Yellow patch, also referred to as cool season brown patch, is caused by Rhizoctonia cerealis, a fungus closely related to the pathogens that cause brown patch and the Rhizoctonia large patch of zoysia grass. Although symptoms can be striking, yellow patch causes only cosmetic damage and does not affect playability. It occurs most often on creeping bentgrass and annual bluegrass. Symptoms are most common on putting greens collars, and surrounds, but they may appear on fairways now and then.
Yellow patch symptoms include small- to medium-sized patches or rings (6 to 12 inches in diameter), usually with yellow margins. Sometimes margins are reddish brown. In severe cases, patches may be distributed uniformly over the putting surface. Unlike brown ring patch (aka, Waitea Patch), which does not affect creeping bentgrass, yellow patch seems to infect bentgrass and annual bluegrass equally.
Yellow patch is a cool season disease. Symptoms most often appear in early to mid-spring, but sometimes occur in mid-fall. Outbreaks will most likely occur under overcast, cool, wet conditions. In the spring, symptoms will readily disappear after a few days of warm, sunny weather. Deliberate attempts to control yellow patch, with or without fungicides is normally not recommended because the disease has only cosmetic effects and symptoms are usually very short-lived. There are several effective fungicides (DMI and QoI) for the rare cases where the chemical option is warranted.
For details and color images, check out the publication BP-118-W at http://www.agry.purdue.edu/turf/pubs/index.htm