilize, your lawn is likely suffering from fertilizer burn. Two main causes are fertilizer spills and under-watering. If you notice a spill while fertilizing, thoroughly clean the spilled area and reapply the fertilizer. Water the fertilized lawn in a pattered fashion, with a sprinkler ensuring even, consistent watering. Pay special attention to the brown areas and apply extra amounts of water until the brown area turns green.