Resistance to white rust in Brassica juncea from an interspecific cross with B. napus
Richard K Gugel, J Philip Raney, Lorne J Duczek,
Gerhard FW Rakow and Todd V Olson
Agriculture and Agri-Food Canada, Saskatoon Research Centre, 107 Science Place, Saskatoon, SK, Canada, S7N 0X2
To improve oil quality in zero erucic acid, low glucosinolate Brassica juncea germplasm, selected lines were crossed with a line of low (2%) linolenic acid B. napus. The resulting interspecific F1 generation was backcrossed several times to elite B. juncea germplasm. Because B. napus is highly resistant to white rust, successive backcross generations of this material were screened for resistance to Albugo candida race 2v; resistance to this race of white rust had not previously been observed in B. juncea. Segregation for resistance to race 2v was observed in the backcross generations and resistant plants were advanced in the breeding program. Through continued backcrossing and crossing, this trait has been introgressed into ‘canola quality’ B. juncea, oriental mustard and brown mustard. Data from greenhouse grown plants is presented and results of field nursery trials of selected lines are discussed. This material is now being used in the Saskatoon Research Centre breeding program to develop improved ‘canola quality’ B. juncea and oriental and brown mustard varieties for western Canada.
KEYWORDS: Albugo candida, white rust, oriental mustard, brown mustard
White rust is an important disease of oriental and brown mustard (Brassica juncea) and Polish canola (B. rapa). Race 2 of Albugo candida attacks B. juncea (Pound and Williams 1963, Petrie 1988). Resistance to the originally isolated strain of this fungus, race 2a, has been identified (Petrie 1988) and incorporated into oriental mustard cultivars such as Cutlass and AC Vulcan. Inheritance studies indicate that the resistance is controlled by a single dominant locus (Tiwari et al. 1988). Recently, another strain of white rust, race 2v, has been identified for which there is no resistance available in B. juncea (Rimmer and Buchwaldt 1995). However, B. napus is highly resistant to both races 2a and 2v. This paper describes the identification of B. juncea lines with resistance to race 2v from an interspecific cross of B. juncea with B. napus.
MATERIALS AND METHODS
In 1992 a line of ‘zero’ erucic acid, low glucosinolate B. juncea (Love et al. 1991, Rakow 1991), J90-4253, which had been screened for resistance to white rust race 2a, was crossed with a line of low linolenic B. napus. The F1 was then backcrossed to J90-4253 and the BC1F1 progeny advanced to BC1F3 via selfing and half-seed selection for low linolenic acid (Raney et al. 1995a). The BC1F3 generation was then either crossed with J92-223, an improved ‘canola quality’ B. juncea (Rakow et al. 1995), or crossed with AC Vulcan.
Cross to J92-223
F1 plants from the cross to J92-223 were selfed and the F2 generation was either selfed or crossed to Commercial Brown. The selfed F3 seed was then backcrossed to J92-223, and the BC1F1 plants from this cross were screened for resistance to white rust race 2v. Resistant plants were selfed, harvested and the seed analyzed for glucosinolate content. This process was repeated on selected low glucosinolate plants at each generation up to and including the BC1F4 generation. Selfed BC1F5 seed from 18 plants rated as resistant was planted in a single row, three replicate field nursery in 1998.
Crosses to AC Vulcan
F1 plants from the cross to AC Vulcan were selfed and the F2 plants were backcrossed to AC Vulcan. BC1F1 plants were half-seed selected for normal erucic acid content and selected plants were screened for resistance to white rust race 2v. Normal erucic acid, white rust resistant plants were backcrossed to AC Vulcan. BC2F1 plants were then screened for white rust resistance, and resistant plants were selfed, harvested and the seed analyzed for glucosinolate content. This process was repeated on selected high glucosinolate plants at each generation up to and including the BC2F3 generation. Selfed BC2F4 seed from 105 plants rated as resistant was planted in a single row, three replicate field nursery in 1998.
