It is most likely caused by poor air circulation which gives fungus an environment in which to thrive. I would separate the plants and remove any leaves or leaf debris that has damage. The brown areas send out spores to spread infection. Remove any plant debris as well, and thin out dense areas so the plants can dry. Bag or burn any debris or infected plant parts, and mulch to keep the soil from splashing on the plants. Finally, don't water overhead. Use a soaker hose to keep the leaves dry.