Races of Puccinia graminis on barley, oat, and wheat in Canada in 2013 and 2014

Abstract

Stem rust, caused by Puccinia graminis, is a catastrophic disease that has and continues to cause yield losses in barley, oat, and wheat crops worldwide. Use of host resistance genes is the most economic, efficient, and environmentally best method to control the disease. Resistance genes have been incorporated into most commercial cultivars and have been effective, for races in the North American populations of P. graminis. However, new races of stem rust are constantly emerging and therefore it is imperative to monitor the virulence structure in the populations to detect new races with increased virulence. Stem rust samples were collected in 2013 and 2014 in Manitoba, Ontario, Quebec, and Saskatchewan to determine the incidence and severity of stem rust in barley (Hordeum vulgare), oat (Avena sativa), and wheat (Triticum aestivum) fields and to characterize the virulence dynamics in the pathogen populations. No stem rust was found in cultivated wheat, and incidence was at trace (<1%) levels in barley fields and very low (0–5%) levels in oat fields in 2013 and 2014. In wheat trap plots and collections from barley, race QFCSC of P. graminis f. sp. tritici was dominant in 2013 (85.9%) and was the only race found in 2014. Races MCCFC, QTHJF, RFCSC, RKQSC, and TMRTF were at low (<5%) levels in 2013. From collections in oat fields and stands of wild barley, nine races of P. graminis f. sp. avenae were found in 2013, with TJS (56.9%) and TJJ (23.5%) the most frequent. In 2014, we found only six races and TJS (74.5%) and TJJ (19.1%) were the most frequent. No new races of stem rust were detected in Canada in 2013 or 2014.

Résumé

La rouille noire, causée par Puccinia graminis, est une maladie dévastatrice qui a causé, et qui continue de causer, des pertes de rendement chez l’orge, l’avoine et le blé partout dans le monde. L’utilization des gènes de résistance de l’hôte se révèle la méthode de lutte la plus économique, efficace et écologique contre cette maladie. Les gènes de résistance ont été incorporés dans la majorité des cultivars commerciaux et ont été efficaces contre les races des populations nord-américaines de P. graminis. Toutefois, de nouvelles races de rouille noire émergent constamment et il est en conséquence impératif de surveiller la structure de virulence des populations afin de détecter de nouvelles races à la virulence accrue. Des échantillons de rouille noire ont été collectés en 2013 et 2014 au Manitoba, en Ontario, au Québec et en Saskatchewan pour déterminer l’incidence et la gravité de la rouille noire dans des champs d’orge (Hordeum vulgare), d’avoine (Avena sativa) et de blé (Triticum aestivum) ainsi que pour caractériser la dynamique de virulence dans les populations d’agents pathogènes. En 2013 et 2014, le blé ne portait aucun signe de rouille noire et l’incidence de la maladie était à l’état de trace (<1%) chez l’orge ainsi que très faible (0 à 5%) chez l’avoine. Dans des parcelles de culture piège de blé et des collections d’orge, la race QFCSC de P. graminis f. sp. tritici dominait en 2013 (85.9%) et était la seule trouvée en 2014. En 2013, les races MCCFC, QTHJF, RFCSC, RKQSC et TMRTF affichaient de faibles taux (<5%). Dans des collections provenant de champs d’avoine et de parcelles d’orge agréable, neuf races de P. graminis f. sp. avenae ont été trouvées en 2013, dont TJS (56.9%) et TJJ (23.5%) étaient les plus courantes. En 2014, nous n’avons trouvé que six races, et TJS (74.5%) ainsi que TJJ (19.1%) étaient les plus courantes. Aucune nouvelle race de rouille noire n’a été détectée au Canada en 2013 et 2014.

