Ultrastructural Investigation of Resistant and Susceptible Maize Inbreds Infected withErwinia stewartii

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Abstract
Anatomical changes associated with the development of Stewart''s wilt were studied in resistant (C123) and susceptible (B14A) maize inbred lines by using light and transmission electron microscopy. Pathogen populations increased at similar rates in leaves of resistant and susceptible plants. When leaves of maize plants at the tasseling stage were inoculated with E. stewartii, lesions expanded 3-4 times more rapidly in B14A than in C123. In both lines, microscopic examination of lesions showed that pit membranes became coated with material resembling bacterial exopolysaccharide (EPS) while pathogen populations in vessels remained very low. As populations increased in vessels, many became occluded totally with bacterial cells and EPS. Four other types of material, assumed to be of host origin, also were found in vessels of infected plants. These materials, which could be differentiated on the basis of ultrastructural appearance and histochemical staining reactions, were found more frequently in C123 than in B14A, and they might possibly function in the localization of the pathogen.