First report of Xanthomonas campestris infecting invasive garlic mustard in the United States.
Garlic mustard (Alliaria petiolata) is an aggressive nonnative cruciferous (Brassica) plant from Eurasia that was introduced into North America around the 19th century. The biennial Brassica herb has since rapidly invaded and displaced native flora in much of the eastern and midwestern United States and Canada, thriving in woodlands, hedgerows, lawns, and other shady environments. In May of 2019, foliar leaf symptoms developed on garlic mustard along the edge of a forested property in Frederick County, Maryland. Initial leaf symptoms were chlorotic angular V-shaped lesions originating from the leaf margins, characteristic of a bacterial infection. As the disease progressed, leaves wilted and lesions became necrotic, covering entire leaf margins. The disease was patchy along the property and nearby forest edge, but the disease was observed on both second-year mature flowering plants and first-year immature rosettes. Disease incidence in the affected area was 75 to 80%. Based on the results of multilocus sequence analysis and pathogenicity tests, the causal agent was identified as X. campestris. This is thought to be the first confirmed report of X. campestris naturally infecting garlic mustard in the USA.