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 Pantoea stewartii |
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Pantoea stewartii causes Stewart's bacterial wilt and leaf blight disease on maize. It is the most important bacterial disease of corn crops in the Midwest and Northeast. The disease is insect vectored by the corn flea beetle, Chaetocnema pullicaria. In spring, adult beetles emerge from the soil and inoculate young corn seedlings. The pathogen exists in corn and related grasses during the summer, until a few generations of beetles complete the larval and pupal stages in late summer, when secondary transmission occurs. Interestingly, the pathogen lacks a prominent epiphytic or saprophytic stage, which is unusual for a bacterium that can be readily cultured and mechanically transmitted in the laboratory.
Stewart's wilt disease was first reported in 1897 on sweet corn in New York. Due to the wide host range of the flea beetle, one can envision how this pathogen crossed over to modern, susceptible corn varieties and became a problem soon after they were introduced into the United States. Stewart's wilt may therefore be a classic example of how a new disease has "emerged" due to changing agricultural practices. This theory is supported by the observations that the disease does not occur in Central and South America, where maize originated. Moreover, teosinte and Zea diploperennis, the ancestors of modern corn, are highly susceptible, whereas North American Indian corn is highly resistant.
P. stewartii grows in the intercellular spaces of the leaves causing "water-soaked" lesions during early infection and invades the xylem vessels leading to subsequent wilting and death of seedlings. In older plants, the characteristic long lesions on mature leaves are also due to vascular blockage and usually originate from beetle feeding wounds. The water-soaking (Wts) symptoms are due to the loss of cell membrane function and accumulation of fluids in the tissue. The Wts symptoms are due to effector proteins delivered into the corn cells by a Hrp type III secretion system. Wilting is the result of production of an extracellular polysaccharide (EPS) slime, called stewartan, which occludes the xylem vessels during the systemic phase of the infection.
The genome of P. stewartii is approximately 5 MB, including an impressive array of plasmids. The GC content is between 50-58%. The uniformity in the PFGE patterns of different P. stewartii strains is striking and reflects the clonal nature of this species. This low genetic variability means that the genome sequence of DC283 should be representative of most strains in this species.