The alternative host with the most: Understanding the ecology of Xanthomonas hortorum pv. carotae on non-carrot crops in central Oregon.

Bacterial blight of carrot, caused by Xanthomonas hortorum pv. carotae ( Xhc ), is an economically important disease in carrot ( Daucus carota subsp. sativus ) seed production. The objectives of this study were to determine if Xhc was present on non-carrot crops grown in central Oregon and if detected, evaluate its ability to colonize alternative hosts. Surveys of three carrot seed fields and adjacent fields of rye ( Secale cereale ), alfalfa ( Medicago sativa ), parsley root ( Petroselinum crispum var. tuberosum ), and Kentucky bluegrass ( Poa pratensis ) demonstrated that Xhc was present on non-carrot crops. Greenhouse experiments were conducted to determine the ability of Xhc to colonize crops cultivated in the region. Carrot, alfalfa, curly parsley ( Petroselinum crispum ), Kentucky bluegrass, mint ( Mentha x piperita ), parsley root, roughstalk bluegrass ( Poa trivialis ), and wheat ( Triticum aestivum ) plants were spray-inoculated with Xhc and then destructively sampled at 1-, 7-, 14-, and 28- or 25- days post-inoculation. Xhc populations were quantified using viability quantitative PCR and dilution plating. A significant (P≤0.03) effect of crop was observed at 1-, 14-, and 28- or 25 days in both experiments. While carrot hosted the most Xhc at the final timepoint, other crops supported epiphytic Xhc populations including wheat and both bluegrasses. Mint, parsley root, and alfalfa hosted the least Xhc . Bacterial blight symptoms were observed on carrots, but not on non-carrot crops. This suggests that crops grown in central Oregon have the potential to be asymptomatically colonized by Xhc and may serve as reservoirs of the pathogen in carrot seed production systems.