June 26, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of pathogenicity groups 3 and 4 of Leptosphaeria maculans on canola in North Dakota
C. A. Bradley, Department of Plant Pathology, North Dakota State University, Fargo 58105; and P. S. Parks, Y. Chen, and W. G. D. Fernando, Department of Plant Science, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. Plant Dis. 89:776, 2005; published on-line as DOI: 10.1094/PD-89-0776C. Accepted for publication 18 Apr 2005.

Blackleg, caused by _Leptosphaeria maculans_ (Desmaz) Ces. & de Not (anamorph = _Phoma lingam_), is an economically important disease of canola (_Brassica napus_ L.) worldwide and was 1st detected in North Dakota in 1991 (3). _L. maculans_ can be categorized into one of several pathogenicity groups (PGs) on the basis of the interaction phenotypes in differential canola cvs. Westar, Glacier, and Quinta by using a standard screening protocol in the greenhouse (4). With this system, PG1 strains are weakly virulent and PG2, PG3, and PG4 are highly virulent.

The predominant strains of _L. maculans_ in North Dakota are PG1 and PG2 (3). In cooperation with the Oilseed Pathology Lab in the Department of Plant Science, University of Manitoba, blackleg-infested canola stubble was collected arbitrarily from fields in North Dakota during August and September of 2003. Isolates of the pathogen were obtained by plating surface-sterilized (2 percent NaOCl), collected stubble on V8 agar containing 0.03 percent chloramphenicol at 22 C under continuous cool-white fluorescent light. Pycnidiospores were harvested from single pycnidia after 14 days of incubation with the Miracloth filtering method (2) and stored at -20 C.

Each isolate was passed once through cv. Westar to maintain virulence. Isolates were confirmed as being _L. maculans_ by the presence of characteristic pink pycnidia formed on V8 agar and the characteristic symptoms caused on inoculated cotyledons of cv. Westar. The PG test was performed using a standard screening protocol (4) and was repeated 3 times for each isolate. For each isolate, 12 7-day-old cotyledons of each differential cultivar were wound inoculated with 10 microliters of a pycnidiospore suspension (10 million spores/ml). Disease severity on cotyledons was assessed 12 days after inoculation with a 0 to 9 scale (0 to 2 = resistant; 3 to 6 = intermediate; and 7 to 9 = susceptible).

A total of 106 isolates were obtained from the stubble collected from 47 fields. Of these isolates, 3 were characterized as PG1, 94 as PG2, 6 as PG3, and one as PG4; 2 isolates could not be characterized according to the PG system as described (4). PG3 isolates originated from 2 fields in Cavalier County and one field in Ward County. The PG4 isolate was from Cavalier County.

To our knowledge, this is the 1st time highly virulent strains of PG3 and PG4 have been detected in North Dakota. PG3 and PG4 strains of _L. maculans_ were found only recently in western Canada (1,2).

The discovery of these PGs in North Dakota and western Canada has immense implication to canola breeding programs and blackleg control, since these PGs may cause greater levels of blackleg severity on canola cultivars that are resistant to only PG2 type isolates.

References:
(1) Y. Chen and W. G. D. Fernando. Plant Dis. 89:339, 2005.
(2) W. G. D. Fernando and Y. Chen. Plant Dis. 87:1268, 2003.
(3) H. A. Lamey and D. E. Hershman. Plant Dis. 77:1263, 1993.
(4) A. Mengistu et al. Plant Dis. 75:1279, 1991.

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[Of the 106 isolates, the majority 94 were PG2 (89 percent), 3 were PG1 (3 percent), 3 were PG3 (one percent) and one was PG4 (one percent). But the most significant aspect of this report is that these PGs have to be considered in plant breeding programs.

Most isolates of _L. maculans_ in western Canada belong to PG2, but PG3 isolates were recently found in Manitoba, and, in this report, PG4. PG3 isolates are aggressive to Q2 and Defender, and additional susceptible cultivars will undoubtedly be identified in future. This does not mean that all varieties will be attacked by PG3, because genes conferring resistance to PG3 have been probably incorporated into many current canola varieties.

Varieties originating from areas where PG3 is endemic have long been used to breed Canadian canola, so resistance genes have almost certainly been introduced. PG3 distribution in western Canada is currently unknown, but the strain may already be present in Canadian blackleg nurseries, so many varieties may have already been
"pre-screened" for resistance. The appearance of PG4 may be evidence of pathogen population changes occurring under high-selection-pressure exerted by resistance genes in commercial cultivars, or through importation of PG4 isolates with canola seed.

Australian plant breeders identified a new source of blackleg resistance _Brassica sylvestris_ [Bs], a wild relative of canola. The resistance was successfully incorporated into canola, resulting in an almost immune response to blackleg. After further breeding, Pacific Seeds released varieties for the Australian market in 2000, and their adoption by growers has led to substantially reduced yield losses associated with blackleg. However, in early 2003, Pacific Seeds issued a press release stating that blackleg stem cankers had been observed on these varieties which had previously been immune to blackleg. In addition to the 2 sites identified by Pacific Seeds
(Port Lincoln, SA and Cudal, NSW), Agriculture WA also reported the breakdown of Bs resistance at Mt. Barker in Western Australia. During 2003, the strain of blackleg able to attack the Bs resistance spread across Eyre Peninsula, causing an estimated cost of 20 million dollars to local farmers. At some locations, varieties based on this type of resistance are now very susceptible to blackleg and may experience almost complete yield loss. Blackleg strains able to attack plants containing the Bs resistance appear to be unable to infect varieties containing polygenic resistance. This development is of considerable concern to farmers and pathologists to the say the least.

Links: <http://www.ext.nodak.edu/extpubs/plantsci/crops/pp1024w.htm>
<http://www.grdc.com.au/growers/res_upd/south/04/marcroft.htm>
- Mod.DH]

[see also in the archive:
Blackleg, canola, path.gr.4 - Canada (MB): 1st report 20050302.0648 Blackleg, canola - Argentina: 1st report 20050319.0807 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 2000
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Blackleg, canola - Australia (NSW) 20000615.0980]