Mowing at the proper height stimulates a healthy growing lawn to resist fungus. The lawn should not be cut lower than 3". Low cuts and dull mower blades will scalp the leaves and create wounds in the tissue. Such wounds limit the natural ability of grass to resist infection.
Mow your lawn when the grass is dry, particularly if you already have a fungal disease. Keep the mower blades sharp. As stated above, a dull blade inhibits the grass to ward off fungus. Sharp blades, on the other hand, cut cleanly and allow the plant to heal and recover quickly. If you have fungus, try to mow the affected areas last to avoid spreading it.