Keywords:

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

Mots clés:

• avoine
• blé
• dynamique de virulence
• orge
• race
• rouille noire
• virulence

Introduction

Epidemics of stem rust have caused substantial yield losses in barley (Hordeum vulgare L.), oat (Avena sativa L.), and wheat (Triticum aestivum L. and Triticum turgidum L.) in Canada since the early 20th century. Stem rust disease is caused by the fungal pathogens Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn. (Pgt) on barley and wheat, and Puccinia graminis Pers.:Pers. f. sp. avenae Eriks. & E. Henn. (Pga) on oat. No significant losses have occurred since 1955, which mainly has been attributed to the development and release of rust resistant varieties. Recently, new highly virulent races have been detected in Canada on oat (Fetch et al. 2018) and in Kenya (Newcomb et al. 2016), Ethiopia (Olivera et al. 2015), and Sicily (Bhattacharya 2017) on wheat. Studies have been conducted since 1919 on the virulence dynamics of Pgt and Pga in Canadian populations (Johnson and Green 1957) to provide a historic record and to warn of new virulent races that may arise. The objective of this paper is to determine the virulence spectrum in Pgt and Pga populations in Canada during 2013 and 2014.

Materials and methods

Collection and processing of isolates

Infected stems were collected in Manitoba and eastern Saskatchewan by car in August and September in 2013 and 2014. Commercial fields of barley, oat, or wheat, patches of wild barley (Hordeum jubatum L.), or stands of wild oat (Avena fatua L.) were sampled at about 20 km intervals. Stem rust incidence and severity was recorded. Several (3–5) stems infected with Pgt or Pga were collected at each location. Samples were labelled with a unique identifier and put in paper envelopes. Subsequently, samples were removed from envelopes and air-dried at room temperature for 2–3 days and then put into a refrigerator at 3–5°C until ready for processing. To ensure spore viability, samples were processed within 30 days of collection. Additionally, packets of seed of susceptible spring wheat (‘Klein Titan’, ‘Little Club’, ‘Morocco’), barley (‘Wolfe’), and oat (‘Triple Crown’, ‘Makuru’) lines were mailed to cooperators in Ontario and Quebec in April of each year to establish trap-plot nurseries and provide stem rust samples for race analyses.

Processing of each sample was as reported previously (Fetch et al. 2018). For each sample, urediniospores were transferred to pots containing 8–10 seedling leaves of ‘Little Club’ wheat (for Pgt) or ‘Pc94' oat (for Pga) using a sterile spatula sprayed with distilled water. Inoculated plants were incubated overnight for 16 h at 20°C and near 100% relative humidity using a dew box misted with an ultrasonic humidifier. The dew box was constructed using stainless steel sides and bottom, with a clear Plexiglas top and door. In the morning, the dew box door was opened 1 cm to slowly dry the plants, and T5 fluorescent lights (200 μmol·m⁻²·s⁻¹) above the dew box were turned on for 2–3 h to finish the infection process. Pots were transported to a greenhouse at 20 ± 4°C with a 14 h light: 10 h dark photoperiod supplemented with high-pressure sodium lights. A plastic lamp cover was placed on each pot, and light positive air pressure was injected through a hole in the base of the cover to prevent cross-contamination of isolates. Two isolates (A and B) of 2–3 large random pustules were made from each collection at 14 days post-inoculation using a mini-cyclone spore collector (G-R Manufacturing, Manhattan, KS) fitted with a size 00 gelatin capsule, which was connected to a Gast vacuum pump (Fisher Scientific catalogue 01-093-5) using a DeVilbiss model 630–605 bleeder-type cut-off valve (http://www.drivemedical.com). Isolate A was used for race phenotyping and isolate B was for repeat tests or increases. Several bulk collections each consisting of 20 survey samples of Pgt were also made using a cyclone spore collector.

