First report on the
presence of Leptosphaeria maculans pathogenicity group-3, the
causal agent of blackleg of canola, in Manitoba
WGD Fernando, Y Chen, Department of
Plant Science, University of Manitoba, Winnipeg, MB R3T 2N2,
Canada. Plant Dis 2003; 87: 1268, published online as
D-2003-0808-02N, 2003. Accepted for publication 23 Jul 2003.
Blackleg, caused by _Leptosphaeria
maculans_ (Desmaz.) Ces. & De Not. (anamorph = _Phoma lingam_)
(Tode:Fr.) Desmaz.), is an economically important and serious
disease of canola (_Brassica napus_ L.) in Australia, Europe,
and Canada.
L. maculans_ isolates can be categorized
into four pathogenicity groups (PGs) on the basis of the
interaction phenotypes (IP) on the differential canola cvs.
Westar, Glacier, and Quinta (1) by using a standard screening
protocol in the greenhouse.
PG1 isolates are weakly virulent, and
PG2, PG3, and PG4 isolates are highly virulent. In Manitoba,
[the] _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 three differentials).
The Oilseed Pathology Lab in the
Department of Plant Science, University of Manitoba examines the
pathogenic variability of blackleg isolates obtained from
Manitoba each year. In 2002, the blackleg-resistant cv. Q2, was
found to be severely infected in Roland, Manitoba. The canola
stubble collected from a coop trial plot (Roland, Manitoba) and
a farm in East Selkirk (60 km northeast of Winnipeg, Manitoba)
was isolated for the blackleg fungus.
Small pieces of stubble were cut from
the pseudothecia-forming section and surface-sterilized with 1
per cent sodium hypochlorite solution for 3 to 5 min and then
rinsed in sterile distilled water. V8 agar medium containing 1
per cent streptomycin sulphate was used to culture the isolates
under continuous cool-white fluorescent light for 14 days. Pure
cultures of the pathogen were isolated and characterized as _L.
maculans_ by means of colony morphology, pycnidia, and
microscopic observations of pycnidiospores.
Pycnidiospores that formed on V8 plates
were flooded with 10 ml of sterile distilled water and then
harvested by filtering through sterilized Miracloth and kept at
-20 deg C. The isolates were passed once through cv.
Westar to maintain their virulence. The
PG test was performed with the 3 differential cultivars. 2
additional cultivars, Q2 (resistant to PG2 isolates) and
Defender (moderately resistant to PG2 isolates), were included
for comparisons.
12 cotyledons of each differential
cultivar (7 days old) grown in Metro Mix were wound-inoculated
with a 10-microliter droplet of pycnidiospore suspension (10 000
000 pycnidiospores per ml). Inoculated cotyledons were
maintained in the greenhouse (16/21 deg C night/day and a 16-h
photoperiod). The experiment was repeated twice. Disease
severity on cotyledons was assessed 12 days postinoculation by
using a 0 to 9 scale (2).
All five isolates from Roland and East
Selkirk were highly virulent on Glacier (6.4 to 7.7), Q2 (7.1 to
8.2), and Defender (7.2 to 8.4), but intermediately virulent on
Quinta (4.5 to 5.4). This clearly indicated that these
isolates were of PG3. Isolates of PG2 have been predominant in
Manitoba for the past 25 years, and highly virulent isolates
belonging to PG3 had not been detected previously.
To our knowledge, this is the first
report of the presence of PG3 in _L. maculans_ in Manitoba.
References:
(1) A Mengistu, et al. Plant Dis 1991; 75: 1279.
(2) PH Williams. Crucifer Genetics Cooperatives (CrGC) Resource
Book. University of Wisconsin-Madison, 1985.
