Races of Puccinia graminis on barley, oat and wheat in Canada in 2006

Abstract

Stem rust, caused by Puccinia graminis, is a disease of cereal crops worldwide. In Canada, it is primarily controlled using resistant cultivars. Tracking the virulence structure in the pathogen populations is essential to detect new virulent races that may occur. Surveys of barley (Hordeum vulgare), oat (Avena sativa) and wheat (Triticum aestivum) fields and trap nurseries were conducted to provide incidence and severity information and to identify the virulence structure in the pathogen populations. Stem rust samples were collected in Manitoba and Saskatchewan in 2006. Stem rust was not found on cultivated wheat and was at trace levels in barley and oat fields. From wheat and barley samples, four races of P. graminis f. sp. tritici were found and QFCSC (89%) was predominant. Thirteen races of P. graminis f. sp. avenae were found in 2006. From cultivated oat samples, the predominant races were TJJ (67%), TJS (17%), and TGD (9%). From wild oat samples, the predominant races were TJJ (40%), TGD (23%), and TGB (12%). Two new races of P. graminis f. sp. avenae (TGN and TJN) with virulence to Pg12 but avirulence to Pg13 were detected on both cultivated and wild oat both in Manitoba and Saskatchewan in 2006.

Résumé

La rouille noire, causée par Puccinia graminis, est une maladie des céréales répandue á l’échelle de la planète. Au Canada, on la combat á l'aide de cultivars résistants. Afin de déceler l'occurrence de nouvelles races virulentes, il importe de suivre l’évolution de la structure de virulence chez les populations d'agents pathogènes. Des études ont été menées dans des champs d'orge (Hordeum vulgare), d'avoine (Avena sativa) et de blé (Triticum aestivum), de même que dans des pépinières-pièges, afin d'obtenir de l'information sur l'incidence ainsi que sur la gravité de la maladie et de déterminer la structure de virulence chez les populations d'agents pathogènes. Des échantillons de rouille noire ont été collectés au Manitoba et en Saskatchewan en 2006. Le blé cultivé en champs était exempt de rouille noire et l'on en a trouvé que des traces sur l'orge et l'avoine. On a trouvé quatre races de P. graminis f. sp. tritici sur les échantillons de blé et d'orge, et QFCSC était la plus courante (89%). Treize races de P. graminis f. sp. avenaeont été trouvées en 2006. Dans les échantillons d'avoine cultivée en champ, TJJ (67%), TJS (17%) et TGD (9%) prédominaient. Dans les échantillons de folle avoine, TJJ (40%), TGD (23%) et TGB (12%) prédominaient. Deux nouvelles races de P. graminis f. sp. avenae (TGN et TJN), virulentes á l’égard de Pg12, mais non virulentes á l’égard de Pg13, ont été détectées chez l'avoine cultivée et la folle avoine, et ce, au Manitoba comme en Saskatchewan en 2006.

Keywords:

• barley
• oat
• race
• stem rust
• virulence
• virulence phenotype
• wheat

Mots clés:

• avoine
• blé
• orge
• phénotype de virulence
• race
• rouille noire des céréales
• virulence

Introduction

Stem rust of wheat (Triticum aestivum L. and Triticum turgidum L.) and barley (Hordeum vulgare L.), caused by Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn., can be a devastating disease of these crops. Epidemics of stem rust on wheat occurred frequently in the early 1900s in North America (Roelfs, 1978), but the last major epidemics occurred from 1953 to 1955 (Peturson, 1958). In Canada, all spring wheat cultivars currently recommended for production in the prairie region are resistant to North American races of stem rust (Fetch, 2009). However, about 80% of Canadian wheat lines are susceptible to an exotic race of stem rust (TTKSK, also known as Ug99) (Fetch, 2007). This race was first reported in Uganda in 1999 (Pretorius et al., 2000) and is virulent to most stem rust resistance genes currently used in wheat cultivars (Jin et al., 2007). It has spread to Kenya (2000), Ethiopia (2003), Yemen (2007) and Iran (2008). There are two known variants in Kenya (TTKST, TTTSK) with added virulence to Sr24 (Jin et al., 2008) or Sr36 (Jin et al., 2009), and a variant in South Africa (PTKST) with avirulence to Sr21 (Pretorius et al., 2010). To date, these races have not been detected in North America.

