ABSTRACT
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.
ABSTRACT
Fields of kale, spinach mustard, and turnip were severely damaged by bacterial leaf spots during 1994 to 1996. Symptoms included circular to angular necrotic lesions with yellow halos and water-soaking on the abaxial leaf surface. Yellow, mucoid strains isolated from leaf spots were identified as Xanthomonas campestris using Biolog. Four strains caused black lesions on stems of cabbage seedlings in an excised cotyledon assay, leaf spots and sunken dark lesions on petioles of spray-inoculated crucifers, and leaf spots on spray-inoculated tomato. These strains were classified as X. campestris pv. armoraciae. Most other strains from leafy crucifers and all strains from a cabbage field caused black rot in the cotyledon assay and in spray-inoculations. Many of these strains also caused leaf spots on collard and kale but not stem and petiole lesions. The strains causing black rot were classified as X. campestris pv. campestris. Cluster analysis of Biolog profiles yielded a small group that contained local strains of both pathovars, and a large group comprised of reference and local strains of each pathovar, and some local, nonpathogenic strains. Five fingerprint groups were identified by rep-polymerase chain reaction using the BOXA1R primer. Local and reference strains of each pathovar occurred in two of the groups. Two pathovars of X. campestris are involved in the leaf spot diseases. Both pathovars were recovered within several fields, and also were recovered along with Pseudomonas syringae pv. maculicola. This is the first report of Xanthomonas leaf spot caused by X. campestris pv. armoraciae in Oklahoma.
ABSTRACT
During 1995 and 1996, bacterial leaf spots severely damaged fields of kale, spinach mustard, and turnip in Oklahoma. Symptoms were small, brown, necrotic spots with irregular edges surrounded by chlorotic halos. Lesion margins were often water-soaked on the abaxial surface. The spots enlarged and coalesced, causing extensive leaf yellowing and necrosis. Nineteen strains of a fluorescent Pseudomonas spp. were isolated from symptomatic plants. LOPAT tests and carbon source oxidation using Biolog GN MicroPlates were used to classify the strains as P. syringae. Cluster analysis of carbon source oxidation profiles for the local strains and selected reference strains of P. syringae pv. maculicola and pv. tomato produced one group with 79.5% similarity. In spray inoculations, all local strains caused chlorotic or water-soaked lesions on collards, kale, cauliflower, and tomato. A few local strains caused necrotic lesions on mustard. Most local strains caused one of the three lesion types on turnip and spinach mustard. Reference strains of P. syringae pv. maculicola caused similar symptoms. All but three of the local strains produced coronatine in vitro. The local strains were thus classified as P. syringae pv. maculicola, the cause of bacterial leaf spot of crucifers. Two distinct groups of P. syringaepv. maculicola were identified by repetitive sequence-based polymerase chain reaction (rep-PCR) with both REP and BOXA1R primers. Three subgroups within each group were further identified using the BOXA1R primer. Except for two strains of P. syringae pv. tomato which were pathogenic on crucifers, the pathovars maculicola and tomato had different genetic fingerprints. The pathogen was recovered from seven of ten fields sampled during 1994 to 1996. In five of the fields with P. syringae pv. maculicola, pathovars of Xanthomonas campestris were also isolated from lesions forming a bacterial disease complex. This is the first report of bacterial leaf spot caused by P. syringaepv. maculicola on leafy crucifers in Oklahoma.
