Primary infections occur during cool, wet weather when the ascospores germinate and penetrate the host tissues directly. If high moisture and cool temperatures prevail in the fall and winter, secondary infections can occur when fungus mycelium spreads to other leaves and stems. Researchers don't know how far the wind can carry the infective spores. The effect of temperature on growth and aggressiveness of the fungus is well documented. This suggests a wide range of growth potential and good survivability under natural field conditions.
Free water is necessary for successful infection.
If the host tissues dry out, the disease stops. It takes about 36 hours for the ascospores to penetrate the leaves. These requirements make fall and winter in Missouri the most conducive to infection and serious injury to alfalfa and red clover. Spring-seeded alfalfa is more likely to encounter dry weather. It has free water on the leaves and stems less often.