RESISTANCE TO WHITE RUST IN BRASSICA JUNCEA FROM A INTERSPECIFIC CROSS WITH B

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

ABSTRACT

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

INTRODUCTION

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.

Other methods

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

Plants Plants

Backcross Population Generation Screened Resistant % Resistant

J92-223^a BC₁F₁ 237 37 15.6

BC₁F₂ 57 18 31.6

BC₁F₃ 55 27 49.1

BC₁F₄ 70 60 85.7

AC Vulcan^b BC₁F₁ 40 10 25.0

BC₂F₁ 285 118 41.4

BC₂F₂ 165 76 46.1

BC₂F₃ 243 194 79.8

Commercial Brown^c 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 % Oil^a Allyl^b Tot. A^c Tot. G^d %Allyl^e C18:1^f C18:3^g C22:1^h

J92-223 BC₁F₅

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)

CONCLUSION

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.

ACKNOWLEDGEMENTS

This project was supported in part by grants from the Canadian Mustard Association and the Agri-Food Innovation Fund.

REFERENCES

Love, H.K., Rakow, G., Raney, J.P. and Downey, R.K. 1991. Breeding improvements towards canola quality Brassica juncea. Proceedings of the Eight International Rapeseed Congress, Saskatoon, Canada, 1, 164‑169.

Petrie, G.A. 1988. Races of Albugo candida (white rust and staghead) on cultivated Cruciferae in Saskatchewan. Canadian Journal of Plant Pathology 10, 142-150.

Pound, G.S. and Williams, P.H. 1963. Biological races of Albugo candida. Phytopathology 53, 1146-1149.

Rakow, G. 1991. Canola quality mustard. Proceedings of the Special “Cropportunites” Conference, Saskatoon, Canada, pp. 55-59.

Rakow, G., Raney, P. and Males, D. 1995. Field performance of canola quality Brassica juncea. Proceedings of the Ninth International Rapeseed Congress, Cambridge, UK, 2, 428-430.

Raney, P., Rakow, G. and Olson, T. 1995a. Development of zero erucic, low linolenic Brassica juncea utilizing interspecific crossing. Proceedings of the Ninth International Rapeseed Congress, Cambridge, UK, 2, 413-415.

Raney, P., Rakow, G. and Olson, T. 1995b. Development of low erucic, low glucosinolate Sinapis alba. Proceedings of the Ninth International Rapeseed Congress, Cambridge, UK, 2, 416-418.

Rimmer, S.R. and Buchwaldt, L. 1995. Diseases. In: Kimber, D. and McGregor, D.I. (eds.) Brassica oilseeds production and utilization. CAB International, Oxford, UK, pp. 111-140.

Thies, W. 1971. Schnelle und einfache Analysen der Fettsäurezusammensetzung in einzelnen Raps-kotyledonen I. Gaschromatographische und papierchromatographische Methoden. Zeitschrift für Pflanzenzüchtung 65, 181-202.

Thies, W. 1980. Analysis of glucosinolates via “on-column” desulfation. Proceedings of Symposium “Analytical Chemistry of Rapeseed and its Products”, Winnipeg, pp. 66-71.

Tiwari, A.S., Petrie, G.A. and Downey, R.K. 1988. Inheritance of resistance to Albugo candida race 2 in mustard (Brassica juncea (L.) Czern.). Canadian Journal of Plant Science 68, 297-300.