1. Introduction
Resistance to rust diseases in bread wheat cultivars (
Triticum aestivum L., 2n = 6× = 42, AABBDD) is a common challenge due to the constant evolution of virulence in rust pathogens.
Pst causes stripe rust (also known as yellow rust) on more than 20 grass and crop species including wheat, barley (
Hordeum vulgare L.), triticale (×
Triticosecale Wittmack), and rye (
Secale cereale L.) [
1]. Even though the extent of yield loss caused by stripe rust depends on factors such as pathogen aggressiveness, cultivar resistance level, and environmental variables, it has been reported that the global annual yield loss is estimated to be 1%, equivalent to USD 1 billion [
2,
3]. Globally, stripe rust is highly prevalent, with an occurrence rate of 88%. Moreover, the majority of wheat cultivars exhibit compatibility with this disease due to the presence of susceptible genes [
3,
4]. The prevalence of the pathogen is closely associated with the availability of suitable host plants and favorable weather conditions [
5].
Wheat stripe rust can be managed through the utilization of cultivars carrying resistance genes, the application of fungicides, and the implementation of improved agronomic practices. Among these strategies, the deployment of resistant
Yr genes stands out as the most effective approach due to its environmental friendliness, cost-effectiveness, and ease of implementation [
6]. At present, over 80
Yr genes have been identified from various domesticated and wild species of wheat, comprising 54 genes conferring all-stage resistance (ASR) and 24 genes providing adult-plant resistance (APR) or high-temperature adult-plant (HTAP) resistance [
7].
The diverse genetic makeup of
Pst populations, resulting from mechanisms such as sexual recombination, mutation, and somatic hybridization, is responsible for virulence variation within the pathogen of stripe rust [
8,
9,
10]. The wind-mediated dispersal of
Pst urediniospores across wheat-growing environments also contributes to the genetic diversity of the pathogen [
11]. Over time, these diversifications facilitate the emergence or re-emergence of highly virulent races of
Pst, which are capable of overcoming genetic resistance in wheat cultivars. Subsequently, this results in recurrent disease epidemics [
3,
9,
12]. For instance, the emergence of new races within the
Pst population has led to disease outbreak, due to the breakdown of resistance for genes
Yr7,
Yr8,
Yr9,
Yr10,
Yr17, and
Yr27 in various countries [
12,
13,
14]. High levels of virulence have been reported in numerous characterized
Yr genes, with the exceptions of
Yr5 and
Yr15 [
7,
15]. The genetic variabilities and virulence spectra of
Pst populations have been extensively studied across countries, including Australia [
11], South Africa [
16], Canada [
14], Russia [
1], the USA [
15,
17,
18], China [
19], and Turkey [
20].
From 1950 to 2000, China witnessed seven significant epidemics of stripe rust [
21]. Since then, numerous research studies have been undertaken to monitor the virulence dynamics and genetic diversity of
Pst populations. The analysis of virulence variation in
Pst populations utilized a set of 19 differentials of wheat cultivars established in 2010, along with
Yr single-gene differentials [
7]. Chen et al. [
22] identified 41 races of
Pst from a total of 4714 isolates and they also observed changes in the virulence of CYR33 (Chinese yellow rust) on
YrSu. Additionally, the race composition of a
Pst population across various provinces of the country was determined [
23,
24]. In the hotspot provinces of China, more than 750
Pst races/pathotypes have been identified. These include various predominant and virulent races, namely CYR1, CYR8, CYR10, CYR13, CYR16, CYR17, CYR18, CYR19, CYR23, CYR25, CYR28, CYR29, CYR30, CYR33, and the L13 and SY11 pathotype groups [
7]. The implementation of molecular markers for genotyping
Pst populations has been widely employed in China. Using SSR markers, Chen et al. [
24] identified shared multi-locus genotypes with the
Pst populations in the provinces of Yunnan and Guizhou, which demonstrated a significant genetic resemblance between these geographical areas. Hu et al. [
25] conducted a study to assess the genetic differentiation among 961
Pst isolates, obtained from various wheat-growing regions in the provinces of Gansu, Shaanxi, Sichuan, and Tibet. Their research findings explicitly indicated the existence of gene flow between
Pst populations. Similarly, Yan et al. [
26] confirmed genetic differentiation within individuals of isolates, while a small proportion of genetic variation was observed between the
Pst populations of Yunnan and the Yangtze river. Furthermore, the simple nucleotide polymorphism (SNP) and amplified fragment length polymorphism (AFLP) techniques are commonly utilized in China for genotyping
Pst populations [
27,
28]. In China, the provinces of Gansu, Sichuan, Guizhou, and Yunnan have been identified as epidemiological zones, exhibiting frequent variations in the virulence of stripe rust [
21].
Understanding the temporal and spatial dynamics of Pst populations is essential for the implementation of timely and effective disease management strategies in these high-risk regions. In this study, we characterized and compared the virulence patterns of two groups of Pst isolates collected during the epidemic seasons of 2016 and 2023. The specific objectives of this study were to (1) analyze the pathotypes of Pst collections based on the Chinese differentials, (2) determine the frequency of virulence and effective genes using Yr single-gene differentials, and (3) assess the genetic diversity among Pst populations using molecular markers.
