Helminthosporium leaf spot is caused by several fungi in the genus Bipolaris. The pathogens are favored by cool to moderate temperatures (60° to 70°F) during wet or humid weather. The pathogen is likely spread by splashing rain, irrigation water, and wind. It affects mainly Kentucky bluegrass, bentgrass, ryegrass, tall fescue and creeping red fescue. Lesions may be in the form of small brown flecks, linear streaks, or oval or irregularly shaped spots. The number and size of lesions may increase, coalesce and form long streaks. Lesions are initially dark brown but become tan or gray-brown with a faint dark brown border. During wet weather, lesions develop a dense gray-brown appearance. Black conidial and mycelial growth may be apparent on individual grains in the head. Warm wet weather is ideal for development. It is most common when temperatures are ranging from 80° to 90°F and there are extended periods of rain.