SCLEROTINIA STEM ROT RESISTANCE IN CRUCIFERS

 

 

Ginette S_guin-Swartz and Chrystel Lefol

 

Agriculture and Agri-Food Canada, Saskatoon Research Centre, 107 Science Place, Saskatoon, Saskatchewan S7N 0X2 Canada.

 

 

ABSTRACT

 

 

Stem rot, caused by Sclerotinia sclerotiorum, is prevalent in canola crops in western Canada. No cultivar resistant to the disease is yet available to producers. Leaf resistance to sclerotinia stem rot has been identified in plants of the wild crucifer Erucastrum gallicum (dog mustard). The objective of the research was to develop homozygous stem rot resistant dog mustard lines for cytological studies and for introgression of the resistance into cultivated brassicas. Plants exhibiting increased resistance or susceptibility to stem rot were selected by inbreeding and recurrent selection for three generations. Stem rot resistance was evaluated using a cut-leaf bioassay. One and 7 day-old petals were placed on excised leaves and inoculated with ascospores. Leaf lesion area (LLA) was measured 4 and 6 days post-inoculation (dpi). Over three generations of selection for increased resistance to stem rot, the average LLA decreased from 6.1 to 0.4 cm2 at 6 dpi with 1 day-old petals and from 8.5 to 3.8 cm2 at 6 dpi with 7 day-old petals, with some plants exhibiting no lesion development in both sets of experiments. The resistance appears to be polygenically controlled.

 

 

KEYWORDS: Erucastrum gallicum, dog mustard, disease resistance, wild crucifer

 

 

INTRODUCTION 

 

 

Sclerotinia stem rot, caused by Sclerotinia sclerotiorum (Lib.) de Bary, is endemic in the canola growing regions of western Canada. The fungus attacks all cultivars of Brassica­rapa­L. and B. napus L. canola. Extended crop rotations and fungicide applications are the only stem rot control measures available to canola growers. Yield losses due to the disease in the range of 5 to 10% are common (Fang and Platford, 1995; Platford, 1996). The primary impact of cultivars resistant to stem rot will be in stabilising canola yields. In addition, reduced fungicide applications will have a positive impact on the environment.

 

In 1996, we discovered a novel source of genetic resistance to stem rot (Lefol C. et­al., 1997a) in the crucifer Erucastrum gallicum (Willd.) O.E. Schulz (dog mustard). Using a cut-leaf bioassay, plants exhibiting increased resistance to stem rot were selected by inbreeding and recurrent selection for three generations.

 

 

MATERIAL AND METHODS

 

Plants of a dog mustard population collected near Lanigan, Saskatchewan, Canada, were grown in the greenhouse. Leaves of 8 week-old plants (n=40, 4 leaves/plant) were infested in­situ with 4 day-old petals inoculated with ascospores of S.­sclerotiorum clone 321 (Kohli et­al., 1995). One of the surviving plants (plant LA1) was selfed and its progeny was raised under controlled environment conditions (16 h photoperiod, 18ĄC). The reaction to stem rot of the plants was assessed using a cut-leaf bioassay.

 

The cut-leaf bioassay consisted in placing one petal inoculated with a 10 μl droplet of ascospores (5x103 spores/ml) on each of eight excised leaves per plant (four leaves with 1 day-old petals and four leaves with 7 day-old petals). To obtain 7 day-old petals, 1 day-old flowers were bagged and the petals were harvested 6 days later. Inoculated leaves were incubated in 100% relative humidity at ambient temperature. Leaf lesion area (LLA) was determined 4 and 6 days post-inoculation (dpi).

 

Self-seed was produced on a progeny plant of LA1 (plant LA1.5) that exhibited stem rot resistance with the cut-leaf bioassay. The reaction of the self-progenies of three plants (plants LA1.5.3, LA1.5.5, and LA1.5.7) was then determined.

 

 

RESULTS AND DISCUSSION

 

Cut-leaf bioassays using 7 day-old petals

 

Average leaf lesion area following inoculation with senescing petals is presented in Table 1. The comparison of the LLA means was performed with an unpaired t-test. At 4 dpi, significant differences (P<0.05) were found among the progenies. The progeny of LA1.5 was more resistant to stem rot than the progeny of LA1. The progenies of LA1.5.3, LA1.5.5, and LA1.5.7 were more resistant than the progeny of LA1, but more sensitive than the progeny of LA1.5. There were also significant differences (P<0.05) among progenies at 6 dpi. The progeny of LA1.5 was again more resistant than the progeny of LA1, but that the progenies of LA1.5.3, LA1.5.5, and LA1.5.7 were the most stem rot resistant. Among these progenies, the progeny of LA1.5.7 was the most resistant. Overall, after three generations of selection for increased resistance to stem rot, average LLA at 6 dpi had decreased from 8.5 to 3.8 cm2.