4. Discussion
The frequent outbreaks of wheat stripe rust in China led to the constant replacements of cultivars [
21,
44]. The northwestern (including Gansu) and the southwestern regions (including Yunnan, Guizhou, and Sichuan) of China are among the largest over-summering and over-wintering regions of
Pst [
45]. Monitoring the phenotypic and genetic diversity of
Pst in these regions is essential to understand the pathogen’s population structure and to identify effective
Yr genes for breeding programs. The virulence profiles of the
Pst isolates on the Chinese differentials revealed a wide distribution of pathotypes across provinces. Approximately 19% of pathotypes (44% of isolates) were detected more than once within or among provinces. This phenomenon is likely attributed to the migration of spores between provinces.
Pst pathotypes, including
V2,
8,
14;
V3,
7,
10,
19;
V2,
4,
8,
14; and
V2,
4,
8,
10,
14,
19, were consistently found in both old and new isolates. The presence of ancestral isolates in the current populations of
Pst indicates their capacity to adapt to diverse environmental factors in the past eight years. In contrast, pathotypes such as
V0;
V2,
8;
V2,
8,
19; and
V2,
8,
14,
19 were prevalent in old isolates, but were not detected in new ones, indicating a potential genetic drift in these specific pathotypes. This result is consistent with the observation of the previous report [
21].
It was observed that the newly identified isolates exhibited a higher frequency of virulence to the Chinese and
Yr single-gene differentials compared to the old isolates, thereby suggesting a shift in the virulence. Two possible explanations can be derived. First, mutations, as a mechanism of virulence variation, give rise to new isolates through evolution [
19]. In most cases, mutations increase the magnitude of virulence [
46]. Second, the newly emerged isolates of
Pst were found to be more aggressive than the old isolates [
17]. However, the high virulence uniformity observed in the old isolates from Sichuan province should not be underestimated. This uniformity in virulence could increase the risk of stripe rust outbreaks, since the virulence profiles on
Yr genes remain consistent. It was observed that the
Pst populations collected from different provinces during the 2016 or 2023 epidemic seasons demonstrate a higher virulence similarity within their respective populations. Conversely, a lower degree of virulence similarity was observed between the
Pst populations collected during the two epidemic seasons. This suggests that the changes introduced between 2016 and 2023 have a greater impact on
Pst virulence dynamics compared to the environmental variations among the provinces at a given time. Previous studies have revealed that the populations of
Pst from Yunnan and Guizhou are closely related, while those from Sichuan and Gansu exhibit distinct clustering patterns [
47], which is consistent with our findings. Similarly, in Europe,
Pst isolates collected before 2011 from the United Kingdom and France showed minimal genetic diversity. Nevertheless, these isolates were genetically distinct from those collected in 2013 [
48].
With the exception of trace sporulation (IT 1) on a 0–4 scale [
22], incompatibility was observed between the
Pst isolates and Zhong 4 (unknown gene),
Yr5, and
Yr15. This suggests their effectiveness against
Pst isolates. The effectiveness of Zhong 4,
Yr5, and
Yr15 has been extensively documented [
15,
29,
44,
49]. However, instances of virulence to
Yr15 in Afghanistan [
50] and
Yr5 in India, Australia, Tajikistan [
11], Turkey [
51], and Syria [
52] have been reported. In China, virulence on
Yr5 was detected in the Shaanxi and Qinghai provinces [
53]. Due to their close geographical proximity, the incursion of
Yr5 virulent isolates through wind from Shaanxi and Qinghai to Sichuan, Guizhou, and Gansu provinces is likely to occur. Thus, continuously monitoring the
Pst population structures is a crucial task. Virulence to
Yr76,
Yr50,
Yr32,
Yr24, Yr10,
Yr17,
YrSp,
YrTr1, and
Yr26 was reached at a moderate level in the studied provinces. The moderate virulence signals a potential risk of complete resistance loss under high selection pressure. Other
Yr genes displayed a province-specific effectiveness. For instance,
Yr6 and
Yr21 exhibited a moderate level of effectiveness against the
Pst population in the Sichuan province. However, their resistance was overcome by the isolates found in the remaining provinces.
Yr40 was effective against the Guizhou isolates, but ineffective to the Sichuan and Yunnan isolates. This location-specific efficacy suggests the limited applicability of resistance genes across provinces. High selection pressure on
Yr45,
YrSu,
Yr43,
YrA,
Yr25,
Yr7,
Yr9,
Yr29,
YrJu4, and
YrRes was observed across the
Pst isolate groups. Among them,
Yr7,
Yr9,
Yr29,
YrSu, and
YrA have been deployed widely in Chinese wheat cultivars [
54]. However, the continuous cultivation of these susceptible cultivars may lead to an accumulation of inoculum, thereby increasing disease pressure over time. Therefore, it is essential to replace these cultivars at the regional level with alternative cultivars that possess
Yr5 and
Yr15. Compared to the 2016 survey conducted by Chen et al. [
55], dynamics in virulence were observed among
Yr genes. Consistency with our results was noted for virulence to
Yr5,
Yr7,
Yr43, and
Yr44. While increases in virulence were observed for
Yr8,
Yr17,
Yr24,
Yr32, and
YrTr1,
Yr6,
Yr9,
YrSp, and
Yr76 exhibited a clear decline in virulence. Likewise, our results showed a relatively lower frequency of virulence compared to previous studies in the Yunnan and Guizhou provinces [
24]. This dynamic in frequency of virulence could be attributed to factors such as mutation, selection, sexual recombination, and migration [
56].