Determination of physiological races

Methods to identify the physiological race of each isolate were as previously described (Fetch et al. 2018). Briefly, five sets of wheat single-gene differential lines (Sr5, Sr21, Sr9e, Sr7b; Sr11, Sr6, Sr8a, Sr9g; Sr36, Sr9b, Sr30, Sr17; Sr9a, Sr9d, Sr10, SrTmp; Sr24, Sr31, Sr38, Sr54 [= SrMcN]) were used for Pgt pathotyping (Jin et al. 2008), and three sets of oat single-gene differential lines (Pg1, Pg2, Pg3, Pg4; Pg6, Pg8, Pg9, Pg10; Pg12, Pg13, Pg15, Pg16) were used for pathotyping of Pga (Fetch and Jin 2007). Oat line ‘Pg-a’ was also planted, as this gene has been deployed in some oat cultivars. Conetainers (RL98) were filled with moist #4 Sunshine Mix, packed to a uniform depth of 4 cm and five seeds of the line was put into the cone. Additional moist mix was added to fill the cone, then packed to a uniform depth of 2 cm to provide even emergence of seedlings. Plastic racks with planted cones were moved to a rust-free greenhouse with environmental conditions described above. Individual cones were labelled with their respective resistance gene and carefully watered until saturated.

Inoculation and disease rating were done as previously reported (Fetch et al. 2018). At 8–9 days after planting, seedling leaves of differential lines were inoculated with 3–4 mg of a rust isolate in a 00 gelatin capsule containing 0.7 mL Bayol oil. Stem rust was immersed into the oil by gently shaking the capsule, then the capsule top was removed and the bottom attached to an inoculator. Inoculum was atomized onto seedlings using an air pump pressurized to 20 kPa, and plants were placed in front of a fan at gentle speed to volatilize the oil. Inoculated plants were incubated in dew cabinets as described above, and moved into a different greenhouse for infection. Seedling infection type (IT) was scored 12–14 days post-inoculation using a 0–4 scale (Stakman et al. 1962). A letter-code nomenclature (Fetch and Jin 2007; Jin et al. 2008) was used to identify each isolate, with ITs <3 considered as a low (resistant) response and those ≥3 as a high response (susceptible). If a line had mixed infection (both resistant and susceptible ITs), single-pustule isolates of each IT were generated to determine the identity of each race.

Additionally, isolates were combined into bulked collections (12–15 per capsule) to quickly detect novel Pgt virulence on highly effective Sr genes and on resistant wheat cultivars. Bulked collections were inoculated on: (i) single-gene differential lines with resistance genes Sr7a, Sr8b, Sr12, Sr13, Sr14, Sr15, Sr16, Sr18, Sr20, Sr22, Sr25, Sr26, Sr27, Sr28, Sr29, Sr32, Sr33, Sr34, Sr35, Sr37, Sr39 or Sr40; (ii) lines RL6076 and Tr129; (iii) the T. aestivum cultivars ‘AC Barrie’, ‘AC Domain’, AC ‘Karma’, ‘AC Michael’, ‘AC Splendor’, ‘AC Vista’, ‘Burnside’, ‘Canthatch’, ‘Columbus’, ‘Harvest’, ‘Kanata’, ‘Lillian’, ‘Little Club’, ‘McKenzie’ ‘Peace’, ‘Selkirk’, ‘Snowbird’, ‘Somerset’, ‘Superb’, and ‘5701PR’; and (iv) the T. turgidum cultivars ‘AC Navigator’, ‘Sceptre’, and ‘Strongfield’. Bulk collections were inoculated, incubated and assessed as described above. This enables the rapid identification of new Pgt races that could threaten wheat production in Canada.

Results and discussion

Environmental conditions

Weather conditions were generally unfavourable for stem rust infection in the Prairie region of Canada in 2013 and 2014 (Agriculture & Agri-Food Canada 2019). Mean temperatures in 2013 and 2014 were mostly normal (−1 to +1°C of average) in the ‘Rust area’ of the Prairies in June, July, and August. Rainfall was inconsistent in both years. In 2013, it was wet (115–150% of normal) to very wet (>150% of normal) in Saskatchewan and western Manitoba but very dry (40–60% of normal) in eastern Manitoba in June, followed by normal (85–115%) rainfall in July. However, it was extremely dry (<40% of normal) in August 2013. In 2014, it was extremely wet (>200% of normal) in most of Saskatchewan and western Manitoba in June and August, but very dry (40–60% of normal) in July. While temperature was normal in both years, very dry conditions in both years would limit adequate dew formation for stem rust infection.

