Virulence phenotypes of Puccinia striiformis in Alberta from 2009–2011

Abstract

One hundred and four isolates of Puccinia striiformis were obtained primarily from central Alberta during 2009–2011 from wheat, barley, foxtail barley and triticale. Isolates were identified as P. striiformis f. sp. tritici (Pst) or P. striiformis f. sp. hordei (Psh) based on virulence on differential seedlings. Twenty-five wheat differentials separated 67 Pst isolates into 12 pathotypes, with a single pathotype representing 48% of Pst isolates. Wheat differentials with resistance genes Yr5 and Yr15 were resistant to all 12 pathotypes, while wheat lines with genes Yr1, Yr10, Yr24, Yr28, YrTyee and YrSP were resistant to 80–98% of the isolates. The remainder of the wheat differentials were susceptible to all Pst isolates. Within the 37 Psh isolates, 12 pathotypes were detected based on 12 barley differentials, half of which were previously unreported in Alberta. The two most common Psh pathotypes combined accounted for 46% of the isolates. One half of the Psh isolates may be the result of hybridization between the two formae speciales since they exhibited virulence on a large number of wheat and triticale lines in addition to barley lines. Psh isolates with these virulence factors were more frequently recovered from six-row than two-row barley. For both formae speciales, there was no apparent association between pathotype and location, with pathotypes being widespread and recovered from multiple locations.

Résumé

De 2009 à 2011, nous avons obtenu, du centre de l'Alberta principalement, 104 isolats de Puccinia striiformis de blé, d'orge, d'orge queue-d’écureuil et de triticale. En se basant sur la réaction des lignées différentielles de semis à la virulence, les isolats ont été identifiés en tant que P. striiformis f. sp. tritici (Pst) ou P. striiformis f. sp. hordei (Psh). Vingt-cinq lignées différentielles de blé ont permis de diviser 67 isolats de Pst en 12 pathotypes, dont un seul représentait 48 % des isolats de Pst. Les lignées différentielles de blé qui possédaient les gènes de résistance Yr5 et Yr15 étaient résistants aux 12 pathotypes, tandis que les lignées possédant les gènes Yr1, Yr10, Yr24, Yr28, YrTyee et YrSP étaient résistants à 80–98 % des isolats. Le restant des lignées différentielles de blé était réceptif à l’égard de tous les isolats de Pst. Parmi les 37 isolats de Psh, en nous basant sur 12 lignées différentielles d'orge, nous avons détecté 12 pathotypes dont la moitié n'avait pas été signalée auparavant en Alberta. Les deux pathotypes de Psh les plus communs représentaient 46 % des isolats. Une moitié des isolats de Psh peut résulter de l'hybridation de deux forma specialis, puisqu'ils affichaient de la virulence à l’égard d'un grand nombre de lignées de blé et de triticale en plus de celles d'orge. Les isolats de Psh affichant ces types de virulence provenaient le plus souvent d'orge à six rangs qu’à deux rangs. En ce qui a trait aux deux forma specialis, il ne semblait pas y avoir de rapport évident entre les pathotypes et le lieu de provenance, ces derniers étant largement disséminés et obtenus de plusieurs endroits différents.

Keywords:

• barley
• Puccinia striiformis f. sp. hordei
• Puccina striiformis f. sp. tritici
• resistance
• stripe rust
• virulence
• wheat

Mots clés :

• blé
• orge
• Puccinia striiformis f. sp. hordei
• Puccinia striiformis f. sp. tritici
• résistance
• rouille jaune
• virulence

Introduction

Stripe (yellow) rust of small grain cereals and grasses is caused by the fungus Puccinia striiformis Westend. The pathogen is an obligate biotroph and is capable of spreading long distances due to its airborne urediniospores. The species has been subdivided into formae speciales based on its specialization on host genera, with P. striiformis f. sp. tritici Eriks. & Henn. (Pst) primarily infecting wheat (Triticum aestivum L.) and P. striiformis f. sp. hordei Eriks. & Henn. (Psh) primarily infecting barley (Hordeum vulgare L.). The formae speciales are further subdivided into pathotypes or races based on their specialization on cultivars within a single host genus (Anikster, 1985).

