Molecular mapping of the crown rust resistance gene rpc1 in barley

ABSTRACT Crown rust of barley, caused by Puccinia coronata var. hordei, occurs sporadically and sometimes may cause yield and quality reductions in the Great Plains region of the United States and Canada. The incompletely dominant resistance allele Rpc1 confers resistance to P. coronata in barley. Two generations, F(2) and F(2:3), developed from a cross between the resistant line Hor2596 (CIho 1243) and the susceptible line Bowman (PI 483237), were used in this study. Bulked segregant analysis combined with random amplified polymorphic DNA (RAPD) primers were used to identify molecular markers linked to Rpc1. DNA genotypes produced by 500 RAPD primers, 200 microsatellites (SSRs), and 71 restriction fragment length polymorphism (RFLP) probes were applied to map Rpc1. Of these, 15 RAPD primers identified polymorphisms between resistant and susceptible bulks, and 62 SSR markers and 32 RFLP markers identified polymorphisms between the resistant and susceptible parents. The polymorphic markers were applied to 97 F(2) individuals and F(2:3) families. These markers identified 112 polymorphisms and were used for primary linkage mapping to Rpc1 using Map Manager QT. Two RFLP and five SSR markers spanning the centromere on chromosome 3H and one RAPD marker (OPO08-700) were linked with Rpc1 and, thus, used to construct a 30-centimorgan (cM) linkage map containing the Rpc1 locus. The genetic distance between Rpc1 and the closest marker, RAPD OPO08-700, was 2.5 cM. The linked markers will be useful for incorporating this crown rust resistance gene into barley breeding lines.     

Sources and genetics of crown rust resistance in barley

ABSTRACT Crown rust, caused by Puccinia coronata var. hordei, is a new disease threat to barley in the Great Plains region of the United States. Deployment of resistant cultivars is the only economically viable option for the control of this disease. Thus, the objective of this study was to investigate the sources and genetics of crown rust resistance in barley. A geographically diverse sample of barley germ plasm collected around the world (526 accessions total) was evaluated at the seedling stage to P. coronata var. hordei, and only 10 accessions (1.9% of the total) were found resistant. These 10 accessions were also resistant at the adult plant stage in a greenhouse test. Three F(2) populations (Bowman x Hor2596, MR x Hor2596, and MD x Hor2596) were developed to study the inheritance of crown rust resistance in the resistant line Hor2596 (CIho 1243). A close fit to a 3:1 ratio of resistant/susceptible plants was observed in all three populations and is consistent with the segregation of a single resistance gene. F(1) plants from the Bowman x Hor2596 population exhibited slightly higher infection types than the resistant parent, indicating incomplete dominance. The locus symbol Rpc1 and allele symbol Rpc1.a were recommended for the crown rust resistance gene in Hor2596. An attempt was made to associate the Rpc1 locus with one of the seven barley chromosomes by analyzing linkage data with previously mapped morphological markers in crosses with multiple recessive (MR) and multiple dominant (MD) morphological marker stocks. However, no close linkages were detected between Rpc1 and the 20 morphological markers present in the marker stocks. The resistant accessions identified in this study should be useful to breeders for developing barley germ plasm with crown rust resistance.     

Effects of recombination on races of a barley powdery mildew population

Analyses of virulence were performed with isolates from ascospore populations and their possible parental populations in order to measure the potential effect of recombination on the pathogenic variation in populations of Erysiphe graminis f. sp. hordei. The conidial isolates and cleistothecia were sampled from a field of spring barley cv. Golden Promise. Compared to the summer conidial populations, higher frequencies of rare races and rare virulence factors together with greater diversity were observed among progeny from ascospores, and the most frequent race was less abundant. The contribution of ascospores to primary infection of barley in autumn could not be measured directly, but analyses of the conidial populations on volunteer seedlings of Golden Promise suggested that sexual recombination was one of the mechanisms that had caused the change in population structure.     

