Xylella fastidiosa. Bacterial leaf scorch (BLS) is an infectious chronic disease caused by the fastidious, gram-negative, xylem-limited bacterium Xylella fastidiosa. This bacterium, which is transmitted by xylem-feeding insects (like leafhoppers), colonizes and physically "clogs" the tree's water conducting tissues or xylem. Water transport becomes disrupted in roots, branches, and leaves due to large amounts of multiplying bacteria and their by-products. The presence of the bacteria also triggers a reaction in the tree that plugs the xylem, further impeding water transport and eventually killing the tree.
Reported hosts include sycamore, mulberry, red maple, sugar maple, sweetgum, American elm, and a number of oaks such as bur, pin, scarlet, red, laurel, water, turkey, bluejack, and shingle oak. Trees infected with Xylella fastidiosa exhibit marginal leaf necrosis, or browning, bordered by a pale halo band separating the dead or scorched tissue from the green tissue. Leaf discoloration begins at the leaf margin and moves toward the midrib. Symptoms recur each year and spread over the tree's crown, thus, reduction in growth and dieback are common in affected trees.
Several leafhopper species have been identified as vectors of this bacterium. However, vectors vary in their efficiencies in acquiring and transmitting the different strains of X. fastidiosa. Vector efficiency may be related to some extent to host affinities or feeding habits. |
