First report of bacterial leaf blight on mustard greens (Brassica juncea) caused by pseudomonas cannabina pv. alisalensis in Mississippi

Authors: Wechter, William - Pat; KEINATH, A - Clemson University; SMITH, J - Clemson University; Farnham, Mark; Bull, Carolee; SCHOFIELD, D - Guild Associates

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2014
Publication Date: 7/10/2014
Publication URL: http://dx.doi.org/10.1094/PDIS-09-13-0966-PDN
Citation: Wechter, W.P., Keinath, A.P., Smith, J.P., Farnham, M.W., Bull, C.T., Schofield, D.A. 2014. First report of bacterial leaf blight on mustard greens (Brassica juncea) caused by pseudomonas cannabina pv. alisalensis in Mississippi. Plant Disease. 98:1151.

Interpretive Summary: In 2010-2013, leaf spot and leaf blight symptoms were observed on mustard greens and turnip greens in Sunflower County, Mississippi. In 2011, a severe outbreak of leaf blight occurred, with more than 80 hectares affected. The affected field, established in 2010, had no prior history of being cropped to brassica leafy greens prior to 2010. Diseased plants were collected from the affected field. Using biochemical, molecular and host range testing, the pathogen was determined to be a specific Pseudomonas type bacterium known to cause leaf blighting. This represents the first report of this pathogen in Mississippi. Over the past 10 years, this bacteria has been found in numerous states in the U.S., as well as, in Europe, Australia, and Japan. As brassica leafy greens production expands onto new lands and into new states, leaf blight caused by this bacteria appears to quickly become a problem. Since resistant cultivars and effective bacteriocides are lacking, growers are extremely concerned and need to be aware about the rapid spread of this disease into existing, as well as new, brassica leafy greens growing regions in the U.S.

Technical Abstract: In 2010, a brassica leafy greens grower in Sunflower County, Mississippi, observed scattered outbreaks of a leaf blight disease on mustard greens (Brassica juncea) in a 180-hectare field. A severe outbreak of leaf blight occurred on mustard greens and turnip greens (Brassica rapa) in the same field, in 2011, with more than 80 hectares affected. The affected field, established in 2010, had no prior history of being cropped to brassica leafy greens prior to 2010. Symptoms appeared as small necrotic spots with associated water-soaking and necrosis of leaves. Lesions were brown to tan with faint chlorotic borders and varied from 4 mm to 3 cm. Lesions coalesced to cover >90% of older leaves. Whole plants of mustard green cultivar Florida Broadleaf were collected in 2011 from an affected field. Leaves were surface disinfested with 0.5% NaOCl and macerated in sterile distilled water, then streaked onto) Pseudomonas Agar F. The majority of growth appeared to be a single colony type. All isolates collected (25 total, one from each plant) were Gram-negative and fluoresced blue-green under UV light. Ten of the 25 isolates were tested for pathogenicity to turnip greens cv. Alamo, and mustard green cv. Florida Broadleaf. A control set of cv. Florida Broadleaf and Topper plants were either non-inoculated, sprayed with water, or inoculated with Pseudomonas cannabina pv. alisalensis (Pca) pathotype strain BS91. All 10 isolates, as well as, the Pca pathotype strain, were pathogenic on both cultivars and caused symptoms similar to those observed in the field. Symptoms were not observed on non-inoculated or water-sprayed plants. Comparative rep-PCR analysis using the BOXA1R primer showed the ten isolates had identical DNA-banding profiles and were identical to that of the Pca BS91. Five isolates tested using a Pca-specific, ‘light-tagged’ reporter bacteriophage gave a strong positive reaction while the negative control, P. syringae pv. maculicola gave no signal. From these tests, we determined the isolated bacteria to be Pca. Blue-fluorescing bacteria re-isolated on PAF from the inoculated ‘Florida Broadleaf’ plants had identical LOPAT and BOXA1R rep-PCR profiles with those of the strains used for inoculations.