中国小麦条锈菌生理小种同工酶分析

 本文首次报道利用聚丙烯酰胺凝胶电泳技术分析我国小麦条锈菌11个主要流行生理小种及美国小麦条锈菌白化菌系夏孢子提取物中酯酶、过氧化物酶、过氧化氢酶、碱性磷酸酶、酸性磷酸酶、谷草酰胺转氨酶、多酚氧化酶的同工酶图谱。结果表明:我国小麦条锈菌不同生理小种具有同工酶表型异质性,而同一生理小种的不同单孢系间或不同地理来源的菌系间同工酶表型相同,即生理小种群体内部可能具有同工酶表型同质性。依据同工酶资料的统计分析结果,将所测的中国小麦条锈菌的11个生理小种划为5个类群:A群(条中8号)、B群(条中10号)、C群(条中17号)、D群(条中19、22、23、25、27、26号)、E群(条中28、29号),它反映出小种间遗传基础的相似性及演化特点。结合对美国条锈菌菌系同工酶测定,并与国外同类研究结果比较,揭示出中国小麦条锈菌具有独特的同工酶表型。同工酶表型分析是研究锈菌毒性变异的一个有力工具,建议建立起中国小麦条锈菌毒性菌系多种同工酶标准图谱,用作研究小麦条锈菌毒性变异和群体遗传学的参照系。     

Genes Governing Resistance to Puccinia hordei in Thirteen Spring Barley Accessions

ABSTRACT Leaf rust, caused by Puccinia hordei, is an important disease of barley in many parts of the world. In the eastern United States, this disease was effectively controlled for over 20 years through the deployment of cultivars carrying the resistance gene Rph7. Isolates of P. hordei with virulence for Rph7 appeared in this region in the early 1990s rendering barley cultivars with this gene vulnerable to leaf rust infection. From a preliminary evaluation test, 13 accessions from diverse geographic locations possessed resistance to P. hordei isolate VA90-34, which has virulence for genes Rph1, 2, 4, 6, 7, 8, and 11. Each of these 13 accessions was crossed with susceptible cvs. Moore or Larker to characterize gene number and gene action for resistance to P. hordei. Additionally, the 13 accessions were intercrossed and crossed to host differential lines possessing genes Rph3, Rph5, and Rph9 to determine allelic relationships of resistance genes. Seedlings of F(1), F(2), and BC(1)F(1) populations were evaluated in the greenhouse for their reaction to P. hordei isolate VA90-34. Leaf rust resistance in six of the accessions including Collo sib, CR270.3.2, Deir Alla 105, Giza 119, Gloria, and Lenka is governed by a single dominant gene located at or near the Rph3 locus. All accessions for which the gene Rph3 was postulated to govern leaf rust resistance, except for Deir Alla 105, likely possess an allele different than Rph3.c found in Estate based on the differential reaction to isolates of P. hordei. The resistance gene in Grit and Donan is located at or near the Rph9 locus. Alleles at both the Rph3 and Rph9 loci confer resistance in Femina and Dorina. In addition to Rph3, Caroline and CR366.13.2 likely possess a second unknown recessive gene for leaf rust resistance. Resistance in Carre 180 is governed by a recessive gene that is different from all other genes considered in this study. Identification of both known and unique genes conferring leaf rust resistance in the barley germplasm included in this study provides breeding programs with the knowledge and opportunity to assess currently used sources of leaf rust resistance and to incorporate new sources of resistance into their programs.     

Temporal Variation in Setosphaeria turcica Between 1974 and 1994 and Origin of Races 1, 23, and 23N in the United States

ABSTRACT Setosphaeria turcica causes northern leaf blight, an economically important disease of maize throughout the world. Survey collections of S. turcica isolates from 1974 to 1994 provided a unique opportunity to examine temporal diversity in the eastern United States. Two hundred forty-two isolates of S. turcica from maize were studied with random amplified polymorphic DNA (RAPD) markers, mating type, and virulence on maize differential inbred lines with known Ht resistance genes to examine changes over time. One hundred forty-nine RAPD haplotypes were identified. Nearly 20% of haplotypes recurred in more than one year. Race 0 isolates declined in frequency from 83% in 1974 to near 50% in the 1990s, most likely in response to the widespread deployment of Ht1 in commercial maize hybrids. Races 23 and 23N were present in the collection at low levels throughout the study period and were also found among isolates from Virginia in 1957. The frequency of MAT1-2 isolates increased sharply after 1979 and was associated with the emergence of race 1 during the same period. RAPD markers were used to investigate the genetic diversity among a subset of isolates collected in the United States from 1976 to 1982, the period in which this dramatic shift in race frequency occurred. Multilocus haplotypes were not exclusively associated with known races of S. turcica. Based on shared haplotypes and cluster analysis, race 1 isolates share greater similarity with race 0 than with 23 or 23N isolates, indicating race 1 probably evolved from multiple lineages of race 0. Sorghum spp.-infecting isolates share greater similarity with one another than with maize-infecting isolates and represent a distinct subgroup.     

