If it's only in a few places you could try straight full strength vinegar using an old pointed top mustard container and point it directly at the root............it will kill a little grass but that will come back before fall.....................if that does not work for you then try fungicide from the lawn & garden section. Keep children & pets away until the next rain. Read the directions on the side of the container and wear old long clothes, hat, gloves, goggles and mask.