Incidence and severity

Stem rust incidence and severity was at very low (<5%) levels in barley and oat fields, and in stands of wild barley (H. jubatum) and wild oat (A. fatua) in both years. This was attributed to widespread application of fungicides to Canadian crops (for Fusarium head blight), dry environmental conditions, and low inoculum pressure from the USA. No stem rust infection was found in cultivated wheat. Stem rust infection was at low levels in the USA in 2013, except in south-eastern South Dakota and a field in North Dakota where severities up to 80% were found in winter wheat (Cereal Disease Laboratory, 2013). In 2014 stem rust was mostly at trace levels in the USA (Cereal Disease Laboratory, 2014). No yield losses were found in 2013 or 2014 in Canada due to stem rust infection.

Physiological specialization

Puccinia graminis f. sp

tritici. Race QFCSC was dominant in 2013 (86% of total isolates, Table 1) and was the only race detected in 2014 in Canada. Race QFCSC also was the only one found in the USA in both 2013 and 2014 (Cereal Disease Laboratory, 2013; Cereal Disease Laboratory, 2014). Additional races found in Canada at low frequency in 2013 were MCCFC (5%), QTHJF and RFCSC (3%), and RKQSC and TMRTF (2%). It was unclear as to why these races were not detected in the USA in 2013, since the Pgt population in Canada is often composed of races in the Midwest region that migrate north along the ‘Puccinia pathway’. As was found in 2012, historic races MCCFC, QTHJF, RKQSC, and TMRTF are likely surviving on wild grass hosts in Canada.

Table 1. Frequencies of races of Puccinia graminis f. sp. tritici obtained from wheat trap plots, cultivated barley and wild barley in Manitoba, Ontario, Quebec, and Saskatchewan, Canada in 2013

		Wheat		Cultivated barley		Wild barley		Total
Race	Virulence spectrum (ineffective Sr genes)	No.	%	No.	%	No.	%	No.	%
Manitoba
MCCFC	5, 7b, 9 g, 17, 10, Tmp, 54	1	7.1	0	0	1	6.3	2	5.3
QFCSC	5, 21, 8a, 9 g, 17, 9a, 9d, 10, 54	11	78.7	7	87.5	12	75.0	30	78.9
QTHJF	5, 21, 11, 6, 8a, 9 g, 9b, 17, 9d, 10, 38, 54	1	7.1	0	0	1	6.3	2	5.3
RFCSC	5, 21, 7b, 8a, 9 g, 17, 9a, 9d, 10, 54	0	0	0	0	2	12.5	2	5.3
RKQSC	5, 21, 7b, 6, 8a, 9 g, 36, 9b, 9a, 9d, 10, 54	1	7.1	0	0	0	0	1	2.6
TMRTF	5, 21, 9e, 7b, 11, 9 g, 36, 9b, 17, 9a, 9d, 10, Tmp, 38, 54	0	0	1	12.5	0	0	1	2.6
Ontario
QFCSC	5, 21, 8a, 9 g, 17, 9a, 9d, 10, 54	3	100	0	0	0	0	3	100
Quebec
MCCFC	5, 7b, 9 g, 17, 10, Tmp, 54	1	100	0	0	0	0	1	100
Saskatchewan
QFCSC	5, 21, 8a, 9 g, 17, 9a, 9d, 10, 54	3	100	1	100	18	100	22	100
Total
MCCFC	5, 7b, 9 g, 17, 10, Tmp, 54	2	9.5	0	0	1	2.9	3	4.7
QFCSC	5, 21, 8a, 9 g, 17, 9a, 9d, 10, 54	17	81.0	8	88.9	30	88.2	55	85.9
QTHJF	5, 21, 11, 6, 8a, 9 g, 9b, 17, 9d, 10, 38, 54	1	4.8	0	0	1	2.9	2	3.1
RFCSC	5, 21, 7b, 8a, 9 g, 17, 9a, 9d, 10, 54	0	0	0	0	2	5.9	2	3.1
RKQSC	5, 21, 7b, 6, 8a, 9 g, 36, 9b, 9a, 9d, 10, 54	1	4.8	0	0	0	0	1	1.6
TMRTF	5, 21, 9e, 7b, 11, 9 g, 36, 9b, 17, 9a, 9d, 10, Tmp, 38, 54	0	0	1	11.1	0	0	1	1.6