Attempted somatic hybridization of Puccinia striiformis f. sp. tritici and P. striiformis f. sp. hordei

Attempts were made to produce somatic hybrids between isolates of Puccinia striiformis f. sp. tritici and f. sp. hordei. A mixed infection was produced on a common susceptible barley host, Fong Tien, using white-spored isolates of P. striiformis f. sp. tritici and yellow-spored isolates off. sp. hordei. Selection was made for non-parental spore colour on selective wheat and barley hosts, and variants thus isolated were analysed for virulence markers, and for isozyme and double-stranded RNA (dsRNA) markers, all of which clearly differentiated the parental isolates. Two white-spored (non-parental) isolates were found on the selective barley host which otherwise resembled the parental f. sp. hordei isolate in virulence, isozyme and dsRNA markers. The most likely explanation of the origin of these isolates is mutation to white spore colour in the f. sp. hordei isolate.     

小麦条锈菌有性生殖与毒性变异的研究进展

小麦条锈病是由条形柄锈菌小麦专化型Puccinia striiformis f.sp.tritici Erikss.et Henn.引起的、最具毁灭性的小麦真菌病害之一,在全世界小麦种植的国家或地区均有发生,其流行常常导致小麦严重的产量损失。利用抗病品种是防治小麦条锈病最为经济、有效的措施。然而,由于病原菌毒性变异不断产生新小种,频繁导致品种抗病性"丧失",继而引发病害流行成灾。小麦条锈菌的有性阶段过去一直未被发现,有性生殖与条锈菌变异及其在病害发生中的作用研究一直处于空白状态。近年来,随着小麦条锈菌转主寄主的发现,国内外学者开展了相关的研究工作,取得了一系列重要的研究进展。本文综述了小麦条锈病的发生症状、病菌的生物学特性、转主寄主、有性生殖与条锈菌毒性变异、转主寄主在条锈病发生中的作用、影响有性生殖发生的因素等方面的最新进展,并展望了深入开展小麦条锈菌有性生殖的研究思路。     

在叶锈菌侵染过程中小麦过氧化物酶同工酶的变化

本文研究了抗叶锈性不同的小麦品种和毒力不同的叶锈菌小种相互作用过程中过氧化物酶(POD)同工酶谱的变化。结果表明:(1)不亲和组合在接种后48小时出现新酶带3,酶带4活性明显增强,120小时后又出现新酶带9。但亲和组合没有新酶带出现,而在接种后96小时酶带4活性也明显增强;(2)慢锈品种在接种后120小时以前,酶带4活性稍有增强,在120小时后其活性骤然增强。这可能是慢锈品种的特点。如果这些特点在更多的品种上得到验证,则POD同工酶谱分析有可能作为鉴定品种抗叶锈性和区分感病或慢锈品种的一种生理生化指标。     

源自陕、甘野生小檗的小麦条锈菌有性生殖后代的毒性分析

 小麦条锈病是我国最具毁灭性的小麦病害之一,其流行常常造成小麦严重减产。种植抗病品种是防治该病害最经济、有效和环保的措施。但是,由于新毒性菌系的出现,抗病品种在种植短短数年内便“丧失”其抗病性。研究证实病原菌毒性变异产生新菌系是导致小麦品种抗病性“丧失”的根本原因。近年来,随着小麦条锈菌转主寄主小檗的确定,发现自然条件下我国小麦条锈菌在野生小檗上可以完成有性生殖。本研究通过对2015年自然发病小檗小麦条锈菌的分离及其单夏孢子堆纯化,利用中国鉴别寄主进行了毒性测定分析。从陕西、甘肃两省的3种感病小檗共分离获得小麦条锈菌菌系8个,其中有1个菌系与已知小种Su11-126的毒性完全匹配,其余7个为新菌系;93个单夏孢子堆群体可分为47个不同的致病类型,包括14个已知小种类型,33个新小种类型;有56个菌系与已知小种的毒性完全匹配,37个为新小种。本研究再次获得了条锈菌自然条件下存在有性生殖并因此导致新菌系产生的证据,证实了野生小檗在我国小麦条锈菌的生活史和病害循环中具有作用。     