 

Leaf lesion area at 4 dpi ranged from 0.3 to 2.3 cm2 for the progeny of LA1, with 60% of the plants having a LLA less than 1.0 cm2. The progeny of LA1.5 ranged from 0 to 0.7 cm2, with all of the plants having a LLA less than 1.0 cm2. Leaf lesion area for progenies of LA1.5.3, LA1.5.5, and LA1.5.7 ranged from 0 to 1.0 cm2, 0 to 4.0 cm2, and 0 to 1.2 cm2, respectively. For these progenies, 95% of the plants had a LLA less than 1.0 cm2.

 

Leaf lesion area at 6 dpi ranged from 2.9 to 16.5 cm2 for the progeny of LA1, with 20% of the plants having a LLA less than 5.0 cm2. The progeny of the LA1.5 ranged from 0 to 21.0 cm2, with 70% of the plants having a LLA less than 5.0 cm2. Leaf lesion area for progenies of LA1.5.3, LA1.5.5, and LA1.5.7 ranged from 0.1 to 17.8 cm2, 0 to 27.2 cm2, and 0 to 5.2 cm2, respectively. For these progenies, 88% of the plants had a LLA less than 5.0 cm2.

 

Table 1. Average leaf lesion area (cm2±s.d.) in selected genotypes of Erucastrum gallicum inoculated with 7 day-old petals colonised by Sclerotinia­sclerotiorum.

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Genotypes                              Number                      Lesion area                      Lesion area

of plants                     4 dpi                               6 dpi

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Progeny of LA1                          10                             1.0±0.45                         8.5±4.30

 

Plant LA1.5                                  1                                  0.6                                  7.0

Progeny of LA1.5                       22                             0.3±0.05                         5.4±6.09

 

Plant LA1.5.3                               1                                  0.3                                  7.0

Progeny of LA1.5.3                    13                             0.3±0.02                         3.9±4.53

 

Plant LA1.5.5                               1                                  0.6                                 13.5

Progeny of LA1.5.5                    19                             0.5±0.98                         4.2±6.40

 

Plant LA1.5.7                               1                                  0.3                                 21.0

Progeny of LA1.5.7                    12                             0.5±0.47                         2.3±1.95

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Cut-leaf bioassays using 1 day-old petals

 

Average leaf lesion area following inoculation with 1 day-old petals was determined for the progenies of plants LA1, LA1.5.3, LA1.5.5, and LA1.5.7 (Table 2). The comparison of the LLA means was performed using an unpaired t-test. At 4 and 6 dpi, no significant differences (P<0.05) were observed between the progeny of plant LA1 and the other progenies. When comparing the progenies of LA1.5.3, LA1.5.5, and LA1.5.7 among themselves, significant differences (P<0.05) were observed, with the progeny of LA1.5.5 being the most stem rot resistant and the progeny of LA1.5.3 being the most susceptible. Over three generations of selection, average LLA at 6 dpi decreased from 6.1 to 0.4 cm2.

 

Leaf lesion area at 4 dpi ranged from 0 to 0.8 cm2 in the progeny of LA1, with 60% of the plants having a LLA less than 0.2 cm2. Leaf lesion area ranged from 0 to 0.2 cm2 in the progeny of LA1.5.3.  With the progenies of LA1.5.5 and LA1.5.7, all of the plants except one (LLA=0.06 cm2) in each progeny had no lesion. After three generations of selection, 100% of the plants had a LLA less than 0.2 cm2.

 

At 6 dpi, LLA ranged from 0.5 to 21 cm2 in the progeny of LA1, with 20% of the plants having a LLA less than 1.0 cm2. Leaf lesion area ranged from 0 to 3.3 cm2 in the progeny of LA1.5.3, 0 to 0.9 cm2 in the progeny of LA1.5.5, and 0 to 2 cm2 in the progeny of LA1.5.7. For the progeny of LA1.5.5, 100% of the plants had a LLA less than 1.0 cm2. For the progenies of LA1.5.3 and LA1.5.7, 83 % of the plants had a LLA less than 1.0 cm2.