The letter code race designation described by Jin et al. (2008) uses 20 single-gene differential lines (Sr5, 21, 9e, 7b, 11, 6, 8a, 9 g, 36, 9b, 30, 17, 9a, 9d, 10, Tmp, 24, 31, 38, and 54) grouped into five subsets of four lines each. The race is determined by the seedling reaction (low or high) on each line.

Virulence frequencies on single-gene differential lines of Pgt isolates collected in Manitoba, Ontario, Quebec, and Saskatchewan in 2013 are presented in Table 2. Genes Sr5, Sr9g, Sr10, and Sr54 were ineffective and genes Sr8a, Sr9a, Sr9d, Sr17, and Sr21 were mostly ineffective (85.7–100% susceptible). Genes Sr9e, Sr24, Sr30, and Sr31 were effective to all isolates, and genes Sr6, Sr7b, Sr9b, Sr11, Sr36, Sr38, and SrTmp were mostly effective. Gene Sr9e is common in durum wheat, and genes Sr6, Sr7b, Sr9b, and Sr11 have commonly been incorporated into many resistant Canadian spring or durum wheat cultivars. Additionally, virulence to genes Sr7a, Sr8b, Sr12, Sr14, Sr15, Sr16, Sr18, Sr20, Sr28, Sr34, and Sr35 was detected in each year from bulked samples from Manitoba and Saskatchewan (data not presented). There were no susceptible pustules found on currently grown Canadian varieties tested, thus no new races were detected that threaten wheat or barley production in Canada in 2013 or 2014.

Table 2. Frequency of virulence of Puccinia graminis f. sp. tritici isolates collected from wheat trap plots, cultivated barley and wild barley in Manitoba, Ontario, Quebec, and Saskatchewan, Canada in 2013 to single-gene stem rust differential lines

	Wheat										Cultivated barley		Wild barley
	Manitoba		Ontario		Quebec		Saskatchewan		Manitoba		Saskatchewan		Manitoba		Saskatchewan
Gene	No.	%	No.	%	No.	%	No.	%	No.	%	No.	%	No.	%	No.	%
Sr5	14	100	3	100	1	100	3	100	8	100	1	100	16	100	18	100
Sr21	13	92.9	3	100	0	0	3	100	8	100	1	100	15	93.8	18	100
Sr9e	0	0	0	0	0	0	0	0	1	12.5	0	0	0	0	0	0
Sr7b	2	14.3	0	0	1	100	0	0	1	12.5	0	0	3	18.8	0	0
Sr11	1	7.1	0	0	0	0	0	0	1	12.5	0	0	1	6.3	0	0
Sr6	2	14.3	0	0	0	0	0	0	0	0	0	0	1	6.3	0	0
Sr8a	13	92.9	3	100	0	0	3	100	7	87.5	1	100	15	93.8	18	100
Sr9g	14	100	3	100	1	100	3	100	8	100	1	100	16	100	18	100
Sr36	1	7.1	0	0	0	0	0	0	1	12.5	0	0	0	0	0	0
Sr9b	2	14.3	0	0	0	0	0	0	1	12.5	0	0	1	6.3	0	0
Sr30	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Sr17	13	92.9	3	100	1	100	3	100	8	100	1	100	16	100	18	100
Sr9a	12	85.7	3	100	0	0	3	100	8	100	1	100	14	87.5	18	100
Sr9d	13	92.9	3	100	0	0	3	100	8	100	1	100	15	93.8	18	100
Sr10	14	100	3	100	1	100	3	100	8	100	1	100	16	100	18	100
SrTmp	1	7.1	0	0	1	100	0	0	1	12.5	0	0	1	6.3	0	0
Sr24	0	0	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	0	0
Sr38	1	7.1	0	0	0	0	0	0	1	12.5	0	0	1	6.3	0	0
Sr54	14	100	3	100	1	100	3	100	8	100	1	100	16	100	18	100