In Canada, stripe rust was first detected in Alberta in 1918 on the wild grass species foxtail barley (Hordem jubatum L.) (Fraser & Conners, 1925). In subsequent years, it was found in British Columbia, southern Alberta and western Saskatchewan on wheat, barley and multiple grass species (Newton & Johnson, 1936). The main area of concern for stripe rust incidence in Canada has been central and southern Alberta, where it has an annual occurrence on cereals. Inoculum appears to overwinter in the Pacific Northwest (PNW) of the USA, and then spreads by wind to Alberta and initiates epidemics (Chen, 2005). There is also evidence that overwintering can occur in central and southern Alberta (Kumar et al., 2011; Gaudet, 2012). Epidemics of wheat stripe rust have occurred in central Alberta since the late 1990s, but were particularly devastating in Alberta in 2005, 2006 and 2011 when severities of up to 100% were observed in some commercial wheat fields, resulting in premature ripening of the crop, yield and quality losses and production of urediniospores on leaves and glumes (McCallum et al., 2006; Puchalski & Gaudet, 2011).

Although Pst has a long history of occurrence within the USA, the geographic range of stripe rust in the eastern USA has expanded since 2000 when a new population of pathotypes entered the USA (Milus et al., 2009). Stripe rust of wheat has increased in importance in central and southeastern USA since 2000 (Chen et al., 2010). Within the USA, the forma specialis continued to mutate and produce new pathotypes with more virulent pathotypes becoming common (Chen et al., 2010). At present, 140 distinct pathotypes of Pst have been detected within the USA (Wan & Chen, 2012) with most races initially detected in the western USA.

Puccinia striiformis f. sp. hordei was found for the first time in the western hemisphere, in Colombia in 1975, and since then has spread throughout western South America, causing yield losses of 30–70% in barley (Dubin & Stubbs, 1986). Since 1991, barley stripe rust has become established and caused considerable damage in the northwestern USA (Marshall & Sutton, 1995). Currently, 82 Psh pathotypes have been identified within the USA (Wan & Chen, 2012).

In Alberta, Canada, wheat and barley are both major crops. Wheat is the most frequently sown field crop, with the majority being spring wheat with limited autumn-sown winter wheat grown primarily in the southern portion of the province (Anonymous, 2012). Spring barley is the second most commonly planted cereal in Alberta with a majority of the area planted to two-row cultivars (Grenier, 2011).

Although major advances have been made in understanding the pathogen populations of wheat stripe rust in other regions, Pst in Canada is rarely studied and Psh is poorly understood. The objectives of this study were to identify pathotypes of Pst and Psh within Alberta and determine their frequency of occurrence in 2009–2011. The study is a continuation of virulence phenotyping begun in 2007 (Kumar et al., 2012), but expanded to include a greater number of isolates and host resistance genes. The pathotypes identified in the current and previous study are compared and recent changes in the pathogen populations in Alberta are discussed.

Materials and methods

Sample collection

A total of 104 isolates of P. striiformis were collected from diseased wheat, barley, foxtail barley and triticale (× Triticosecale Wittm. ex A. Camus.) plants in commercial fields and breeding nurseries at various locations in central and southern Alberta. Fifty-seven isolates were collected from wheat (36 from winter wheat, 17 from spring wheat and 4 from unknown sources) and 36 from barley (22 from six-row, 11 from two-row and 3 from unknown sources) in 2009–2011. Nine additional isolates were collected from foxtail barley in 2010–2011 and two from triticale in 2011. The majority of sampling was performed in 2010 and 2011, coinciding with the increased disease incidence that occurred in those years; only eight collections from 2009 were included. Sampling locations, hosts and years for all isolates are provided in Supplementary Table 1. Samples were collected or sent to our laboratory in paper envelopes. Single pustule isolates were purified and multiplied according to the methods of Kumar et al. (2012).

