Falak I., Thompson P., McNabb W., Grombacher A. and Dwipak S.
Pioneer Hi-Bred Production Ltd., 12111 Mississauga Rd. R.R.#4
Georgetown - Ontario - L7G 4S7 - Canada
Canola quality Brassica rapa grown in Canada is susceptible to blackleg disease caused by Leptosphaeria maculans. Blackleg infected B. rapa plants can suffer significant yield losses. In order to improve this weakness of B.rapa canola two approaches were undertaken.
A source of resistance was identified within Ethiopian B. rapa non-canola germplasm. Through crossing and greenhouse/field selection for resistance, blackleg resistant canola quality lines were developed.
Two blackleg resistant Brassica napus varieties were crossed with B.rapa and via further backcrossing and selection for blackleg resistance, B. rapa lines with a high level of blackleg resistance were developed.
The field disease reaction of greenhouse selected B. rapa lines developed from all three sources was variable and it ranged from moderately susceptible, moderately resistant to resistant when categorized in comparison with the susceptible B.rapa parent. These results imply that resistance to blackleg in B.rapa can be attained by introducing genetic variability through intraspecific crossing with B.rapa or interspecific crossing with B.napus sources of blackleg resistance.
KEYWORDS: Polish canola, Canada , Leptosphaeria maculans, susceptibility, breeding
Blackleg, caused by the fungus Leptosphaeria maculans (Desm.) Ces et ce Not. has resulted in extensive losses in canola crops throughout Canada. The virulent strain of blackleg was first detected in 1975 in east central Saskatchewan. Since then the disease has spread rapidly to most of canola growing areas of Saskatchewan and into Manitoba and Alberta. Considerable effort in breeding for blackleg resistant B. napus has resulted in the development of resistant varieties such as Quantum from the University of Alberta in 1995 and 46A65 by Pioneer Hi-Bred in 1996. Subsequently, several more resistant varieties have been released.
However, progress in breeding for blackleg resistance in B. rapa has lagged behind that of
B. napus for several reasons including the relative lack of genetic variability for the trait, the difficulty of breeding a self-incompatible crop and the regional importance of the crop. Another factor was a relatively slow spread of the virulent blackleg into the major B. rapa growing region located in Alberta.
At present the B. rapa acreage in Canada has plummeted and susceptibility to blackleg, among other factors, is presumed to have contributed to the decline. Various public and private breeding programs attempted to develop B. rapa resistant to blackleg by using different sources of resistance like B.juncea (Gugel et al., 1994) and Erucastrum gallicum (R.K.Gugel, G,Seguin-Swartz, and S.I.Warwick. 1996). We report here on generating B. rapa canola quality material with resistance to blackleg that originates from Ethiopian B. rapa and Canadian/Australian B. napus varieties.
1. Sources of resistance/incorporation into canola quality background
Field resistance to blackleg was identified in a wild Brassica rapa accession originating from Ethiopia, at blackleg nursery in Elgin, Manitoba. B. napus blackleg resistant varieties derived from breeding programs in Canada and Australia were also used as sources of resistance. These three sources were crossed with canola quality B. rapa. For crosses made with the non-canola quality Ethiopian accession, the F2 generation was screened for erucic acid prior to blackleg screening. This was followed by four backcrosses to the canola quality parent with selection for blackleg resistance in the greenhouse at each generation. At the BC4 level, plants were screened for blackleg and resistant plants were intercrossed. Seed from single plants were evaluated in a breeding nursery for agronomic traits and later analyzed for quality traits. The best performing lines were evaluated for blackleg disease resistance and yield.
For the material with the B. napus background, the material went through a backcrossing program to the BC3 level and underwent selection for blackleg resistance in the greenhouse at each generation. Following screening of the BC3s, selected plants were intercrossed. Seed from these single plants underwent erucic acid and glucosinolate analysis. Selected lines were increased to produce seed for field testing.
2. Greenhouse screening for reaction to blackleg
A western Canadian PG2 isolate of blackleg from Saskatchewan, ‘Leroy’, was used for all greenhouse screenings that were performed. Plants were inoculated at growth stage 3.2 (Harper & Berkenkamp, 1975) by injecting a 10 ul droplet of a pycnidiospore suspension (1 x 10 7 spores/ml) into the stem between the second and third node. Blackleg resistant plants were initially selected three weeks post-inoculation and further selected five weeks post-inoculation based on the extent of external lesion development in comparison to the susceptible check, Tobin. Resistant selections were crossed further to the recurrent B. rapa parent.