小麦秆锈菌新小种Ug99及其对我国的影响分析

Ug99(TTKS)是1999年在乌干达首次发现的对最重要小麦抗秆锈病基因Sr31有强毒力的秆锈菌新小种。大量证据显示:该小种不仅具有极其特殊的毒力组合而且传播十分迅速,除在中非乌干达、东非肯尼亚、埃塞俄比亚、苏丹流行外,Ug99现已越过红海、传到了阿拉伯国家也门,以及巴基斯坦的沿海地区,越来越逼近我国。Ug99堪称我国的超毒小种,我国最典型的秆锈菌小种只能分别克服或Sr5或Sr9 e或Sr11单基因抗性,而Ug99不仅具有Sr5、Sr9 e、Sr11的联合毒力,而且还具有Sr21、Sr31、Sr38的联合毒力,而我国从未有小种能克服后者的抗性。1B/1L(含Sr31)易位系曾是我国使用的重要秆锈抗源,必对Ug99高度脆弱,我国118份小麦品种在KARI的测定结果表明,高感品种频率98.3%。一旦Ug99入侵我国,其他流行条件也完全具备。因此,充分作好防范Ug99流行的准备十分必要。     

我国小麦条锈菌体细胞遗传重组的分子证据

 小麦条锈病是影响我国小麦生产的重要病害之一,条锈菌毒性变异是引致小麦品种抗病性丧失的主要原因。本文应用SSR分子标记技术,研究了陇南越夏区小麦条锈菌群体遗传结构,以期寻找条锈菌群体遗传重组的分子证据。研究结果表明,陇南地区小麦条锈菌群体遗传多样性比较丰富而遗传分化较小,遗传变异主要存在于群体内部,而在群体之间,遗传多样性有显著的差异。在11对SSR引物中,有4对能够检测到陇南地区小麦条锈菌群体存在遗传重组,而且重组体出现的频率不同,CPS15揭示的条锈菌重组体出现的频率为20.0%,CPS34揭示的条锈菌重组体出现的频率为18.5%,RJ20揭示的条锈菌重组体出现的频率为12.8%,RJ18揭示的条锈菌重组体出现的频率为15.0%,陇南地区小麦条锈菌群体重组体出现频率平均为16.6%。本文揭示的遗传重组现象表明陇南地区条锈菌体细胞结合十分普遍,由此推测我国小麦条锈菌在自然条件下通过遗传重组而导致毒性变异的可能性。     

Genetic Variation and Virulence on Lr26 in Puccinia triticina

ABSTRACT The genetic relationships between isolates of Puccinia triticina virulent on wheat with the Lr26 resistance gene were studied. The diversity within and between isolates of P. triticina from Israel, Europe, and the United States was determined by virulence on near-isogenic Thatcher lines and by random amplified polymorphic DNA. According to the molecular markers, isolates that were virulent on Lr26 had diversity levels similar to those of Lr26 nonpathogenic isolates. Distances between subpopulations of isolates virulent and avirulent on Lr26 varied and were unrelated to the Lr26 virulence phenotype. Cluster analysis suggested four groups, three of which were closely associated with the geographical origin of the isolates-Israel, the United States, and Europe. All four groups included both Lr26 virulent and avirulent pathotypes. The results showed that Lr26 virulent rust pathotypes are as genetically dissimilar as the rest of the population. The cluster analysis showed that the rust population in Israel includes at least two different subpopulations, both of which contain Lr26 virulent and Lr26 avirulent isolates.     

