AAFC Research Centre, 107 Science Pl., Saskatoon, SK S7N 0X2, Canada

Background and objectives
Stem rot, caused by Sclerotinia sclerotiorum (Lib.) de Bary, is prevalent in canola crops in western Canada. No cultivars resistant to the disease are yet available to producers. Tolerance to sclerotinia stem rot was identified recently in the wild crucifer Erucastrum gallicum (dog mustard) [1]. Stem rot infection in crucifers relies on the growth of the fungus on senescent floral parts that have fallen on leaves and in leaf axils. Lefol etal. [1] observed that in some dog mustard plants, petal colonization by the fungus was limited and leaf lesion development was reduced. Plants exhibiting the petal tolerance trait were selected by inbreeding and recurrent selection for stem rot tolerance.

Materials and methods
Plants of a dog mustard population collected in Saskatchewan were raised in the greenhouse. Leaves of eight week-old plants were inoculated with petals infested with ascospores of S. sclerotiorum clone 321 [2]. Self-progeny of surviving plants LA1 (LA1.1 to LA1.10) and LA6 (LA6.1 to LA6.49) were raised under controlled environment conditions (16 h photoperiod, 18C). Self-progeny was also produced on plants LA1.1, LA1.5, LA1.7, and LA1.10. To test the stem rot reaction of the plants, seven day-old petals were harvested from the LA1 and the LA6 self-progeny plants, and from 20 progeny plants each of LA1.1, LA1.5, LA1.7, and LA1.10. Flowers at anthesis were enclosed individually in glassine bags and petals were harvested after a predetermined time. In addition, one day-old petals were collected from the LA6 self-progeny plants and air-dried at room temperature for 7 and 10 days (artificially aged petals). Petals were placed on excised leaves and inoculated with a 10 l drop of ascospores (5000 spores/ml). The leaves were then incubated at room temperature in a humidity chamber (100% RH). Lesion area was measured four days post-inoculation.

Results and conclusions
Mean lesion area was 1.02 cm2 (s.d.=0.40) for plants LA1.1 to LA1.10 and 0.58 cm2 (s.d.=0.21) for plants LA6.1 to LA6.49. The reaction of two stem rot tolerant plants (LA1.5 and LA1.10) and two susceptible plants (LA1.1 and LA1.7) was investigated further. Mean lesion area was 0.28 cm2 (s.d.=0.22) for LA1.5.1 to LA1.5.20 and 1.41 cm2 (s.d.=0.80) for LA1.10.1 to LA1.10.27. For the self progenies of stem rot susceptible plants, mean lesion area was 1.49 cm2 (s.d.=1.20) for LA1.1.1 to LA1.1.21 and 1.45 cm2 (s.d.=0.90) for LA1.7.1 to LA1.7.21. Both stem rot tolerant and susceptible plants were recovered in the self-progeny of stem rot tolerant plants, but only stem rot susceptible plants were recovered in the self-progeny of susceptible plants. Further generations of inbreeding will be required to derive homozygous stem rot tolerant lines.

The effect of petal age on leaf lesion development was studied in self-progeny of plant LA6 (LA6.1 to LA6.49). Mean lesion area was significantly lower with one day-old petals (0.03 cm2, s.d.=0.01) than with seven day-old petals (0.58 cm2, s.d.=0.21). Mean lesion area obtained with seven day-old artificially aged petals (0.78 cm2, s.d.=1.13) and 10 day-old artificially aged petals (0.50 cm2, s.d.=0.46) was not significantly different from that obtained with naturally aged petals, indicating that artificially aged petals could be used in the stem rot tolerance assays.

1. Lefol C, Seguin-Swartz G, Morrall RAA, 1997. Canadian Journal of Plant Pathology 19, 113.
2. Kohli Y, Brunner LJ, Yoell H, Milgroom MG, Anderson JB, Morrall RAA, Kohn LM, 1995. Molecular Ecology 4, 69-77.