In barley, stem rust epidemics are rare. This is attributable to the use of the durable resistance gene Rpg1 (Steffenson, 1992), and to low inoculum pressure of virulent races. However, in 1988 a stem rust race highly virulent to Rpg1 (race QCCJ) was detected (Martens et al., 1989), and caused localized epidemics in 1990 and 1991 in the northern Great Plains of the USA and Prairie Provinces of Canada (Steffenson, 1992). The frequency of QCCJ-type races is variable in the stem rust population, and is attributed to the area of susceptible winter wheat cultivars grown in the southern USA (Dill-Macky & Roelfs, 1998). No significant losses in barley production due to race QCCJ has occurred in Canada.

Stem rust of oat (Avena sativa L.), caused by Puccinia graminis Pers.:Pers. f. sp. avenae Eriks. & E. Henn., incites sporadic epidemic losses. The most recent epidemic in Canada was in 2002 and caused an estimated $14 million yield loss, attributed to race TJJ (Fetch, 2005). Race TJJ (NA67; Fetch & Jin, 2007) steadily increased in prevalence on wild oat from 2001 to 2003, but declined slightly in 2004 (Fetch, 2008). In 2005, a new race (TJS) was found in one sample in Manitoba (Fetch, 2009). Races TJJ and TJS are virulent to Pg2 and Pg13, which are in combination in most resistant Canadian oat cultivars (Fetch, 2008). Race TJS differs from race TJJ by being virulent to Pg12 and the Pg-a complex. Therefore, all Canadian oat cultivars are vulnerable to an oat stem rust epidemic from race TJS.

Annual stem rust surveys have been conducted in Canada since 1919 (Johnson & Green, 1957) to determine the incidence, severity and virulence spectrum of P. graminis f. sp. tritici and P. graminis f. sp. avenae populations. These surveys enable the detection of new rust races, which could defeat the stack of resistance genes in currently grown cereal cultivars. Surveys also provide a database on virulence changes in P. graminis populations that can be used for population genetic analyses. This research continues the long-term reporting of the incidence, severity and virulence spectrum of P. graminis f. sp. tritici and P. graminisf. sp. avenae populations in Canada.

Materials and methods

Methods for rust survey, inoculation and race determination were as previously reported (Fetch, 2009). Commercial fields, stands of wild barley (Hordeum jubatum L.) and wild oat (Avena fatua L.) and susceptible lines of spring wheat, barley, and oat from trap-plot nurseries were surveyed for stem rust incidence and severity in Saskatchewan and Manitoba from mid July to mid September in 2006. Additional samples from barley and wheat were obtained from collaborators in Ontario and Quebec. Infected stems (about five per site or line) were collected to provide isolates of P. graminis f. sp. tritici and P. graminis f. sp. avenae for determining the virulence spectrum of the 2006 stem rust populations. Samples were air-dried at room temperature for at least 24 h and then stored in a refrigerator at 3–5 °C. Urediniospores were subsequently scraped from stems using a sterile spatula sprayed with distilled water, and transferred to seedling leaves of either ‘Little Club’ wheat (for wheat or barley collections) or ‘Pc94’ oat (for oat collections). Inoculated seedlings were incubated in the dark for 16 h at 20 °C and near 100% relative humidity in a dew chamber. Plants were then moved to a greenhouse bench and covered with a plastic sheet for 2–3 h to provide a period of slow-drying and light (250 μmol·m⁻²·s⁻¹) to enhance infection. The plastic sheet was subsequently removed and plants maintained in the greenhouse at 20 ± 4 °C with a 14 h light:10 h dark photoperiod supplemented with high-pressure sodium lighting. Each pot containing inoculated ‘Little Club’ or ‘Pc94’ seedlings was covered with a plastic lamp cover connected to an air source of light positive pressure to prevent cross-contamination of isolates. At 14 days postinoculation, two single-pustule isolates were made from each sample using cyclone spore collectors and size 00 gelatin capsules. Bulk collections (20 random samples per bulk) were also made from the initial inoculations using the cyclone spore collectors.