Puccinia graminis f. sp

avenae. Race TJS was dominant in both 2013 (57%) and 2014 (74%), and was found in Ontario, Manitoba, and Saskatchewan in both years (Tables 3 and 4). This race is virulent on all genes deployed in commercial oat, and frequency of this race continues to rise in the population of Pga. Race TJJ (NA67) was the next most common race in both years, followed by race TGN. In the USA there were few reports of oat stem rust, with race TJS detected in 2013 (Cereal Disease Laboratory, 2013) and races TJS and TGN detected in 2014 (Cereal Disease Laboratory, 2014). We detected more variability in Canada than was found in the USA, with 8 races detected from 204 samples in 2013 and 6 races from 94 samples in 2014. No new races were detected in Canada in either 2013 or 2014 in Canada.

Table 3. Frequencies of races of Puccinia graminis f. sp. avenae obtained from cultivated and wild oat in Manitoba, Ontario, and Saskatchewan, Canada in 2013

		Cultivated oat		Wild oat		Total
Race	Virulence spectrum (ineffective Pg genes)	No.	%	No.	%	No.	%
Manitoba
TGD (NA29)	1, 2, 3, 4, 8, 15	0	0	1	1.0	1	0.7
TGL (NA28)	1, 2, 3, 4, 8, 12, Pg-a	0	0	1	1.0	1	0.7
TGN (2006)	1, 2, 3, 4, 8, 12, 15, Pg-a	1	2.6	10	10.3	11	8.1
TJD (NA30)	1, 2, 3, 4, 8, 9, 15	0	0	1	1.0	1	0.7
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13, 15	10	26.3	23	23.7	33	24.4
TJN (2006)	1, 2, 3, 4, 8, 9, 12, 15, Pg-a	2	5.3	1	1.0	3	2.2
TJS (2005)	1, 2, 3, 4, 8, 9, 12, 13, 15, Pg-a	25	65.8	60	61.9	85	63.0
Ontario
TJS (2005)	1, 2, 3, 4, 8, 9, 12, 13, 15, Pg-a	1	100.0	0	0	1	100.0
Saskatchewan
SGD (NA63)	1, 2, 3, 8, 15	1	5.6	1	2.0	2	1.8
TGB (NA27)	1, 2, 3, 4, 8	0	0	1	2.0	1	0
TGD (NA29)	1, 2, 3, 4, 8, 15	1	5.6	3	6.0	4	1.8
TGN (2006)	1, 2, 3, 4, 8, 12, 15, Pg-a	3	16.7	8	16.0	11	7.1
TJD (NA30)	1, 2, 3, 4, 8, 9, 15	0	0	3	6.0	3	0
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13, 15	4	22.2	11	22.0	15	25.0
TJN (2006)	1, 2, 3, 4, 8, 9, 12, 15, Pg-a	1	5.6	1	2.0	2	5.4
TJS (2005)	1, 2, 3, 4, 8, 9, 12, 13, 15, Pg-a	8	44.4	22	44.0	30	58.9
Total
SGD (NA63)	1, 2, 3, 8, 15	1	1.8	1	0.7	2	1.0
TGB (NA27)	1, 2, 3, 4, 8	0	0	1	0.7	1	0.5
TGD (NA29)	1, 2, 3, 4, 8, 15	1	1.8	4	2.7	5	2.5
TGL (NA28)	1, 2, 3, 4, 8, 12, Pg-a	0	0	1	0.7	1	0.5
TGN (2006)	1, 2, 3, 4, 8, 12, 15, Pg-a	4	7.0	18	12.2	22	10.8
TJD (NA30)	1, 2, 3, 4, 8, 9, 15	0	0	4	2.7	4	2.0
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13, 15	14	24.6	34	23.1	48	23.5
TJN (2006)	1, 2, 3, 4, 8, 9, 12, 15, Pg-a	3	5.3	2	1.4	5	2.5
TJS (2005)	1, 2, 3, 4, 8, 9, 12, 13, 15, Pg-a	34	59.6	82	55.8	116	56.9

