Detection, Survival, and Sources of Inoculum for Bacterial Diseases of Leafy Crucifers in Oklahoma

Xanthomonas campestris pv. campestris, X. campestris pv. armoraciae, and Pseudomonas syringae pv. maculicola are bacterial pathogens that cause leaf spot diseases on leafy crucifers in Oklahoma. Polymerase chain reaction (PCR) amplification of the cfl gene from the gene cluster encoding the phytotoxin coronatine was used to identify coronatine-producing strains of P. syringae, and the expected 0.65-kb PCR product was detected in 19 strains of P. syringae pv. maculicola originating from diseased crucifers in Oklahoma. A simple, rapid PCR method based on primers from the cfl gene was developed to detect coronatine-producing strains of P. syringae in planta. Pathogenicity tests confirmed the cfl-positive strains to be P. syringae pv. maculicola. To monitor the survival of X. campestris pv. armoraciae and P. syringae pv. maculicola in the field, turnip and collards were inoculated with rifampicin-resistant strains and were buried beneath the soil or left on the soil surface. Both pathogens were recovered from turnip and collard debris up to 2 months following burial, but neither pathogen was recovered from soil after the debris had decomposed. However, both pathogens were recovered from plant debris left on the soil surface for up to 5 months. Four production fields were surveyed for sources of inoculum of the bacterial pathogens from October 1999 to May 2000. X. campestris pv. campestris was isolated from the weed shepherd's purse (Capsella bursa-pastoris) in all fields, and from volunteer turnip and kale in three fields. X. campestris pv. campestris and P. syringae pv. maculicola were isolated from surface debris and regrowth from crop stubble left in one field after harvest in the fall. X. campestris pv. campestris was detected in 6 of 51 lots of crucifer seed assayed. X. campestris pv. armoraciae and P. syringae pv. maculicola were not recovered from weeds, volunteer plants, or seed lots.