They are usually round when caused by fungi and angular when caused by bacteria. Bacteria also usually cause large chlorotic halos and water soaked areas around the lesion. Leaf blights are unrestricted in size and usually irregular in shape and may enlarge as long as conditions remain favorable. Some also infect the twigs but cause little damage. If they did, the disease would be classified as a twig blight.
Infected twigs serve as important sources of inoculum.
Virtually all have a secondary stage such as conidia or bacterial cells. The classic life cycle for an Ascomycetes which cause leaf diseases is that of Venturia inaequalis, the apple scab fungus. The produce the ascomycete stage in the fallen leaves during the winter and early spring, releasing ascospores for primary inoculum as the young leaves emerge. However, virtually all have a strategy for staying in the tree throughout the year and produce imperfect spores as an additional primary inoculum. Conditions favoring the disease a.