September is the worst time of the year for muddy patches on the lawn caused by worms. The worm casts are the deposits that they leave when feeding on decaying vegetation in the soil. They are at their most active when the soil is warm and wet, so this week they've been having a great time. Although the worms are good for the soil, aerating it and incorporation organic matter into it, the muddy casts that are left behind are often the perfect breeding ground for weed seeds, and if they are squashed into the lawn by feet or lawnmowers, they can often kill the grass beheath the mounds, leaving patches in spring.
To lessen the problem, remove worm casts whenever it is dry with a broom (a besom broom is the suggested method) onto the borders as the soil is great for plant growth. Also don't walk on the grass when there is still dew on it, to prevent casts being squashed in. There are no longer any chemical controls for gardeners to use, so just think of the benefits of having lots of worms in your soil. When the weather gets colder, the worms will go further underground, so don't wish them away just yet!