Fortunately, it takes more than the presence of pathogens (disease-causing microorganisms) to bring on infection in lawns. You also need a host--that is, a grass susceptible to a particular pathogen--and environmental conditions that foster disease. Conditions favoring pathogens vary by pathogen but generally include warm weather and extended periods of moisture, from rain, humidity, irrigation, or poor drainage. Other conditions, such as drought and high heat, encourage problems because they reduce the grass's ability to fight off infection. These three elements--pathogen, host, and environment--make up the "Plant Disease Triangle." Any one factor on its own cannot initiate disease. Problems begin when all three come together.