Table 1.  Virulence frequency of Puccinia striiformis f. sp. tritici and f. sp. hordei on wheat and barley differentials

				P. striiformis f. sp. hordei frequency (%)
Differential host	Resistance gene	Differential line	P. striiformis f. sp. tritici frequency (%)	All isolates	Pathotype group A	Pathotype group B
Wheat	Null	Avocet S	100	89	79	100
Wheat	YrA	Avocet R	100	49	0	100
Wheat	Yr1	Yr1/6*Avocet S	12	84	74	94
Wheat	Yr2	Yr2/6*Avocet S	100	14	0	28
Wheat	Yr5	Yr5/6*Avocet S	0	0	0	0
Wheat	Yr6	Yr6/6*Avocet S	100	62	42	83
Wheat	Yr7	Yr7/6*Avocet S	100	2.7	0	5.6
Wheat	Yr8	Yr8/6*Avocet S	100	0	0	0
Wheat	Yr9	Yr9/6*Avocet S	100	0	0	0
Wheat	Yr10	Yr10/6*Avocet S	19	0	0	0
Wheat	Yr15	Yr15/6*Avocet S	0	0	0	0
Wheat	Yr17	Yr17/6*Avocet S	100	0	0	0
Wheat	Yr24	Yr24/3*Avocet S	10	0	0	0
Wheat	YrUnk^a	Yr26/6*Avocet S	100	57	21	94
Wheat	Yr27	Yr27/6*Avocet S	100	0	0	0
Wheat	Yr28	Yr28/3*Avocet S	16	0	0	0
Wheat	Yr31	Yr31/3*Avocet S	100	92	84	100
Wheat	YrSP	YrSP/3*Avocet S	1.5	0	0	0
Wheat	YrCV	YrCV/6*Avocet S	100	0	0	0
Wheat	Yr17, YrTye	Hyak	10	0	0	0
Wheat	Yr10,YrMor	Moro	19	0	0	0
Wheat	YrPa1,YrPa2,YrPa3	Paha	34	0	0	0
Wheat	YrPr1,YrPr2	Produra	100	0	0	0
Wheat	Yr3a,YrS,YrSte	Stephens	100	41	0	83
Wheat	YrTye	Tyee	10	0	0	0
Wheat	Yr2,Yr4a, YrYam	Yamhill	100	0	0	0
Barley	Null	Topper	6.0	100	100	100
Barley	Rps4, rpsHF	Heils Franken	0	0	0	0
Barley	rpsEm1&2	Emir	0	0	0	0
Barley	Rps4, rpsAst	Astrix	0	0	0	0
Barley	rpsHi1&2	Hiproly	0	78	79	78
Barley	rpsVa1&2	Varunda	0	41	79	0
Barley	rps2	Abed Binder 12	1.5	97	95	100
Barley	rpsTr1&2	Trumpf	0	70	63	78
Barley	Rps1.c	Mazurka	0	51	100	0
Barley	Rps1.b	Bigo	0	51	100	0
Barley	Rps3, rpsI5	I 5	0	8.1	16	0
Barley	unknown	Bancroft	0	54	89	17
^a YrUkn was initially believed to be Yr26, but later found not to be Yr26 and different from any known Yr genes (Sharma-Poudyal et al., 2013).

Screening of isolates

A set of 19 wheat isogenic lines with the ‘Avocet’ spring wheat background (Table 1) that was used previously to characterize stripe rust isolates in Alberta (Kumar et al., 2012) was utilized. An additional seven wheat differential cultivars (‘Hyak’, ‘Moro’, ‘Paha’, ‘Produra’, ‘Stephens’, ‘Tyee’ and ‘Yamhill’) that have proven effective in differentiating Pst isolates detected since 2000 in the USA with virulence to Yr8 and Yr9 (Chen et al., 2010) were included with the wheat differential set (Table 1). Twelve barley differential cultivars used to differentiate Psh were used (Table 1). A set of wheat, triticale and barley cultivars (Table 2) grown in Alberta and previously tested with isolates from central Alberta was also used (Kumar et al., 2012). Inoculations were performed following the protocol of Kumar et al. (2012). Briefly, urediniospores of a single isolate were mixed with talcum powder in a 1 : 20 ratio and used to inoculate a tray of seedlings of all differentials and cultivars at the 2-leaf stage. The inoculated trays were maintained at 12 °C night, 16 °C day with a 16 h photoperiod after an initial 24 h incubation period in the dark at 10 °C. Infection types (ITs) were recorded 18–21 days after inoculation based on a 0–9 scale (Line & Qayoum, 1992). The virulence tests were repeated once to confirm the results. An isolate was considered to be avirulent when the IT was in the range of 0–4 and virulent when the IT was 5–9 (Wan & Chen, 2012).

