Bacterial leaf spot of lettuce
Xanthomonas campestris pv. vitians
The first symptoms are small, usually angular, water-soaked spots that appear on mature and fully developed lettuce leaves. These spots soon turn black and become bordered by the leaf veins. As the disease progresses, the spots coalesce into larger necrotic leaf spots. Heavily affected leaves begin to wilt and die. As the leaf spots dry out, they become papery and tear or break out of the leaves. If optimal conditions for further disease development prevail (prolonged periods of wetness), inner leaves of the lettuce head are also infested. It is typical for an infestation that the disease increases significantly at the stage of head formation. The bacteria can also infect the tissue of the stalk, causing stalk rot, and in young plants, stunting and death of the latter. However, the bacteria do not penetrate into the conduits and thus do not cause systemic infections.
If infested plants are boxed ready for transport, secondary rot pathogens can colonize the leaf spots, causing significant postharvest damage. The bacteria can also infect inflorescences during seed multiplication, where the same symptoms are then also visible.
Propagation and transmission
Starting points of infection can be infected seeds, infested weeds, infested crop residues or wild Lactuca species. The bacteria can also occur on lettuce and weeds without infecting them. Only under prolonged periods of wet weather (rainfall, irrigation) do infections and an outbreak of the disease then occur. Water droplets hitting the leaves spread the bacteria from plant to plant.
Last updated: 15.11.2021