Association of xylem-limited bacteria with elm, sycamore, and oak leaf scorch

Abstract

Ultrathin sections of leaves from American elms (Ulmus americana L.), sycamores (Platanus spp.), and two red oak species (Quercus spp.) that exhibited leaf scorch were examined by transmission electron microscopy. Rod-shaped, ripple-walled bacteria resembling the Pierce's disease organism were found consistently in tracheary elements of the primary and secondary veins of diseased plants. Smaller, ripple-walled, densely stained, irregular-shaped bodies (SDB) were found also in a matrix that lined the inner walls or filled the lumina of the tracheary elements. In leaf scorch affected American elms the bacteria were 0.3–0.4 μrn × 0.9–2.4 μm, with rounded ends. Fimbriae-like structures radiated from a few organisms. Bacteria were frequently embedded in a matrix. Pit cavities and the ends of tracheary elements were often filled with the bacteria–matrix complex. Rod-shaped bacteria were not as numerous in the diseased sycamore and oak as in diseased elm; however, SDB's were more numerous. Bacteria in sycamore (1.0–1.8 μm) and oak (1.0–2.0 μm) were slightly shorter than those in elm, many had numerous fimbriae-like hairs, and some were tapered at one end. Indirect fluorescent antibody staining showed a serological relationship between bacteria extracted from elm and oak and the Pierce's disease bacterium.