Table 2.  Virulence frequency of Puccinia striiformis f. sp. tritici and f. sp. hordei on barley, triticale and wheat cultivars

				P. striiformis f. sp. hordei frequency (%)
Crop	Habit	Cultivar/ genotype	P. striiformis f. sp. tritici frequency (%)	All isolates	Pathotype group A	Pathotype group B
Wheat	spring	AC Barrie	100	0	0	0
Wheat	spring	AC Crystal	99	0	0	0
Wheat	winter	AC Bellatrix	100	89	84	94
Wheat	winter	Radiant	19	0	0	0
Triticale	spring	AC Ultima	97	41	0	83
Barley	spring	Seebe	0	49	89	5.6
Barley	spring	Falcon	3.0	57	42	72
Barley	spring	Condor	0	76	79	72
Barley	spring	Mahigan	93	100	100	100
Barley	spring	HB522	100	100	100	100

Characterization of isolates

Cluster analysis was performed using results from all isolates and their virulence on all differentials and cultivars. A dendrogram was generated using the unweighted pair grouping by mathematical average algorithms (UPGMA), with the simple matching coefficient of similarity in the program NTSYS-pc (Rohlf, 2000). Isolates were classified as either Pst or Psh based on which host species they preferentially infected. Pst isolates with identical virulence spectra on wheat differentials were considered the same Pst pathotype and Psh isolates with the same virulence spectrum on barley differentials were considered the same Psh pathotype. A number was assigned to each pathotype following the numbering system started by Kumar et al. (2012). For Psh, when the pathotype was identical to a race previously identified in the USA, the race designation given was the same as previously described, since the barley differential set used here was identical to that used previously (Chen, 2004, 2007). The virulence frequency on each differential and cultivar was calculated for each forma specialis separately. The virulence frequency for a forma specialis on a differential is the percentage of isolates within the forma specialis that was virulent on the differential. The Kosman diversity (KW) (Kosman, 1996; Kosman & Leonard, 2007) within the formae speciales was calculated using the program Virulence Analysis Tool (VAT) (Schachtel et al., 2012). It was calculated as KW = Ass _max (A,A)/nk, where Ass _max (A,A) is the maximum value of the sum of distances between ‘n’ matched pairs of isolates within population A, ‘n’ is the number of isolates in A, and ‘k’ is the number of differentials tested. The Kosman diversity was used due to its ability to incorporate both pathotype and virulence frequency as well as the degree of differences between pathotypes.

Results

Cluster analysis clearly separated the two formae speciales (Fig. 1). Of the 104 isolates analysed, 67 were determined to be Pst and 37 were Psh. Fifty-five Pst isolates were recovered from wheat, two from triticale, seven from foxtail barley, and three from cultivated barley. Thirty-three of the Psh isolates were recovered from barley, two from foxtail barley and the remaining two from wheat. The KW was 0.089 for Pst and 0.25 for Psh, indicating lower diversity within Pst than Psh.

Fig. 1. Similarity dendrogram based on simple matching coefficient of 104 Puccinia striiformis isolates collected from within Alberta based on virulence or avirulence to wheat, barley and triticale differentials and cultivars. Vertical lines to the right show the two formae speciales Pst (Puccinia striiformis f. sp. tritici) and Psh (Puccinia striiformis f. sp. hordei) as well as the two pathotype groups (A & B) within Puccinia striiformis f. sp. hordei.