Race typing of individual samples was done using differential lines (8–10 seeds/line) planted in fibre flats (20 × 30 cm) filled with soil. For samples from wheat or barley, five sets of single-gene differential lines (Sr5, Sr21, Sr9e, Sr7b; Sr11, Sr6, Sr8a, Sr9g; Sr36, Sr9b, Sr30, Sr17; Sr9a, Sr9d, Sr10, SrTmp; Sr24, Sr31, Sr38, SrMcN; Fetch & Dunsmore, 2004), were used to determine the physiological race using the letter-code nomenclature for P. graminis f. sp. tritici (Jin et al., 2008). For samples from oat, three sets of single-gene differential lines (Pg1, Pg2, Pg3, Pg4; Pg6, Pg8, Pg9, Pg10; Pg12, Pg13, Pg15, Pg16) were used to determine the physiological race of each isolate using the letter-code nomenclature for P. graminisf. sp. avenae (Fetch & Jin, 2007). Oat isolates were also inoculated onto the lines ‘Pg-a’, ‘Alpha’, ‘Omega’, (Pg-a complex; Martens et al., 1981) and ‘Wisconsin X-1588’ (Pg10; Harder, 1999). Bayol oil (Esso Canada, Toronto) was added to each gelatin capsule containing a single-pustule isolate and inoculated onto a flat of eight-day-old seedlings of differential lines using a rust inoculator (G-R Manufacturing, Manhattan, Kansas, USA) pressurized to 20 kPa by an air pump. Inoculated plants were incubated as described previously, then moved to a greenhouse at 20 ± 4 °C with a 14 h light:10 h dark photoperiod supplemented by artificial lighting. Infected plants were assessed for infection type (IT; Stakman et al., 1962) at 14 days postinoculation. Single-pustule isolates were identified to race based on their reaction on the differential lines (Fetch & Jin, 2007; Jin et al., 2008).

In addition to race typing of individual stem rust isolates, bulked collections of random P. graminis f. sp. tritici samples (20 samples per bulk) were inoculated on wheat lines and cultivars that have demonstrated effective resistance to the common races of P. graminis f. sp. tritici. This was done to efficiently detect virulence on lines considered to be broadly resistant. Bulked samples were inoculated, incubated, and assessed as previously described on: (i) lines with resistance genes Sr22, Sr24, Sr25, Sr26, Sr27, Sr29, Sr31, Sr32, Sr33, Sr35, Sr37, Sr39 and Sr40; (Fetch & Dunsmore, 2004) (ii) lines Mida–McMurachy–Exchange, Kenya Farmer, BW259, BW264, ES54, HY644 and RL6076; (iii) the T. aestivum cultivars ‘Little Club’, ‘Glenlea’, ‘Somerset’, ‘Superb’, ‘5701PR’, ‘Peace’, ‘Lillian’, ‘AC Domain’, ‘AC Vista’, ‘AC Barrie’, ‘Alsen’, and ‘McKenzie’; and (iv) the T. turgidum cultivars ‘Avonlea’ and ‘Navigator’.

Results and discussion

Incidence and severity

From April through August 2006, average (85–115% of normal) rainfall occurred in Saskatchewan, and below average (60–85% of normal) to well-below average (40–60% of normal) rainfall occurred in Manitoba. Temperatures were near normal (0 to +1 °C above average) for May, June and August and slightly above normal (+1 to +3 °C above average) in July across the Prairies (Agriculture & Agri-Food Canada, 2010). These conditions would be slightly dry for stem rust infection. No stem rust infection was detected in cultivated spring wheat fields in Canada in 2006. Trace infection was found on susceptible wheat and barley lines in trap-plot nurseries and on cultivated barley and on wild barley (H. jubatum) in 2006. Stem rust severity was also at trace levels on wheat and barley in the USA in 2006 (Long et al., 2007). No losses in wheat or barley were attributed to stem rust in 2006.

Stem rust was also at trace severity in commercial oat fields and on wild oat collections in the eastern Prairie region in 2006. Stem rust on oat in the USA was generally at low levels (trace–40%) in the upper Midwest, but up to 100% severity was found in Texas (Long, 2006). Low stem rust inoculum arriving from the USA along with dry weather conditions led to the low levels of stem rust on oat in 2006.

