Evaluation of Disinfectant Seed Treatments to Reduce Xanthomonas hortorum pv. carotae in Carrot Seed Lots

Bacterial blight of carrot, caused by the plant pathogenic bacterium Xanthomonas hortorum pv. carotae (Xhc), is a common disease of carrot wherever the crop is grown. The disease can affect carrot foliage, stems, umbels, and roots and can be seed-borne. Symptoms of bacterial blight include small, irregular, chlorotic areas on leaves that can manifest into water-soaked, necrotic lesions. Lesions can also occur on stems and petioles. Floral infections can result in blighted umbels, reduced seed yield, and reduced germination rates of harvested seed. Once established, Xhc is difficult to control and disease prevention is challenging because Xhc is seedborne and seed treatments with hot water or disinfectants may not entirely eradicate the pathogen. The seed-borne nature of Xhc makes it a major concern not only to the hybrid carrot seed industry in the Pacific Northwest but also to regions that import carrot seed for root production. Epiphytic populations can reach high levels on plants in the field, resulting in seed that is infected or infested by the pathogen. Seed lots that are highly infested with Xhc (>105 CFU/g seed) necessitate seed treatment to reduce the risk of bacterial blight occurring in commercial root crop production. Seed treatments are usually in the form of hot water treatment (52°C for 25 minutes) which can be effective but can reduce germination and/or shelf life of seed lots. Germination can be reduced further if seed lots need to be treated multiple times to reduce infestation levels below the 105 CFU/g threshold that was established for carrot seed planted in the Central Valley of California. Chemical seed treatments, which can remove bacterial pathogens that are borne on the seed surface, may provide alternative or additional methods for reducing Xhc in carrot seed lots. The objective of this project was to evaluate chemical disinfectants as seed treatments to reduce Xhc levels in carrot seed lots.

Series/Report Number: COARC2016

Chapter Number: 18