The virulence frequencies of the Pst isolates on the 27 wheat differentials ranged from 0–100% (Table 1). Differentials ‘Avocet S’, YrA, Yr2, Yr6, Yr7, Yr8, Yr9, Yr17, YrUnk, Yr27, Yr31, YrCV, ‘Produra’, ‘Stephens’ and ‘Yamhill’ were susceptible to all isolates. Differentials Yr5 and Yr15 were resistant to all isolates. The remaining differentials were susceptible to up to 34% of the isolates. The wheat and triticale cultivars included were susceptible to 97–100% of isolates except for the Yr10 containing ‘Radiant’ which was susceptible to 19% of isolates (Table 2). Most barley differentials and cultivars were resistant to all Pst isolates (Tables 1 and 2). The barley differentials ‘Topper’ and ‘Abed Binder 12’ were only susceptible to 6% and 1.5% of the isolates, respectively, and the cultivar ‘Falcon’ was susceptible to only 3% of the isolates. ‘Mahigan’ and HB522 were highly susceptible, however, and were infected by 93% and 100% of Pst isolates, respectively.

There were 12 pathotypes among the 67 Pst isolates examined in this study (Table 3). The most common one (Pst pathotype 7) that was represented by 32 isolates (48%) was avirulent on Yr1, Yr5, Yr10, Yr15, Yr24, Yr28, YrSP, ‘Hyak’, ‘Moro’, ‘Paha’ and ‘Tyee’ and virulent on the other 15 wheat differentials. The next most common pathotype (Pst pathotype 14) was from nine isolates (13.4%) and was identical to Pst pathotype 7 except for the additional virulence on ‘Paha’. The remainder of the pathotypes were virulent on one to four more differentials than the most common pathotype (Table 3). All Pst pathotypes represented by more than one isolate were recovered from multiple counties. All Pst pathotypes were recovered from wheat except for the single isolate of pathotype 20 from triticale. The most common pathotype (Pst pathotype 7) accounted for all collections of Pst from cultivated barley and the other collection from triticale. Three pathotypes were present in the seven Pst isolates collected from foxtail barley (Table 3). Only the most common pathotype was recovered in all 3 years of sampling. Pathotypes with virulence towards ‘Paha’ were not recovered until 2010. Pathotypes with virulence on Yr1 and/or ‘Hyak’ and ‘Tyee’ were not recovered until 2011.

Table 3.  Virulence spectra and frequency of Puccinia striiformis f. sp. tritici pathotypes sampled from 2009 to 2011 in Alberta

	Differential genotype																											Host of origin
Pathotype	Null	YrA	Yr1	Yr2	Yr5	Yr6	Yr7	Yr8	Yr9	Yr10	Yr15	Yr17	Yr24	YrUnk	Yr27	Yr28	Yr31	YrSP	YrCV	Hyak	Moro	Paha	Produra	Stephens	Tyee	Yamhill	# ^b	Spring wheat	Winter wheat	Wheat (unknown)	Triticale	Foxtail barley	Barley	Year of 1^st detection inAlberta ^c
4	V ^a	V		V		V	V	V	V	V		V		V	V		V		V		V		V	V		V	5		5					2010 ^d
5	V	V		V		V	V	V	V	V		V	V	V	V	V	V		V		V		V	V		V	4	1	3					2009
6	V	V		V		V	V	V	V			V		V	V	V	V		V				V	V		V	2		2					2010 ^e
7	V	V		V		V	V	V	V			V		V	V		V		V				V	V		V	32	9	10	4	1	5	3	2007
14	V	V		V		V	V	V	V			V		V	V		V		V			V	V	V		V	9	4	4			1		2010
15	V	V		V		V	V	V	V	V		V		V	V		V		V		V	V	V	V		V	2		2					2010
16	V	V	V	V		V	V	V	V			V		V	V		V		V	V		V	V	V	V	V	6	1	4			1		2011
17	V	V		V		V	V	V	V	V		V	V	V	V	V	V		V		V	V	V	V		V	2		2					2011
18	V	V		V		V	V	V	V			V		V	V	V	V		V			V	V	V		V	2		2					2011
19	V	V		V		V	V	V	V			V	V	V	V	V	V		V			V	V	V		V	1	1						2011
20	V	V	V	V		V	V	V	V			V		V	V		V		V				V	V		V	1				1			2011
21	V	V	V	V		V	V	V	V			V		V	V		V	V	V	V		V	V	V	V	V	1	1						2011
^aV virulent.
^bTotal number of isolates collected of a pathotype.
^cRefers to this paper or Kumar et al. (2012).
^dDetected earlier in 2009 British Columbia, Canada (Kumar et al., 2012).
^eDetected in 2005 in Manitoba, Canada (Kumar et al., 2012).