Physiological specialization

Puccinia graminis f. sp. tritici. There were 57 viable single-pustule isolates from 278 collections and 125 locations in 2006 for evaluation of P. graminis f. sp. tritici virulence on wheat and barley. Additionally, there were 59 collections identified as P. graminis Pers.:Pers. f. sp. secalis Eriks. & E. Henn. from Hordeum jubatum L. that were discarded, based on resistant ITs found on ‘Little Club’. Race QFCSC was predominant and was identified from Saskatchewan (three isolates), Manitoba (23 isolates), Ontario (22 isolates), and Quebec (four isolates) in 2006 (Table 1). This race was also the most common in the United States in 2006 and was identified on 25 of the 27 isolates tested (Long, 2006). Race QCCJ, which was last detected in Canada in 2003 (Fetch, 2005), was detected in Manitoba (two isolates), Ontario (one isolate), and Quebec (one isolate) and remains at a low frequency in the P. graminis f. sp. tritici population. Race RKQSC was detected at only one location on wild barley in Saskatchewan.

Table 1.  Frequencies of races of Puccinia graminisf. sp. tritici obtained from wheat trap plots, cultivated barley, and wild barley in Saskatchewan, Manitoba, Ontario and Quebec, Canada in 2006

		Wheat		Cultivated barley		Wild barley		Total
Race	Virulence spectrum (ineffective Sr genes)	No.	%	No.	%	No.	%	No.	%
Saskatchewan
QFCSC	5, 21, 8a, 9g, 17, 9a, 9d, 10, McN	0	0	0	0	3	75	3	75
RKQSC	5, 21, 7b, 6, 8a, 9g, 36, 9b, 9a, 9d, 10, McN	0	0	0	0	1	25	1	25
Manitoba
QCCJC	5, 21, 9g, 17, 9d, 10	0	0	0	0	2	13	2	8
QFCSC	5, 21, 8a, 9g, 17, 9a, 9d, 10, 7a, 8b, McN	3	100	7	100	12	80	22	88
RCCJC	5, 21,7b, 9g, 17, 9d, 10	0	0	0	0	1	7	1	4
Ontario
QCCJC	5, 21, 9g, 17, 9d, 10	0	0	1	20	0	0	1	4
QFCSC	5, 21, 8a, 9g, 17, 9a, 9d, 10, 7a, 8b, McN	18	100	4	80	0	0	22	96
Quebec
QCCJC	5, 21, 9g, 17, 9d, 10	1	20	0	0	0	0	1	20
QFCSC	5, 21, 8a, 9g, 17, 9a, 9d, 10, 7a, 8b, McN	4	80	0	0	0	0	4	80
Total
QCCJC	5, 21, 9g, 17, 9d, 10	1	4	1	8	2	10	4	7
QFCSC	5, 21, 8a, 9g, 17, 9a, 9d, 10, 7a, 8b, McN	25	96	11	92	15	80	51	89
RCCJC	5, 21,7b, 9g, 17, 9d, 10	0	0	0	0	1	5	1	2
RKQSC	5, 21, 7b, 6, 8a, 9g, 36, 9b, 9a, 9d, 10, McN	0	0	0	0	1	5	1	2

Only four races of P. graminis f. sp. tritici were found in 2006, continuing the trend in low diversity that has been found the past two years. One factor is that only 57 isolates were pathotyped in 2006, which is comparable to that in recent years (Fetch, 2008). If more viable isolates of P. graminis f. sp. tritici were obtained, the likelihood of detecting more races would increase. It is difficult to obtain isolates of P. graminis f. sp. tritici because all Canadian spring wheat cultivars registered for the Rust area have a high level of resistance to stem rust, thus isolates were obtained only from wild barley and trap plots of susceptible wheat lines. Low inoculum pressure from the USA, along with dry weather conditions in Manitoba and eastern Saskatchewan (known as the ‘Rust area’) were another factor. Race QCCJ, which was last detected in 2003, was found in low frequency (7%) in 2006 and must be surviving on wild hosts although it was not detected in the USA in 2006 (Long et al., 2007). Race RKQS, which was last detected in 2002, was found in one sample from Oxbow, SK and apparently is surviving in low frequency.

