Wet spots

Farmers are used to see lesions called wet spots, on the underside of tobacco leaves. They are first wet, then brown, isolated or confluent, sometimes localised along the veins (figure 1). In general, the spots lighten quite rapidly (figure 2). Their appearance is certainly linked to the presence of water on the underside of the leaf which is more sensitive than the upper side. Presumably water could play either a role of a magnifying glass on sunny days and cause burns or bursting of cell islets, leading to tissue necrosis. This natural phenomenon is difficult to control.