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Potential for seed transmission of Xanthomonas vasicola pv. vasculorum on maize collected from fields in the United States

Silvina Arias: Iowa State University

<div>Bacterial leaf streak of maize, caused by <em>Xanthomonas vasicola</em> pv<em>. vasculorum</em> (<em>Xvv</em>), was first reported in the U.S. in 2016 and spread quickly to multiple states, leading to speculation that it was introduced on seeds. However, the potential for seed transmission of <em>Xvv</em> is unknown. A preliminary study demonstrated that <em>Xvv </em>was readily transmitted to seedlings from seeds that had been soaked in concentrated bacterial suspensions, but seed transmission from naturally-contaminated seeds has not been shown. In 2016, we obtained open-pollinated seeds from 37 moderately to heavily-diseased commercial production fields in Colorado, Nebraska, and Iowa, but were able to isolate live <em>Xvv </em>cells from just two of the 37 seed lots. In an initial grow-out study, 4000 seeds from a popcorn field with disease incidence of approximately 90% infected plants were planted in a greenhouse and grown for four weeks. No disease symptoms were observed in this period, but TaqMan PCR results of 10-seedling pooled samples suggested that <em>Xvv </em>may have been present, but asymptomatic, in seedlings. Twenty samples had Ct values <31, and two samples were below Ct=24, in the range of the positive controls. Although the risk of seed transmission seems to be low, it is critical to evaluate the potential for seed-to-plant transmission from naturally diseased fields in order to protect trade and prevent movement of the pathogen if seed is proven as a pathway. Additional grow-out plantings are in progress.</div>