The letter code race designation described by Fetch and Jin (2007) uses 12 single-gene differential lines (Pg1, 2, 3, 4, 6, 8, 9, 10, 12, 13, 15, and 16) grouped into three subsets of four lines each. The race is determined by the seedling reaction (low or high) on each line. The equivalent North American (NA) race code or year of discovery is provided in parentheses.

Table 4. Frequencies of races of Puccinia graminis f. sp. avenae obtained from cultivated and wild oat in Manitoba, Ontario, and Saskatchewan, Canada in 2014

		Cultivated oat		Wild oat		Total
Race	Virulence spectrum (ineffective Pg genes)	No.	%	No.	%	No.	%
Manitoba
NGB (NA23)	1, 3, 8	0	0	1	2.4	1	1.4
TGB (NA27)	1, 2, 3, 4, 8	1	3.1	0	0	1	1.4
TGN (2006)	1, 2, 3, 4, 8, 12, 15, Pg-a	1	3.1	1	2.4	2	2.7
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13, 15	6	18.8	8	19.0	14	18.9
TJS (2005)	1, 2, 3, 4, 8, 9, 12, 13, 15, Pg-a	24	75.0	32	76.2	56	75.7
Ontario
TJS (2005)	1, 2, 3, 4, 8, 9, 12, 13, 15, Pg-a	1	100	0	0	1	100.0
Saskatchewan
TGN (2006)	1, 2, 3, 4, 8, 12, 15, Pg-a	0	0	1	8.3	1	5.3
TJG (NA76)	1, 2, 3, 4, 8, 9, 13	1	14.3	0	0	1	5.3
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13, 15	2	28.6	2	16.7	4	21.1
TJS (2005)	1, 2, 3, 4, 8, 9, 12, 13, 15, Pg-a	4	57.1	9	75.0	13	68.4
Total
NGB (NA23)	1, 3, 8	0	0	1	1.9	1	1.1
TGB (NA27)	1, 2, 3, 4, 8	1	2.2	0	0	1	1.1
TGN (2006)	1, 2, 3, 4, 8, 12, 15, Pg-a	1	2.2	2	3.7	3	3.2
TJG (NA76)	1, 2, 3, 4, 8, 9, 13	1	2.2	0	0	1	1.1
TJJ (NA67)	1, 2, 3, 4, 8, 9, 13, 15	8	17.8	10	18.5	18	19.1
TJS (2005)	1, 2, 3, 4, 8, 9, 12, 13, 15, Pg-a	34	75.6	41	75.9	70	74.5

The letter code race designation described by Fetch and Jin (2007) uses 12 single-gene differential lines (Pg1, 2, 3, 4, 6, 8, 9, 10, 12, 13, 15, and 16) grouped into three subsets of four lines each. The race is determined by the seedling reaction (low or high) on each line. The equivalent North American (NA) race code or year of discovery is provided in parentheses.