The virulence frequencies of the P. graminis f. sp. tritici isolates from 2006 are presented in Table 2. All isolates were virulent to Sr5, Sr9d, Sr9g, Sr10 and Sr21, and most were virulent to Sr17, Sr8a, Sr9a and SrMcN. Stem rust resistance in Canadian spring wheat is mainly derived from ‘Renown’ (Sr2, Sr7b, Sr9d, Sr17) or ‘Neepawa’ (Sr5, Sr7a, Sr9b, Sr12, Sr16) wheat (Kolmer et al., 1991), but the number and identity of genes in current varieties is unknown. Genes Sr6, Sr7a, Sr7b and Sr9b are the most likely to condition effective resistance in Canadian spring wheat against the stem rust races in the North American population, including race QFCSC. However, the cultivars ‘AC Cadillac’ and ‘Peace’ were recently identified to have a non-catalogued resistance gene temporarily designated as SrCad (Fetch et al., 2009; Hiebert et al., 2010) that is effective to all North American races and also to race TTKSK (Ug99).

Table 2.  Frequency of virulence of Puccinia graminis f. sp. tritici isolates collected from wheat trap plots, cultivated barley, and wild barley in Saskatchewan, Manitoba, Ontario, and Quebec, Canada in 2006 to single-gene stem rust differential lines

	Wheat						Cultivated barley				Wild barley
Resistance	Manitoba		Ontario		Quebec		Manitoba		Ontario		Saskatchewan		Manitoba
Gene	No.	%	No.	%	No.	%	No.	%	No.	%	No.	%	No.	%
Sr5	3	100	18	100	5	100	7	100	5	100	4	100	15	100
Sr9d	3	100	18	100	5	100	7	100	5	100	4	100	15	100
Sr9g	3	100	18	100	5	100	7	100	5	100	4	100	15	100
Sr10	3	100	18	100	5	100	7	100	5	100	4	100	15	100
Sr21	3	100	18	100	5	100	7	100	5	100	4	100	15	100
Sr17	3	100	18	100	5	100	7	100	5	100	3	75	15	100
Sr8a	3	100	18	100	5	100	7	100	4	80	4	100	12	80
Sr9a	3	100	18	100	5	100	7	100	4	80	4	100	12	80
SrMcN	3	100	18	100	5	100	7	100	4	80	4	100	12	80
Sr7b	0	0	0	0	0	0	0	0	0	0	1	25	1	7
Sr6	0	0	0	0	0	0	0	0	0	0	1	25	0	0
Sr9b	0	0	0	0	0	0	0	0	0	0	1	25	0	0
Sr36	0	0	0	0	0	0	0	0	0	0	1	25	0	0
Sr9e	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Sr11	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Sr24	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Sr30	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Sr31	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Sr38	0	0	0	0	0	0	0	0	0	0	0	0	0	0
SrTmp	0	0	0	0	0	0	0	0	0	0	0	0	0	0

Bulked samples of collections from Saskatchewan and Manitoba were inoculated onto lines considered broadly resistant in order to detect new virulent races. With the exception of the Sr35 differential, all lines and cultivars were resistant (ITs 0; to 12) to the bulked samples (data not shown). Race QCCJ is virulent to Sr35, thus explaining the susceptible response found on this single-gene line. No virulence was found in the 2006 P. graminis f. sp. tritici population that would threaten wheat or barley production in Canada.

Puccinia graminis f. sp. avenae. There were 405 viable single-pustule isolates from 228 field collections from 161 locations in 2006 for evaluation of P. graminis f. sp. avenae virulence on oat. There were 11 races identified in Saskatchewan and 12 in Manitoba (Table 3). The most prevalent races from cultivated oat samples were TJJ (67% each), TJS (11% and 18%) and TGD (11% and 9%) in Saskatchewan and Manitoba, respectively. The prevalent races from wild oat samples were TJJ (16% and 44%), TGD (21% and 24%) and TGB (26% and 10%) in Saskatchewan and Manitoba, respectively. Races TJJ and TJS are virulent on all oat cultivars (except ‘Stainless’) in Canada, while races TGB and TGD are avirulent on resistant Canadian oat cultivars such as ‘Ronald’ and ‘Leggett’, which have the Pg2+Pg13 combination (Mitchell Fetch et al., 2003, 2007). Race TJG, which is also virulent to all Canadian cultivars except ‘Stainless’ and has been stable at 3–5% in the P. graminisf. sp. avenae population, was not found in samples from cultivated oat and was only in two samples from wild oat in 2006. In the USA, the races identified in 2006 were DBD, TGD,TJJ, TJS and TGN (Long, 2006).

