Bacterial Blight in Carrot Seed Crops in the Pacific Northwest

Abstract

Carrot (Daucus carota subsp. sativus) seed crops in Oregon and Washington were surveyed in 2001-02 and 2002-03 for development of Xanthomonas campestris pv. carotae, causal agent of bacterial blight. For each state and season, 20 plants were sampled from each of 7 to 12 direct-seeded crops twice in the fall or winter and three times from spring to summer; and from each of 2 to 4 steckling (root-to-seed) crops three times from spring to summer. X. campestris pv. carotae was detected in 1 of 15 and 6 of 32 stock seed lots planted in the fall in Oregon and Washington, respectively, and in 2 of 6 steckling shipments planted in each state in the spring. The pathogen was detected at 102 to 108 CFU/g foliage in 1 of 22 and 10 of 14 direct-seeded crops within 2 months of planting in 2001 and 2002, respectively. The prevalence of X. campestris pv. carotae then increased through the season in most seed crops, although bacterial blight symptoms were not observed until April in Oregon and July in Washington in both seasons. In August 2002 and 2003, X. campestris pv. carotae was detected in all 10 and 13 crops surveyed in Oregon, respectively; and in 11 of 12 and 7 of 10 crops in Washington, respectively. The pathogen was typically less prevalent in steckling versus direct-seeded crops. X. campestris pv. carotae was detected in 20 of 22 and 19 of 23 harvested seed lots in Oregon and Washington, respectively, at populations ranging from 1.3 × 101 to 1.4 × 108 CFU/g seed. Airborne X. campestris pv. carotae, detected ≤1,600 m downwind of crops being threshed in Oregon in September of 2003 and 2004, may provide a source of inoculum for newly planted seed crops between overlapping biennial seasons for carrot seed production. Despite the prevalence of this pathogen in the Pacific Northwest, carrot seed lots free of X. campestris pv. carotae were detected, demonstrating the ability to produce clean seed in this region by adhering to recommended practices for management of bacterial blight.

Genome sequencing and comparative analysis of the carrot bacterial blight pathogen, Xanthomonas hortorum pv. carotae M081, for insights into pathogenicity and applications in molecular diagnostics

“…In this region, epiphytic populations of Xhc on umbels can infect developing seeds without eliciting foliar disease. As a consequence, Xhc can be unknowingly abundant on seeds harvested from production fields, which then serves as the inoculum for bacterial blight on carrots grown in other regions more conducive for disease development (du Toit et al ., 2005). The asymptomatic ‘epidemic’ of epiphytic colonization of the seed crops frequently necessitates hot water treatment of seed lots to reduce the seed‐borne populations of Xhc .…”

“…The production of commercial carrot crops depends greatly on planting seeds with zero or low contamination of Xhc . The semi‐arid climates in which carrot seeds are produced permit the epiphytic growth of Xhc (du Toit et al ., 2005). The bacteria associated with seeds provide the inoculum for bacterial blight when carrots are grown in warm and humid regions.…”

“…Sensitive, specific and facile methods are therefore needed to detect Xhc in seed lots (Meng et al ., 2004). The development of cost‐effective management methods against Xhc is also greatly desired (du Toit et al ., 2005). A sequenced genome can be an important resource for these purposes and has been used in the development of molecular markers to distinguish between two important Xanthomonas rice pathogens (Lang et al ., 2010).…”

Tunnel production enhances quality in organic carrot seed production

“…Also, disease pressure has been greatly reduced in the tunnel system compared with field situations, resulting in fewer fungi on the harvested seed. This is achieved by, e.g., introducing controlled drip watering from the soil surface and using cuttings for transplanting instead of direct seeding (Gilbertson, 2002;Du Toit et al, 2005;Crowe et al, 2006).…”

“…This is also a means of controlling X. campestris pv. carotae as the pathogen has been found to be less prevalent in cutting versus direct seeded crops (du Toit et al, 2005).…”