Virulence frequencies on single-gene differential lines of Pga isolates collected on cultivated and wild oat in Manitoba, Ontario, Quebec, and Saskatchewan and the Pg-a complex are presented in Tables 5 and 6. All isolates were virulent to Pg1 and Pg3, and avirulent to Pg6, Pg10, and Pg16 in both 2013 and 2014. The frequency of virulence was very high to Pg2, Pg4, Pg8, and Pg15 (97–100%) in both years. Virulence to Pg12 (66–76%) and Pg-a (52–57%) has steadily continued to increase, concurrent with the increased prevalence of race TJS. Virulence to Pg9, Pg12, Pg13, and Pg-a rose substantially from 2011 (30%, 66%, 28%, and 57%, respectively) to 2014 (95%, 78%, 95%, and 78%, respectively) due to race TJS. Genes Pg6, Pg10, and Pg16 are effective to race TJS, but have not been useful in oat breeding programmes. Until new stem rust genes in oat are discovered that provide resistance to highly virulent races such as TJS and TJJ, fungicide use will increase in oat crops.

Table 5. Frequencies of virulence of Puccinia graminis f. sp. avenae isolates collected from cultivated and wild oat in Manitoba, Ontario, and Saskatchewan, Canada in 2013 to single-gene stem rust differential lines and the Pg-a gene complex

	Cultivated oat						Wild oat
	Manitoba		Ontario		Saskatchewan		Manitoba		Saskatchewan		Total
Resistance Gene	No.	%	No.	%	No.	%	No.	%	No.	%	No.	%
Pg1	38	100	1	100	18	100	97	100	50	100	204	100
Pg2	38	100	1	100	18	100	97	100	50	100	204	100
Pg3	38	100	1	100	18	100	97	100	50	100	204	100
Pg4	38	100	1	100	17	94.4	97	100	49	98.0	202	99.0
Pg6	0	0	0	0	0	0	0	0	0	0	0	0
Pg8	38	100	1	100	18	100	97	100	50	100	204	99.2
Pg9	37	97.4	1	100	13	72.2	85	87.6	37	74.0	173	84.8
Pg10	0	0	0	0	0	0	0	0	0	0	0	0
Pg12	28	73.7	1	100	12	66.7	72	74.2	31	62.0	144	70.6
Pg13	35	92.1	1	100	12	66.7	83	86.6	33	66.0	164	80.4
Pg15	38	100	1	100	18	100	96	99.0	49	98.0	202	99.0
Pg16	0	0	0	0	0	0	0	0	0	0	0	0
Pg-a	28	73.7	1	100	12	66.7	72	74.2	31	62.0	144	70.6

Table 6. Frequencies of virulence of Puccinia graminis f. sp. avenae isolates collected from cultivated and wild oat in Saskatchewan, Ontario, and Manitoba, Canada in 2014 to single-gene stem rust differential lines and the Pg-a gene complex

			Cultivated oat						Wild oat
Resistance	Manitoba		Ontario		Saskatchewan		Manitoba		Saskatchewan		Total
Gene	No.	%	No.	%	No.	%	No.	%	No.	%	No.	%
Pg1	32	100	1	100	7	100	42	100	12	100	94	100
Pg2	32	100	1	100	7	100	41	97.6	12	100	93	98.9
Pg3	32	100	1	100	7	100	42	100	12	100	94	100
Pg4	32	98.9	1	100	7	100	41	97.6	12	100	93	98.9
Pg6	0	0	0	0	0	0	0	0	0	0	0	0
Pg8	32	100	1	100	7	100	42	100	12	100	94	100
Pg9	30	93.8	1	100	7	75.0	40	95.2	11	91.7	89	94.7
Pg10	0	0	0	0	0	0	0	0	0	0	0	0
Pg12	25	78.1	1	100	4	57.1	33	78.6	10	83.3	73	77.7
Pg13	30	93.8	1	100	7	100	40	95.2	11	91.7	89	94.7
Pg15	31	96.9	1	100	6	85.7	41	97.6	12	100	91	96.8
Pg16	0	0	0	0	0	0	0	0	0	0	0	0
Pg-a	25	78.1	1	100	4	57.1	33	78.6	10	83.3	73	77.7

Acknowledgements

Dr Andre Comeau from St. Foy, Quebec, is thanked for providing samples from infected wheat.