Virulence Phenotypes of a Worldwide Collection of Puccinia triticina from Durum Wheat

ABSTRACT A total of 78 isolates of Puccinia triticina from durum wheat from Argentina, Chile, Ethiopia, France, Mexico, Spain and the United States and 10 representative isolates of P. triticina from common wheat from the United States were tested for virulence phenotypes on seedling plants of 35 near-isogenic lines of Thatcher wheat. Isolates with virulence on lines with leaf rust resistance genes Lr10, Lr14b, Lr20, Lr22a, Lr23, Lr33, Lr34, Lr41, and Lr44 represented the most frequent phenotype. Cluster analysis showed that P. triticina from durum wheat from South America, North America, and Europe had an average similarity in virulence of 90%, whereas isolates from Ethiopia were <70% similar to the other leaf rust isolates collected from durum wheat. Of the 11 isolates from Ethiopia, 7 were avirulent to Thatcher and all near-isogenic lines of Thatcher. The isolates from common wheat had an average similarity in virulence of 60% to all leaf rust isolates from durum wheat. P. triticina from durum wheat was avirulent to many Lr genes frequently found in common wheat. It is possible that P. triticina currently found on durum wheat worldwide had a single origin, and then spread to cultivated durum wheat in North America, South America, and Europe, whereas P. triticina from Ethiopia evolved on landraces of durum wheat genetically distinct from the cultivated durum lines grown in Europe and the Americas.     

Virulence and Molecular Polymorphism in International Collections of the Wheat Leaf Rust Fungus Puccinia triticina

ABSTRACT Collections of Puccinia triticina, the wheat leaf rust fungus, were obtained from Great Britain, Slovakia, Israel, Germany, Australia, Italy, Spain, Hungary, South Africa, Uruguay, New Zealand, Brazil, Pakistan, Nepal, and eastern and western Canada. All single-uredinial isolates derived from the collections were tested for virulence polymorphism on 22 Thatcher wheat lines that are near-isogenic for leaf rust resistance genes. Based on virulence phenotype, selected isolates were also tested for randomly amplified polymorphic DNA (RAPD) using 11 primers. The national collections were placed into 11 groups based on previously established epidemiological zones. Among the 131 single-uredinial isolates, 105 virulence phenotypes and 82 RAPD phenotypes were described. In a modified analysis of variance, 26% of the virulence variation was due to differences in isolates between groups, with the remainder attributable to differences within groups. Of the RAPD variation, 36% was due to differences in isolates between groups. Clustering based on the average virulence distance (simple distance coefficient) within and between groups resulted in eight groups that differed significantly. Collections from Australia-New Zealand, Spain, Italy, and Britain did not differ significantly for virulence. Clustering of RAPD marker differences (1 - Dice coefficient) distinguished nine groups that differed significantly. Collections from Spain and Italy did not differ significantly for RAPD variation, neither did collections from western Canada and South America. Groups of isolates distinguished by avirulent/virulent infection types to wheat lines with resistance genes Lr1, Lr2a, Lr2c, and Lr3 also differed significantly for RAPD distance, showing a general relationship between virulence and RAPD phenotype. The results indicated that on a worldwide level collections of P. triticina differ for virulence and molecular backgrounds.     

Diversity and Complexity of Erysiphe graminis f.sp. hordei Collected from Barley Cultivar Mixtures or Barley Plots Treated with a Resistance Elicitor

Powdery mildew populations were analysed to determine the effects of a resistance elicitor and cultivar mixtures on genetic complexity and diversity. Isolations were made from a range of spring barley monocultures and mixtures in a field trial, and characterised for virulence and RAPD profile. In a second trial, isolates were taken from a single mixture from untreated and resistance elicitor-treated areas and from the components of the mixture in monoculture. The mildew population was not only highly heterogeneous for virulence characteristics, but also proved heterogeneous within pathotypes for molecular markers, indicating the major impact of sexual recombination on population structure and the lack of clonal dominance. Various diversity measurements were compared and the value of dissimilarity measurement for revealing genetic distance within a population was highlighted. There was a trend towards increasing complexity as the season progressed, but there was no consistent relationship between cultivar or mixture, disease control treatment, fertiliser treatment, replicate or position in trial, and pathogen genotype. Whilst the resistance elicitor did reduce mildew by 78% in the first trial, and there was no interaction with fertiliser level in its expression, control was substantially less in the second trial. There were no differences between mildew isolates from elicitor and control treatments. It was felt that more effective and consistent resistance elicitors need to be developed before it can be stated that they are unlikely to be eroded by selecting resistant or adapted mildew genotypes.     