Amongst Psh isolates, virulence to differentials ‘Topper’, ‘Hiproly’, ‘Abed Binder 12’ and ‘Trumpf’ was very common, occurring in 70–100% of isolates (Table 1). Virulence towards ‘Varunda’, ‘Mazuraka’ and ‘Bigo’ was moderately common (41–51%). Virulence to ‘I 5’ was rare (8%) and ‘Emir’, ‘Astrix’ and ‘Heils Franken’ were resistant to all isolates. The barley cultivar ‘Mahigan’ and line HB522 were susceptible to all Psh isolates and ‘Seebe’, ‘Falcon’ and ‘Condor’ were susceptible to 49–76% of Psh isolates (Table 2). The majority of wheat differentials were resistant to all Psh isolates except for ‘Avocet S’, YrA, Yr1, Yr2, Yr6, Yr7, YrUnk, Yr31 and ‘Stephens’ which were susceptible to up to 92% of isolates (Table 1). The wheat cultivars ‘AC Barrie’, ‘AC Crystal’ and ‘Radiant’ were resistant to all Psh isolates whereas ‘AC Bellatrix’ was susceptible to 89% of them (Table 2). The spring triticale ‘AC Ultima’ was susceptible to 41% of Psh isolates. Virulence on YrA, Yr2, Yr7, ‘Stephens’ and ‘AC Ultima’ occurred only in Psh isolates avirulent on barley differentials ‘Varunda’, ‘Mazurka’ and ‘BigO’.

Twelve Psh pathotypes were detected (Table 4). The two most common Psh pathotypes (18 and 2) accounted for 24% and 22% of isolates, respectively. Each of the remaining pathotypes accounted for 2.7–8.1% of Psh isolates. All Psh pathotypes recovered more than once occurred in multiple counties except for Psh pathotype 3. All three isolates of that pathotype were recovered from Lacombe County. All Psh pathotypes were recovered exclusively from cultivated barley except the single isolate of Psh pathotype 7 from foxtail barley and three of the nine isolates of Psh pathotype 18, two from winter wheat and one from foxtail barley. None of the samples examined from 2009 were Psh. All Psh pathotypes recovered more than twice occurred in both 2010 and 2011 except the second most common pathotype, which was only recovered in 2010 and all three isolates of Psh pathotype 17 that were recovered in 2011.

Table 4.  Virulence spectra and frequency of Puccinia striiformis f. sp. hordei pathotypes sampled from 2009 to 2011 in Alberta

			Differential genotype													Host of origin
Pathotype group	Pathotype	Race	Topper	Heils Franken	Emir	Astrix	Hiproly	Varunda	Abed Binder 12	Trumpf	Mazurka	Bigo	I 5	Bancroft	Number of isolates	Two-row barley	Six-row barley	Barley – unknown variety	Foxtail barley	Winter wheat	Year of first detection in Alberta	Year of first detection in the USA
A	2	PSH-76	V ^a				V	V	V	V	V	V		V	8	1	5	2			2008	2006
	3	−	V				V	V	V		V	V		V	3		3				2008	−
	7	−	V					V	V	V	V	V		V	1				1		2007	−
	11	−	V				V		V		V	V			1	1					2008	−
	14	−	V								V	V			1	1					2008	−
	16	−	V						V		V	V		V	2	2					2010	−
	17	PSH-69	V				V	V	V	V	V	V	V	V	3	3					2011	2003
B	15	PSH-33	V						V						2		2				2008	1996
	18	PSH-52	V				V		V	V					9	1	5		1	2	2010	1998
	19	PSH-56	V				V		V	V				V	3	1	2				2010	2001
	20	PSH-46	V						V	V					2		2				2010	1996
	21	PSH-51	V				V		V						2	1	1				2011	1998
^aV virulent

