August 11, 2005
From: ProMED-mail <promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited]

First* outbreak of bacterial leaf spot caused by Xanthomonas campestris on canola in Argentina
S. Gaetan, Facultad de Agronomia, Universidad de Buenos Aires, A. San Martin 4453, 1417, Argentina; and N. Lopez, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina. Plant Dis. 89:683, 2005; published on- line as DOI: 10.1094/PD-89-0683B. Accepted for publication 31 Mar 2005.

Canola (_Brassica napus_) is an alternative crop to wheat in Argentina, and approximately16 000 ha are grown commercially in southern Buenos Aires Province. During 2003, experimental field plots of canola plants located at
Agronomy Faculty, University of Buenos Aires, in Buenos Aires were severely damaged by bacterial leaf spot. Average disease incidence across 25 2- to 5- month-old canola cultivars was 58 percent (range = 27-89 percent).

During 2004, identical infection associated with blackened veins (8-12 percent of plants) was found in 2 commercial fields and experimental crops (rosette and flowering stages) in Tres Arroyos in southern Buenos Aires Province. Symptoms observed on adaxial surfaces consisted of v-shaped necrotic lesions on leaf margin surrounded by yellow halos. Yellow bacterial ooze was found on young lesions. The advanced phases of the disease included lesion enlargement, foliar chlorosis, and death of leaves. The disease developed from the lower leaves to the apex, resulting in complete leaf necrosis and defoliation.

10 samples (5 plants per sample) with lesions were arbitrarily collected from 2003 to 2004 from commercial and experimental canola crops. Diseased leaf tissue was surface sterilized in 0.50 percent sodium hypochlorite for 30 s and rinsed in sterile distilled water (SDW). Leaf sections were macerated in SDW, and the extract was streaked onto nutrient agar. Plates were incubated at 28 C for 3 days. Resultant colonies were yellow, mucoid, and convex. Gram-negative, aerobic, and rod-shaped bacteria were obtained. 8 strains were biochemically characterized using API 20NE (BioMerieux, Marcy l'Etoile, France) and identified as _Xanthomonas campestris_ (1).

Strains hydrolyzed starch, gelatine, and aesculin and were positive for catalase and negative for oxidase, nitrate reduction, ureasa, and triptophanase. Strains were capable of utilizing D-glucose, D-mannose, D- maltose, malic acid, and N-acetyl-glucosamine. _X. campestris_ pv. _campestris_ 8004 was used as a reference strain (2).

Pathogenicity and host range for 3 isolates were completed by injecting a bacterial suspension (10 million CFU/ml) into leaves of 2-week-old canola plants (cvs. Eclipse, Impulse, Master, and Mistral), cabbage (B. oleracea_ var. _capitata_), and cauliflower (_B. oleracea_ var. _botrytis_) seedlings (2- leaf stage). The experiment (4 inoculated and 2 control plants for each cultivar and each strain) was conducted in a greenhouse at 24 C and 75 percent relative humidity. Inoculated and control plants were enclosed in a plastic bag for 48 h after inoculation.

Chlorotic patches on the leaves followed by a dry, brown necrosis spread beyond the initial injected area were observed in inoculated plants 8 days after inoculation. Enlarged spots caused death of leaves. The pathogen was
successfully reisolated. Control plants, inoculated only with SDW, remained symptomless. The results suggest that the bacterium represents a potential threat to canola production in Argentina and indicate the need for further study to identify the pathovar involved in canola leaf spots.

*To our knowledge, this is the 1st report of an outbreak of _X. campestris_ causing leaf spot of canola and in which the bacteria affecting canola commercial crops was biochemically characterized and host range was carried
out in Argentina.

References:

(1) N. W. Schaad et al. Laboratory Guide for Identification of Plant Pathogenic Bacteria. 3rd ed. The American Phytopathological Society, St. Paul. MN, 2001.

(2) P. Turner et al. Mol. Gen. Genet. 195:101, 1984.

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[Canola is the major oilseed crop grown in the Prairie Provinces in Canada and its coverage area is expanding. It is also an economically important and serious disease of canola (_Brassica napus_) in Australia, France, Germany, USA and the United Kingdom. It is the most serious disease of canola/rapeseed in the prairies, and may cause major crop losses in some years. The infections of blackleg may occur on cotyledons, leaves, stems and pods. Stem canker is the most serious symptom, as it can girdle the stem, causing plant lodging leading to yield loss. 3 disease prevention methods -- crop rotation, genetic resistance and seed treatment with fungicide have proven to be effective.

Isolates of canola can be categorized into 4 pathogenicity groups (PGs) on the basis of the interaction phenotypes (IP) on the differential canola cultivars Westar, Glacier, and Quinta by using a standard screening protocol in the greenhouse. Isolates in PG1 are weakly virulent as they generally cause superficial lesions on the leaves. However, isolates in PG2, PG3, and PG4 are highly virulent because they can produce stem canker at the base of the canola plant, causing significant yield loss. In Manitoba, _L. maculans_ population consists mainly of PG2 (virulent on cv. Westar; avirulent on cvs. Glacier and Quinta) and a few PG1 isolates (avirulent on all 3 differential hosts). PG3 isolates (virulent on cv. Westar and Glacier; avirulent on Quinta) are found in Europe, Australia, USA and eastern Canada. The existence of PG3 in western Canada had not been established until its isolation in 2002).

Links:
<http://www.ext.nodak.edu/extpubs/plantsci/crops/pp1024w.htm>
<http://www.pioneer.com/Australia/Products/Canola/blackleg.htm>
- Mod.DH]

[see also in the archive:
Blackleg, canola - Argentina: 1st report 20050319.0807 Blackleg, canola, path.gr.4 - Canada (MB): 1st report 20050302.0648 Blackleg, canola - USA (ND): 1st report 20050626.1807 Blackleg, canola - Australia (NSW) 20000615.0980 2003
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Blackleg, canola - Brazil (Rio Grande do Sul) 20030225.0478 Blackleg, canola - Australia (WA, NSW, SA, VIC) 20030503.1105 Blackleg, canola - Canada (Manitoba) 20030918.2367 2001
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Blackleg, root infection, canola - Australia (SA, NSW) 20010819.1959]