 

 

 

 

 

Table 2. Average lesion area (cm2±s.d.) on excised leaves of Erucastrum gallicum genotypes inoculated with 1 day-old petals colonised by Sclerotinia sclerotiorum.

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Genotypes                              Number                      Lesion area                      Lesion area

of plants                     4 dpi                               6 dpi

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Progeny of LA1                          10                           0.32±0.32                       6.07±5.90

 

Progeny of LA1.5.3                    13                           0.04±0.08                       0.77±1.10

 

Progeny of LA1.5.5                    19                           0.01±0.01                       0.21±0.32

 

Progeny of LA1.5.7                    12                            0.01±0.01                       0.43±0.73

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Fresh petals of dog mustard and B.­napus canola genotypes have been shown to be less conducive to the growth of the stem rot fungus than senescing petals (Lefol C. et­al., 1997b,c), which would explain the lower LLA values obtained following inoculation with 1 day-old petals.

 

Little is known on sources of resistance to sclerotinia stem rot in crucifers and on the mode of inheritance. Baswana et­al. (1991) observed polygenic recessive resistance to stalk rot in B.­oleracea var.­botrytis (cauliflower) genotypes. Dickson and Petzoldt (1994) reported a major recessive gene to white mold in B.­oleracea var.­sabellica (collards) and recessive, possibly polygenic, resistance in B.­oleracea var.­sabauda (Savoy cabbage). The results obtained in this study indicate that wild crucifers can be a source of stem rot resistance and that highly stem rot resistant genotypes can be developed. Transferring the stem rot resistance of dog mustard to crop brassicas may be possible since the species can be hybridised to B.­rapa canola (Lefol E. et­al., 1997).

 

 

ACKNOWLEDGEMENTS

 

The authors wish to acknowledge the Agricultural Development Fund, Regina, Canada, for financial support.

 

 

REFERENCES

 

Baswana, K.S., Rastogi, K.B. and P.P. Sharma. 1991.  Inheritance of stalk rot resistance in cauliflower (Brassica oleracea var. botrytis L.). Euphytica, 57: 93-96.

 

Dickson, M.H. and R. Petzoldt. 1994. White mold or Sclerotinia sclerotiorum resistance in B.­oleracea. Cruciferae Newsletter, No. 16: 120-121.

 

 

Fang, J. and R.G. Platford. 1995. Distribution, prevalence and incidence of canola diseases in Manitoba 1994. Canadian Plant Disease Survey, 75: 145-147.

 

Kohli, Y., Brunner, L.J., Yoell, H., Milgroom, M.G., Anderson, J.B., Morrall, R.A.A. and L.M. Kohn. 1995. Clonal dispersion and spatial mixing in populations of the plant pathogenic

fungus Sclerotinia sclerotiorum. Molecular Ecology, 4: 69-77.

 

Lefol, C. and G. S_guin-Swartz. 1998. 7th International Congress of Plant Pathology, 9-16 August 1998, Edinburgh, UK.

 

Lefol, C., S_guin-Swartz, G. and R.A.A. Morrall. 1997a. Resistance to Sclerotinia sclerotiorum in a weed related to canola. Canadian Journal of Plant Pathology, 19: 113.

 

Lefol, C., S_guin-Swartz, G. and R.A.A. Morrall. 1997b. Effect of petal age on infection by Sclerotinia sclerotiorum in Erucastrum gallicum. Book of Abstracts, p. 198, Brassica 97, 23-27 September 1997, Rennes, France.

 

Lefol, C., S_guin-Swartz, G. and R.A.A. Morrall. 1997c. Sclerotinia stem rot resistance in canola petals. Canadian Journal of Plant Pathology, 19: 327.

 

Lefol, E., S_guin-Swartz, G. and R.K. Downey. 1997. Sexual hybridisation in crosses of cultivated Brassica species with the crucifers Erucastrum­gallicum and Raphanus raphanistrum: Potential for gene introgression. Euphytica, 95: 127-139.

 

Platford, R.G. 1996. Distribution, prevalence and incidence of canola diseases in Manitoba in 1995. Canadian Plant Disease Survey, 76: 103-105.