Associations among alleles at pairs of avirulence loci enhance the durability of resistance against stripe rust. Several
Yr gene pairs exhibited positive association, with a VV proportion of less than 10%. For instance, there was a significant association between avirulence to
Yr24 and
Yr26 in the
Pst isolates. These
Yr genes have been frequently used as sources of resistance genes in wheat breeding programs in China [
57]. However, the emergence of virulent races to
Yr24 and
Yr26 has been reported in the Sichuan, Gansu, Shaanxi, Ningxia, and Qinghai provinces, posing a threat to wheat production [
55,
58]. This underscores an urgent need for pyramiding
Yr24 and
Yr26, as the co-selection likelihood of these
Yr genes by the pathogen is low. Similarly, 22
Yr gene pairs were found to be positively associated with a lower proportion of virulence compared to their individual virulence frequency. This demonstrates that pyramiding these genes could serve as a viable strategy to extend the duration of resistant cultivars in production.
Both the phenotypic and genotypic results revealed the presence of genetic diversity in the
Pst population. The comparison among provinces showed that the
Pst population of Gansu exhibit a higher genetic diversity. This can be attributed to favorable environmental conditions that support over-summering and over-wintering, along with the presence of alternative hosts like
Berberis spp., which encourage sexual recombination within the population [
59]. Previous studies have confirmed the existence of genotypic diversity within the
Pst population in Gansu province [
27,
60,
61]. Moreover, the Yunnan
Pst population exhibited a high level of pathotypic and genotypic diversity. The cultivation of various wheat genotypes throughout the year in Yunnan likely increases the genetic diversity of
Pst [
61]. The cultivation of various wheat cultivars frequently leads to the deployment of diverse
Yr genes, thereby augmenting the overall diversity of the pathogen population. A diverse pathogen population increases the likelihood of the emergence of more virulent pathotypes through mechanisms such as mutation, recombination, and host selection [
8,
9,
10]. Consequently, this can lead to the occurrence of novel disease epidemics. To mitigate this risk, regions with high pathogen diversity should implement a broader range of breeding programs, aiming to develop resistant cultivars that can effectively combat the disease. On the contrary, the
Pst isolates from Guizhou province exhibited the lowest diversity, presumably due to unfavorable weather conditions, narrow host range, geographic barriers, and a small sample size.
The results of AMOVA revealed that the majority of genetic differentiation occurred within populations, despite limited genetic variation observed among populations. This is consistent with a previous study [
25]. Several factors influence the level of genetic differentiation in
Pst populations. Gene flow increases genetic variation within a population, while decreasing genetic differentiation between populations [
25]. The lack of distinct clustering among
Pst populations in the PCoA signifies the likelihood of gene flow among provinces. It is well-known that mutation and somatic recombination can augment genetic variation within populations [
9]. Similarly, studies have confirmed that sexual recombination significantly increases the degree of genetic variability within
Pst populations [
8,
25]. However, natural selection and genetic drift contribute to genetic differentiation between populations [
19]. This suggests that the collective effect of these factors could determine the genetic variation among
Pst populations.
Despite limited genetic variation observed among provinces, the
Pst populations of Gansu and Sichuan were found to be genetically distant. The urediniospores of
Pst exhibit a migratory behavior towards the mountainous regions of Gansu during the summer season, to undergo over-summering in favorable conditions. Similarly, during the winter season, these spores move to the lower-elevation areas of Sichuan for over-wintering, taking advantage of the cold conditions prevalent in that region [
62]. Gradually, the
Pst populations could potentially limit or even stop such migratory behavior, undergoing location-specific evolutionary changes to cope with temperature fluctuations. Consequently, this could result in a divergence in their genetic makeup. Recent evidence has confirmed distinct clustering patterns among
Pst isolates from Sichuan and Gansu [
47].
In conclusion, it can be deduced that the newly evolved isolates of Pst demonstrate a higher virulence complexity, leading to the ineffectiveness of 14.2 out of 31 Yr genes, compared to older isolates, which rendered only 12 Yr genes ineffective. All Pst isolates were avirulent to lines with Yr5, Yr15, and Zhong 4. These resistant Yr genes can be recommended for stripe rust breeding programs in China. The Pst population exhibited higher pathotypic and genotypic diversities across the studied provinces of China. These findings underscore the necessity to develop multiple resistant cultivars or employing gene-pyramiding strategies to mitigate the risks associated with the diversity of the pathogen.