Table 3.  Frequencies of races of Puccinia graminis f. sp. avenae obtained from cultivated and wild oat in Saskatchewan and Manitoba, Canada in 2006

		Cultivated oat		Wild oat		Total
Race	Virulence spectrum (ineffective Pg genes)	No.	%	No.	%	No.	%
Saskatchewan
NGB (NA16)	1, 3, 8	0	0	1	2.6	1	2.1
SGB (NA23)	1, 2, 3, 8	0	0	2	5.3	2	4.3
TGB (NA27)	1, 2, 3, 4, 8	1	11.1	10	26.3	11	23.4
TGD (NA29)	1, 2, 3, 4, 8, 15	1	11.1	8	21.1	9	19.1
TGL (NA28)	1, 2, 3, 4, 8, 12	0	0	1	2.6	1	2.1
TGN (new)	1, 2, 3, 4, 8, 12, 15	0	0	1	2.6	1	2.1
TJD (NA30)	1, 2, 3, 4, 8, 9, 15	0	0	4	10.5	4	8.5
TJG (NA76)	1, 2, 3, 4, 8, 9, 13	0	0	1	2.6	1	2.1
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13, 15	6	66.7	6	15.8	12	25.5
TJN (new)	1, 2, 3, 4, 8, 9, 12, 15	0	0	1	2.6	1	2.1
TJS (new)	1, 2, 3, 4, 8, 9, 12, 13, 15	1	11.1	3	7.9	4	8.5
Manitoba
SGB (NA23)	1, 2, 3, 8	0	0	6	2.6	6	1.7
TDD (25)	1, 2, 3, 4, 9, 15	0	0	1	0.4	1	0.3
TGB (NA27)	1, 2, 3, 4, 8	2	1.6	23	9.9	25	7.0
TGD (NA27)	1, 2, 3, 4, 8, 15	11	8.8	55	23.6	66	18.4
TGL (NA28)	1, 2, 3, 4, 8, 12	0	0	1	0.4	1	0.3
TGN (new)	1, 2, 3, 4, 8, 12, 15	1	0.8	3	1.3	4	1.1
TJB (NA68)	1, 2, 3, 4, 8, 9	1	0.8	1	0.4	2	0.6
TJD (NA30)	1, 2, 3, 4, 8, 9, 15	2	1.6	15	6.4	17	4.7
TJG (NA76)	1, 2, 3, 4, 8, 9, 13	0	0	4	1.7	4	1.1
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13, 15	84	67.2	102	43.8	186	52.0
TJN (new)	1, 2, 3, 4, 8, 9, 12, 15	2	1.6	6	2.6	8	2.2
TJS (new)	1, 2, 3, 4, 8, 9, 12, 13, 15	22	17.6	16	6.9	38	10.6
Total
NGB (NA16)	1, 3, 8	0	0	1	0.4	1	0.2
TDD (25)	1, 2, 3, 8	0	0	8	3.0	8	2.0
TGB (NA27)	1, 2, 3, 4, 9, 15	0	0	1	0.4	1	0.2
TGD (NA27)	1, 2, 3, 4, 8	3	2.2	33	12.2	36	8.9
TGL (NA28)	1, 2, 3, 4, 8, 15	12	9.0	63	23.2	75	18.5
TGN (new)	1, 2, 3, 4, 8, 12	0	0	2	0.7	2	0.5
TJB (NA68)	1, 2, 3, 4, 8, 12, 15	1	0.7	4	1.5	5	1.2
TJD (NA30)	1, 2, 3, 4, 8, 9	1	0.7	1	0.4	2	0.5
TJG (NA76)	1, 2, 3, 4, 8, 9, 15	2	1.5	19	7.0	21	5.2
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13	0	0	5	1.8	5	1.2
TJN (new)	1, 2, 3, 4, 8, 9, 13, 15	90	67.2	108	39.9	198	48.9
TJS (new)	1, 2, 3, 4, 8, 9, 12, 15	2	1.5	7	2.6	9	2.2
TDD (25)	1, 2, 3, 4, 8, 9, 12, 13, 15	23	17.2	19	7.0	42	10.4