The Psh pathotypes formed two large pathotype groups based on their virulence on barley and wheat (Fig. 1, Table 4). The first group (A) contained seven pathotypes, all of which were virulent on ‘Marzurka’ and ‘Bigo’; isolates in pathotype group B were avirulent on these differentials. Additionally, the average number of virulence factors towards wheat and triticale per isolate in Psh pathotype group A was 3.8 and 9.6 in pathotype group B. Within Psh, it was pathotype group B where virulence on YrA, Yr2, Yr7, ‘Stephens’ and ‘AC Ultima’ was confined. Of the 19 isolates that were in pathotype group A, eight were recovered from two-row barley, eight from six-row barley, one from foxtail barley, and two from barley with an unknown phenotype. For the isolates in pathotype group B, 12 were recovered from six-row barley, three from two-row barley, one from foxtail barley, and two from winter wheat.

Discussion

Virulence analysis in this study showed that, as expected, the primary cause of stripe rust in wheat in Alberta was Pst and Psh in cultivated barley. Foxtail barley was found to be infected more often by Pst than Psh. This may not be indicative of foxtail barley being more susceptible to Pst than Psh. It could be a result of foxtail barley being exposed to greater amounts of Pst than Psh inoculum, as wheat stripe rust epidemics occurred during the sampling period whereas barley stripe rust was relatively rare.

All the Pst isolates had virulence on both Yr8 and Yr9, a characteristic distinctive of a group of invasive isolates that entered North America sometime around 2000 and spread across the continent (Chen et al., 2002; Hovmøller et al., 2008). In addition to Yr8 and Yr9, all pathotypes also had virulence towards ‘Stephens’ and ‘Yamhill’. This virulence combination was not detected until 2002 in the USA (Chen et al., 2010). The ability of isolates to infect the Yr8, Yr9 and ‘Paha’ differentials was first detected in the western USA in 2004 (Chen et al., 2010). By 2008, isolates with these virulence factors were widespread across the USA (Wan & Chen, 2012). Although not detected until 2010, it is possible that pathotypes with those phenotypes were already present in Alberta, as ‘Paha’ was previously not used as a differential with Alberta isolates (Su et al., 2003; Kumar et al., 2012). Pathotypes with virulence on ‘Hyak’ and ‘Tyee’ as well as Yr1, Yr8 and Yr9 were not detected until 2007 in the western USA (Chen et al., 2010). As of 2009, isolates with these virulence characteristics had not spread east of Idaho, USA (Wan & Chen, 2012). Their detection in 2011 is indicative of the relatively short period of time that is necessary for stripe rust to spread between regions. Although the differentials ‘Hyak’ and ‘Tyee’ were not previously used in Alberta, the pathotypes appear to have not been present. This is because isolates with virulence towards Yr1, Yr8 and Yr9 that these pathotypes also possess were not detected prior to this time in Alberta (Su et al., 2003; Kumar et al., 2012). Of the 12 Pst pathotypes detected in this study, five were detectable only due to the addition of ‘Paha’ and ‘Hyak’/‘Tyee’ as differentials.

Only 19% of Pst isolates were able to overcome the Yr10 resistance gene deployed in the winter wheat cultivar ‘Radiant’. Although this percentage is higher than the 5% reported previously from the same area (Kumar et al., 2012), isolates virulent on Yr10 are still a minority in central Alberta. In the present study, all but one of the 13 isolates virulent on Yr10 were recovered from winter wheat, most commonly ‘Radiant’. The unequal ratio of recovery from winter wheat vs. spring wheat was not as pronounced for other Pst isolates, being approximately 1 : 1 for the two most abundant Pst pathotypes (pathotypes 7 and 14). The relatively rare recovery of isolates virulent on Yr10 from spring wheat suggests that isolates virulent on Yr10 may be less fit on non-Yr10 containing cultivars than the more common Pst pathotypes.

In Pst the low KW value and small differences between pathotypes is indicative of the shared ancestry the isolates have to the invasive strains that entered the USA in 2000 and the stepwise mutation that the species progressed through. The isolates with virulence on ‘Hyak’ and ‘Tyee’ may be a partial exception to this. These isolates have genotypes quite distinct from the other Pst isolates (Holtz et al., 2012). Also, virulence phenotyping of American isolates with these virulence patterns has resulted in the hypothesis that the isolates may have resulted from somatic hybridization between Pst isolates from the population that arrived in 2000 and the pre-existing North American Pst population (Chen et al., 2010).

