Late blight

In wet conditions Late Blght can quickly destroy a whole crop because of its rapid reproduction - total defoliation can be as quick as two to three weeks. Lesion growth is not stopped at veins, therefore, one lesion may take over a whole leaf.  Stems, petioles and leaves will be infected with lesions. Lesions appear first as small flecks, quickly growing larger, and typically are surrounded by a light (yellow) ring. White mycelium will be visible on underside of leaf during times of high humidity.