Crosses to Commercial Brown
F1 plants from the cross to Commercial Brown were selfed and the F2 plants were half-seed selected for normal erucic acid content. Selected plants were screened for resistance to white rust race 2v, and normal erucic acid, white rust resistant plants were backcrossed to Commercial Brown. BC1F1 plants were screened for white rust resistance, and selected plants were crossed to Commercial Brown. BC2F1 plants were then screened for white rust resistance, and resistant plants were selfed, harvested and the seed analyzed for glucosinolate content. This process was repeated on selected high glucosinolate plants in the BC2F2 generation. BC2F3 seed from 77 plants rated as resistant was planted in a single row, three replicate field nursery in 1998.
White rust evaluation method
Seedlings were sprayed with a suspension of A. candida race 2v zoospores (10,000/ml) 12-14 days after planting, when the plants were in the first leaf stage. Immediately after inoculation, plants were incubated in the dark in a misting chamber (100% RH) for 24 hours. Disease severity was recorded 10 days after inoculation using a 0 to 5 scale: 0 (no visible disease), 1 (Small, pinpoint necrotic flecks on upper leaf surface), 2 (Small pinpoint necrotic flecks on both upper and lower leaf surfaces), 3 (Few minute scattered pustules on upper leaf surface), 4 (Few small pustules on both upper and lower leaf surfaces), 5 (Many pustules on lower leaf surface). Plants with a ‘0’ rating were classified as resistant and advanced to the next generation. Commercial Brown was used as a susceptible check in all three populations, AC Vulcan was used as a susceptible check with material crossed to AC Vulcan, and J92-223 was used a susceptible check with material crossed to J92-223.
The fatty acid composition of seed samples was determined by gas chromatography of the methyl esters (Thies 1971, Raney et al. 1995b). The glucosinolate content of seed was determined by gas chromatography of the trimethylsilyl derivatives of the desulphoglucosinolates (Thies 1980, Raney et al. 1995b). Total aliphatic glucosinolate content is reported as the sum in µmoles/g seed of allyl, 3-butenyl, 4-pentenyl, 2-hydroxy-3-butenyl and 2-hydroxy-4-pentenyl glucosinolates. Oil content of harvested nursery rows was measured on dried, intact seed using a continuous-wave, low-resolution nuclear magnetic resonance instrument. The field nurseries consisted of single rows three metres long with three replicates. AC Vulcan and Forge were included as checks for the AC Vulcan backcross lines. Commercial Brown and Scimitar were included as checks for the Commercial Brown backcross lines. J92-223 was included as a check for the J92-223 backcross lines.
RESULTS AND DISCUSSION
Selection for resistance to white rust race 2v
Resistance to white rust race 2v was readily observed in progeny of all three crosses (Table 1). As expected, due to the dominant nature of the white rust resistance trait (Tiwari et al. 1988), F1 plants were resistant. The F1 plants and some selfed plants in successive generations segregated susceptible progeny as well as resistant. Other selfed resistant plants segregated only resistant progeny. As expected with continued selfing, the percentage of plants carrying resistance increased.
Table 1. Number of plants resistant to white rust race 2v observed at each generation in three backcross populations of Brassica juncea
Backcross Population Generation Screened Resistant % Resistant
J92-223a BC1F1 237 37 15.6
BC1F2 57 18 31.6
BC1F3 55 27 49.1
BC1F4 70 60 85.7
AC Vulcanb BC1F1 40 10 25.0
BC2F1 285 118 41.4
BC2F2 165 76 46.1
BC2F3 243 194 79.8
Commercial Brownc F2 131 8 6.1
BC1F1 141 55 39.0
BC2F1 180 60 33.3
BC2F2 300 132 44.0
a. Crosses to J92-223
b. Crosses to AC Vulcan
c. Crosses to Commercial Brown
A wide variation in performance was observed in the 1998 field nursery. BC1F5 lines derived from J92-223 generally performed poorly; no line was rated equal to J92-223. The performance of BC2F4 lines derived from AC Vulcan was significantly better; in fact, some lines were equal to or better than AC Vulcan. BC2F3 lines derived from Commercial Brown ranged in performance from poor to equal to Commercial Brown. Seed quality data for the best performing lines of each backcross population is presented in Table 2. The oil content of all J92-223 BC1F5 lines was lower than the parent; however, there were lines with nearly equivalent glucosinolate content and fatty acid composition. As expected, all lines had zero or nearly zero erucic acid contents. AC Vulcan BC2F4 and Commercial Brown BC2F3 lines were equivalent to their respective backcross parent lines in essentially all seed quality traits. These lines have been entered in replicated yield trials in 1999 to verify our initial observations.