Barley stripe rust was significantly more diverse than Pst with a higher KW despite being sampled less frequently and in only 2 of 3 years. Within Psh, there seem to be two distinct pathotype groups, A and B, represented by a nearly equal number of isolates. Of the seven pathotypes in pathotype group A, five have been detected previously in Alberta (Kumar et al., 2012), but only two have been reported in the USA (Chen, 2008). Of the pathotypes in pathotype group B, only one has been reported previously in Alberta, but all five have been detected in the USA (Chen, 2008; Kumar et al., 2012). Although some wheat genotypes such as ‘Avocet S’ and ‘AC Bellatrix’ are highly susceptible to almost all Psh isolates, it is in group B where much of the virulence towards wheat was found. Also, they were the only Psh isolates virulent on triticale. Although it is not atypical for Psh isolates to be virulent on some wheat genotypes (Chen et al., 1995), the number of virulence factors towards wheat detected here seems abnormally high. All the Psh isolates in pathotype group B were found to have microsatellite alleles previously detected only in Pst isolates (Holtz et al., 2012). This is suggestive that these isolates could be hybrids between Pst and Psh. Similar reports of stripe rust isolates that appear to be somatic hybrids between Pst and Psh based on their virulence on differentials and alleles at microsatellite loci have been described recently in the USA (Cheng & Chen, 2009).

It is not clear why members of pathotype group B increased in frequency over the short duration of the study. They were undetected in the 2 years prior to the present study (Kumar et al., 2012) and increased to nearly half of the isolates collected a few years later. This abundance may be overemphasized due to the ad hoc sampling strategy employed. The number of samples collected from six-row barley was significantly higher in 2011 than 2010. If isolates in pathotype group B are rarer on two-row barley than six-row barley, their numbers may have been overstated by sampling mostly from what could be a preferred host. Despite this, it is possible that there is a fitness penalty to having unnecessary virulence genes towards barley, and Psh isolates without these are better pathogens. Due to barley stripe rust being a new disease in Canada, there has been no necessity to breed stripe rust resistance into extant cultivars. Therefore, it is likely few resistance genes are present in Canadian barley cultivars. Unnecessary virulence factors can result in large fitness costs in Pst (Bahri et al., 2009). If unnecessary virulence factors impose a fitness cost in Psh, it would seem counterintuitive that Psh isolates rarely recovered from wheat, yet with the ability to infect many wheat cultivars, would be so abundant. It is possible that the ability to infect additional wheat cultivars may provide some advantage in the life cycle of Psh. Winter wheat could potentially act as an overwintering host for these Psh isolates in the absence of winter barley. Additionally, it is possible exogenous Psh inoculum arriving early in the season could infect winter wheat, produce secondary inoculum, and then spread to barley after it has been seeded and emerged. Thereby, the virulence towards wheat may increase the isolates fitness by allowing it to survive year-round and reproduce more frequently.

From the results of the pathotyping of Pst within Alberta, it is clear that the pathogen population is similar to what has been present in recent years, but there was a trend towards increased virulence. Due to the rapidity of erosion of seedling resistance genes to this pathogen, it is likely that durable adult plant resistance and gene pyramiding are necessary to provide long-term resistance (Chen et al., 2010). Puccinia striiformis f. sp. hordei appeared to be much more diverse than Pst, attributable in part to the arrival of a group of pathotypes that are potentially hybrids between the two formae speciales. Despite this apparent blurring of the boundaries between the formae speciales, these isolates appear to be de facto Psh isolates, recovered from cultivated barley in the vast majority of cases. Within both formae speciales, there was preliminary evidence of specialization on certain wheat and barley classes. Future studies should focus more carefully on the host and temporal dynamics of the stripe rust populations in the area.

Acknowledgements

The authors wish to thank L. Langford for technical assistance. The authors express gratitude to X.M. Chen and A. Navabi for providing barley and wheat differentials. The funding provided by Alberta Crop Industry Development Fund for this project is gratefully acknowledged.