March 2, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of canola blackleg caused by pathogenicity group 4 of Leptosphaeria maculans in Manitoba
Y. Chen and W. G. D. Fernando, Department of Plant Science, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. Plant Dis. 89:339, 2005; published on- line as DOI: 10.1094/PD-89-0339B. Accepted for publication 13 Dec 2004.

_Leptosphaeria maculans_ (Desmaz.) Ces. & de Not., causal agent of blackleg of canola (_Brassica napus_ L.), was initially placed in several pathogenicity groups (PG) on the basis of the interaction phenotypes (IP) of
_L. maculans_ isolates on the differential canola cvs. Westar (W), Glacier (G), and Quinta (Q) (4). PG1 isolates are weakly virulent and PG2, PG3, and PG4 isolates are highly virulent. In Manitoba, the _L. maculans_ population
consists mainly of PG2 isolates (virulent on W and avirulent on G and Q), a few PG1 isolates (avirulent on W, G, and Q), and PGT (virulent on W and Q, but avirulent on G) (3).

Since the blackleg fungus is known to have a high level of evolutionary potential, the Oilseed Pathology Laboratory at the University of Manitoba, Winnipeg, Canada, examines the pathogenic variability of _L. maculans_ isolates from the Canadian Prairies and North Dakota each year. During 2002, the presence of PG3 (virulent on W and G and avirulent on Q) was reported in Manitoba (1). During 2003, a canola field located at La Riviere, Manitoba, 200 km southwest of Winnipeg, was found to be severely affected by blackleg.

Stubble from this field was arbitrarily collected in mid-April 2004, and 98 single-pycnidia pure cultures were obtained by isolating fungi from surface- sterilized (2 percent sodium hypochlorite), infested residue, cultured on V8 agar at room temperature under cool-white florescent light for 24 h. Pycnidiospores were harvested after 14 days of incubation using the Miracloth filtering method (1). PG testing was performed using the three differential
cultivars in the greenhouse. Known PG2, 3, and 4 isolates, 86-12, Liffole-6, and PL30.2, respectively, were included as positive controls.

For each of the 98 isolates, 12 7-day-old cotyledons of each differential cultivar grown in Metro Mix were wound-inoculated with 10 microliters of a pycnidiospore suspension (10 million spores per ml) (1). Inoculated plants
were maintained in the greenhouse (16/21 deg C night/day and a 16-h photoperiod with cool-white florescent light). The experiment was repeated 3 times. 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).

Of the 98 isolates tested, 5 were PG1, 51 were PG2, 24 were PG3, 13 were PGT, and 5 were PG4. The isolates classified as PG4 gave IP reactions of 7 to 9, 7 to 9, and 6.6 to 8.2, on W, G, and Q, respectively. PG3 was reported one year ago, but highly virulent isolates belonging to PG4 have not been previously detected in Manitoba.

To our knowledge, this is the 1st report of the occurrence of PG4 isolates of _L. maculans_, and the 1st report of PG4 causing canola blackleg in Manitoba. The appearance of PG4 may be evidence of pathogen population changes occurring under high-selection-pressure exerted by resistance genes in commercial cultivars (2), or through importation of PG4 isolates with canola seed.

References:
(1) W. G. D. Fernando and Y. Chen. Plant Dis. 87:1268, 2003.
(2) B. J. Howlett. Can. J. Plant Pathol. 26:245, 2004.
(3) M. Keri et al. Can. J. Plant Pathol. 23:199, 2001.
(4) A. Mengistu et al. Plant Dis. 75:1279, 1991.

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[Blackleg in western Canada will continue to change, and new strains will continue to appear in canola fields. Farmers will have to be prepared to adapt to new varieties and means of identifying new strains of the pathogen.
The industry's action plan for coping with new disease strains includes ongoing evaluation of varietal resistance over time, developing a system to identify new strains, and developing polygenic resistance to the blackleg
pathogen. New strains (PG3 and PGT) were identified in 2003. PG3 has been confirmed only in Manitoba so far. Most current commercial varieties appear to have resistance to the new PG strains, but pathologists agreed they can't yet advise growers which ones may be resistant. Disease management depends upon planting canola no more than once in 3 years and choosing varieties rated as resistant or moderately resistant to blackleg. - Mod.DH]

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