Table 2. Seed quality data for top performing white rust resistant lines and checks in the 1998 Brassica juncea field nursery
Backcross/Line % Oila Allylb Tot. Ac Tot. Gd %Allyle C18:1f C18:3g C22:1h
T097-3395-2 39.6 0.3 9.2 13.4 2.1 44.0 7.9 0.1
T097-3395-10 39.2 0.2 14.3 18.2 1.3 45.1 8.0 0.1
T097-3398-1 38.9 0.2 17.7 21.8 0.8 40.0 9.0 0.1
T097-3395-11 37.9 0.2 11.9 16.5 1.1 44.9 9.2 0.1
T097-3395-5 36.8 0.6 15.0 18.2 3.4 43.3 8.1 0.1
J92-223 41.6 0.3 8.7 13.3 2.3 47.7 9.4 0.1
AC Vulcan BC2F4
TO97-3218-5 39.0 123.4 125.7 127.8 96.6 20.2 9.9 23.6
TO97-3218-3 39.1 132.8 135.1 137.8 96.4 21.0 9.6 23.1
TO97-3218-2 38.7 126.3 128.5 130.9 96.5 19.9 10.0 23.9
TO97-3213-1 36.5 144.6 147.0 149.1 97.0 18.3 9.9 25.7
TO97-3220-5 37.7 123.1 125.1 127.3 96.7 21.7 9.1 22.5
AC Vulcan 39.4 126.9 129.7 131.8 96.2 20.4 11.6 23.6
Forge 36.7 116.0 118.0 120.5 96.2 24.8 9.2 18.8
Commercial Brown BC2F3
TO97-2576-16 36.9 118.6 121.0 124.2 95.5 22.4 11.0 22.8
TO97-2577-16 34.7 113.6 115.5 118.2 96.1 21.8 12.7 22.7
TO97-2576-5 36.4 118.0 121.5 124.0 95.2 22.8 10.1 23.1
TO97-2579-19 38.7 101.6 103.2 106.0 95.9 24.7 11.6 20.5
TO97-2576-7 35.9 109.3 111.3 115.0 95.1 20.6 11.2 23.8
Comm.Brown 37.6 97.8 99.3 103.0 95.0 22.9 11.3 22.0
Scimitar 37.2 107.0 108.7 111.3 96.2 19.6 12.2 22.7
a. Percent of dry seed e. Allyl glucosinolate as percent of total glucosinolate
b. Allyl glucosinolate (µmoles/g seed) f. Oleic acid (Percent of total fatty acids in oil)
c. Total aliphatic glucosinolate (µmoles/g seed) g. Linolenic acid (Percent of total fatty acids in oil)
d. Total glucosinolate (µmoles/g seed) h. Erucic acid (Percent of total fatty acids in oil)
Resistance to white rust race 2v has been identified in lines of B. juncea derived from an interspecific cross with B. napus. This trait has been introgressed into ‘canola quality’ B. juncea, oriental mustard and brown mustard through backcrossing and reselection. Use of this resistance in the breeding of new cultivars of both condiment and canola quality mustard will help improve and stabilize production of these crops in western Canada.
This project was supported in part by grants from the Canadian Mustard Association and the Agri-Food Innovation Fund.
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