3. Field screening for reaction to blackleg
In total, 9 selections from the Ethiopian source, 11 selections from the B. napus Australian source and 33 selections from the Pioneer source were tested for disease reaction in the field. Each trial included susceptible B. rapa varieties, Maverick, Reward and Parkland as checks. Trials were planted in a RCB design with three replicates. The field experiments were conducted in Elgin, Manitoba in the 1998 season in a field naturally infested with L. maculans. Plants were also sprayed with a suspension of pycnidiospores of a local PG2 isolate, LM26, prior to the six true leaf stage. Material was rated just prior to physiological maturity, growth stage 5.2 (Harper & Berkenkamp,1975) for the severity of basal stem cankers by examining internal damage. Plants were rated on a 1 – 9 scale as follows:
1 – Plant dead
3 – 51-75% of internal tissue diseased
5 – 25-50% of internal tissue diseased
7 - <25% of internal tissue diseased
9 – Plant clean, no lesions.
RESULTS
In the greenhouse, plants with improved levels of blackleg resistance were identified in each successive backcross generation. The number of selected plants at each backcross varied from 20-40 plants.
A moderate level of blackleg disease was present in the field trials. The most resistant selections from all three sources had a similar effect of reducing disease severity in the field when compared with the check varieties. The disease reaction of selected B. rapa lines developed from all three sources was variable and it ranged from moderately susceptible (MS), moderately resistant (MR) to resistant (R) (Table 1). Of the 9 lines with the Ethiopian background, 3 were significantly more resistant than the highest rated B. rapa check. From the Pioneer source, 19 of the 33 lines were significantly more resistant while from the Australian source, 8 of the 11 lines were significantly more resistant.
Table 1: Blackleg reaction of B. rapa lines derived from three sources of resistance in field trials
B. rapa Source – Ethiopian |
B. napus Source – Pioneer |
B. napus Source – Australian |
||||||
|
Line I.D. |
Blackleg Score |
|
Line I.D. |
Blackleg Score |
|
Line I.D. |
Blackleg Score |
|
|
CS4066 CS4071 CS4067 Maverick Parkland Reward Check mean |
8.2 7.6 7.0 5.6 6.0 6.7 6.1 |
R MRMS
|
CS4390 CS4396 CS4395 Maverick Parkland Reward Check mean |
8.6 7.6 6.7 5.3 4.9 5.8 5.3 |
R MR MS |
CS4386 CS4380 CS4382 Maverick Parkland Reward Check mean |
8.1 7.5 6.6 5.3 4.9 5.8 5.3 |
R MR MS |
Additional field testing of selected lines in 1999 is expected to provide more insight into the level of resistance being attained.
Canadian B.rapa germplasm is susceptible to blackleg. A number of approaches to introduce some genetic variability for this trait have been undertaken. At Pioneer Hi-Bred, resistance to blackleg was introduced from three unrelated sources, from wild Brassica rapa and two Brassica napus varieties. The level of resistance that is attained varies. The most resistant selections from all three sources have a similar effect of reducing disease severity in the field. Segregation for different degrees of resistance/susceptibility in field reaction to blackleg occurred within all three sources. It is presumed that this is a function of a number of genes conferring the trait, their relative efficacy and frequency within a line. A greater level of stability of disease resistance could be achieved by combining these sources of resistance.
A significant reduction in blackleg disease severity within material that was generated should minimize yield losses under disease pressure in the field in Western Canada and provide enhanced yield stability.
REFERENCES
R.K.Gugel, D.S.Hutcheson, G.F.W. Rakow and K.C.Falk. 1994. Transfer of Blackleg Resistance from Brassica juncea Condiment Mustard to B. rapa Canola. Blackleg News 2:3-4
R.K.Gugel, G,Seguin-Swartz, and S.I.Warwick. 1996.Transfer of blackleg resistance from Erucastrum gallicum to Brassica rapa. The Canadian Phytopathological Society. 67th Annual Meeting Saskatoon.
F.R. Harper and B. Berkenkamp. 1975. Revised growth-stage key for Brassica campestris and B. napus . Can. J. Plant Sci. 55: 657-658.