The most important finding in the 2006 survey was the detection of new races (TGN, TJN) with virulence to Pg12. What is even more interesting is that both of these races, along with a new variant of TJS, conditioned a mesothetic (IT = X⁻) reaction on the Pg-a differential line and resistant (IT = 1) reactions on both the Omega and Alpha sources of Pg-a resistance (Martens et al., 1981). The previous variant of TJS was virulent (IT = 4) on Pg12, Pg-a, Omega and Alpha (Fetch, unpublished data). This indicates that the resistant response in the Pg-a differential line, Omega and Alpha is not attributable to the Pg12 donor of Pg-a resistance, but to a second gene or suppressor that likely came from the Avena sterilis L. parent. Races TGN, TJN and TJS variant were recovered from several samples, on both cultivated and wild oat, in both Manitoba and Saskatchewan in 2006. Races TGN and TJN may have arisen as single-step mutations of race TGD and TJD, respectively, with additional virulence to Pg12. Alternatively, TJN may have arisen from a single-step mutation from TJS with avirulence to Pg13. The TJS variant was found in nine of the 42 samples of race TJS, and appears to be a single step mutation to avirulence to the gene or suppressor from A. sterilis. Races TGN and TJN are avirulent to Pg13 and are not a current threat to oat production in Canada, but only need to acquire virulence to Pg13 to pose a threat to oat production.

The virulence frequencies of P. graminis f. sp. avenae isolates on the 12 single-gene differential oat lines and the Pg-a complex are presented in Table 4. All isolates from cultivated oat and almost all from wild oat were virulent to Pg1, Pg2, Pg3, Pg4 and Pg8. Virulence was common to Pg9 (77%–89% and 40%–62%) and Pg15 (89%–98% and 61%–85%) in cultivated and wild oat, respectively. Virulence frequency to Pg13, found in most of the resistant Canadian cultivars, remained high in collections from cultivated oat (78–85%) but was lower in collections from wild oat (26–52%). No isolates were found in 2006 with virulence to Pg6, Pg10 or Pg16, but 58 isolates (races TGN, TJN and TJS) with virulence to Pg12 and 35 with virulence on the Pg-a complex were found. The Pg-a complex is used by Canadian oat breeders and is in the cultivar ‘Stainless’, but is less useful as race TJS increases in frequency in the P. graminis f. sp. avenae population. Resistance from the diploid species Avena strigosa Schreb. (gene Pg6) has not yet been transferred into an A. sativa background, thus leaves only genes Pg10, Pg16 and the adult plant resistance gene Pg11 as known sources of resistance to all races in the P. graminis f. sp. avenae population. New oat cultivars with an effective pyramid of these genes with Pg13 are desirable to protect Canadian oat crops against stem rust.

Table 4.  Frequencies of virulence of Puccinia graminis f. sp. avenae isolates collected from cultivated and wild oat in Saskatchewan and Manitoba, Canada in 2006 to single-gene stem rust differential lines and the Pg-a gene complex

	Cultivated oat						Wild oat
Resistance	Saskatchewan		Manitoba		Total		Saskatchewan		Manitoba		Total
gene	No.	%	No.	%	No.	%	No.	%	No.	%	No.	%
Pg1	9	100	125	100	134	100	38	100	233	100	271	100
Pg2	9	100	125	100	134	100	37	97.4	233	100	270	99.6
Pg3	9	100	125	100	134	100	38	100	233	100	271	100
Pg4	9	100	125	100	134	100	35	92.1	227	97.4	262	96.7
Pg6	0	0	0	0	0	0	0	0	0	0	0	0
Pg8	9	100	125	100	134	100	38	100	232	99.6	270	99.6
Pg9	7	77.8	111	88.8	118	88.1	15	39.5	145	62.2	160	59.0
Pg10	0	0	0	0	0	0	0	0	0	0	0	0
Pg12	1	11.1	25	20.0	26	19.4	6	15.8	26	11.2	32	11.8
Pg13	7	77.8	106	84.8	113	84.3	10	26.3	122	52.4	132	48.7
Pg15	8	88.9	122	97.6	130	97.0	23	60.5	198	85.0	221	81.5
Pg16	0	0	0	0	0	0	0	0	0	0	0	0
Pg-a	0	0	19	15.2	19	14.2	3	7.9	13	5.6	16	5.9

Acknowledgements

Ken Dunsmore, Elaine Martineau and Debrah Witko are thanked for their expert technical work on the collection and virulence phenotyping of survey samples. Dr Andre Comeau from St. Foy, Quebec is thanked for providing samples of wheat.