The margin of the diseased tissue is not distinct. Secondary organisms, such as soft rots, usually finish off the tubers. In contrast, early blight tuber infection is usually characterized by black lesions with distinct borders that do not extend deeply into the tuber flesh. One of the first things that occurs after tuber infection is that the fungus kills the eyes. In a bad foliar infection situation, there can be less of a tuber blight problem since tubers are more mature and can resist infection and late blight dies in the dying foliage.
Late blight likes high moisture conditions at moderate temperatures.
Rain, dew, irrigations are all favorable for rapid growth. Late blight can lay somewhat dormant, or just grow very slowly, on plants for some time, waiting for the right conditions to spread rapidly. Conditions must remain wet for a period of 7-10 hours for sporulation to occur, as such sporulation usually occurs after wet nights or periods of rainfall. In the San Luis Valley conditions may be conducive for sporulation with the heavy dew in the early morning. The duration of the morning dew will be a big factor in late blight sporulation, as well as those occasional rainy periods.