Molecular Mapping of the rps1.a Recessive Gene for Resistance to Stripe Rust in BBA 2890 Barley

ABSTRACT Stripe rust, caused by Puccinia striiformis f. sp. hordei, is one of the most important diseases of barley in the south-central and western United States. Growing resistant cultivars is the best approach for controlling the disease. The barley genotype BBA 2890 has all-stage resistance against all races of P. striiformis f. sp. hordei (PSH) identified thus far in the United States. The resistance in BBA 2890 is controlled by a single recessive gene, rps1.a. The objectives of this study were to identify resistance gene analog polymorphism (RGAP) markers for the all-stage resistance gene rps1.a, to map the gene on a barley chromosome using chromosome-specific simple sequence repeat (SSR) markers, and to determine the presence or absence of the flanking RGAP markers for the gene in 24 barley genotypes. Seedlings of the parents and 200 F(8) recombinant inbred lines (RILs) were tested for resistance to pathogen races PSH-14, PSH-48, and PSH-54 in the greenhouse in 2005. Genomic DNA was extracted from the parents and 150 F(8) RILs. The RGAP technique was used to identify molecular markers for the rps1.a gene. Twelve primer pairs generating repeatable polymorphic bands were selected for genotyping the 150 F(8) RILs. A genetic linkage group was constructed for the resistance gene with 13 RGAP markers and four chromosome-specific SSR markers. The four SSR markers mapped the gene on the long arm of barley chromosome 3H. The closest RGAP marker for the resistant allele was within a genetic distance of 2.1 centimorgans (cM). The closest marker for the susceptible allele was 6.8 cM away from the locus. The two closest RGAP markers for the resistant allele detected polymorphisms in 67 and 71% of the 24 barley genotypes when used individually, and detected polymorphism in 88% of the genotypes when used in combination. This information should be useful in incorporating the resistance gene into barley cultivars and in pyramiding the gene with other resistance genes for superior stripe rust resistance.     

Racial diversity and complexity in regional populations of Erysiphe graminis f.sp. hordei in France over a 5-year period

The evolution of race patterns in three French regional populations of the barley powdery mildew pathogen Erysiphe graminis f.sp. hordei over a 5-year period showed rapid adaptation to newly introduced host resistance genes. In all three regions, the main change consisted of the replacement of initially abundant races by pathotypes differing markedly from them by their virulence gene combinations. This explained the increase in diversity during the first 3 years of the survey, when the second group of pathotypes became more common in the populations, and its subsequent decrease due to the decline of the first group of races. The mean number of virulence genes per isolate did not vary noticeably over time in the three populations, remaining at about four out of 12 genes tested. However, the distribution of the isolates into virulence complexity classes was greatly modified, fitting a binomial distribution by the end of the study, although significant deviations were apparent in the first 2 years (1986 and 1987). The data indicate that selection, migration and recombination are the most important factors shaping race structure and evolution in powdery mildew populations, and that mutation is of limited significance. No convincing evidence was obtained for the existence of stabilizing selection sensu Vanderplank as the mechanism limiting virulence complexity. Implications regarding spatial and temporal deployment of race-specific resistance genes to control powdery mildew are discussed.     

A unique race of the wheat stem rust pathogen with virulence on Sr31 identified in Spain and reaction of wheat and durum cultivars to this race

Wheat stem rust, caused by Puccinia graminis f. sp. tritici, is a re-emerging disease, posing a threat to wheat production. In Spain, stem rust has been rarely detected since the 1970s, but infection was observed in wheat fields in 2018. We analysed six stem rust samples collected in Rota, Cádiz province and one from Monteagudo del Castillo, Teruel province. All the samples from Rota were typed as race TKTTF, whereas the sample from Monteagudo del Castillo, collected in a wheat field adjacent to barberry bushes, was typed as race TKHBK. This race has a unique and significant virulence combination that includes virulence to Sr31, Sr33, Sr53 and Sr59, and is avirulent to Rusty, a durum line developed for universal susceptibility to the wheat stem rust pathogen. TKHBK is the first race outside the Ug99 race group with virulence to Sr31 and the first known race with virulence to Sr59. Genotyping studies indicate that race TKHBK does not belong to the Ug99 or TKTTF race groups and constitutes a previously unknown lineage. Two hundred bread and durum wheat cultivars and breeding lines from Spain were evaluated against TKHBK, TKTTF, and six additional races. Resistance was observed to all the races evaluated. Molecular markers confirmed the presence of Sr7a, Sr24, Sr31, Sr38 and Sr57 in bread wheat, and Sr13 in durum wheat. The re-emergence of wheat stem rust in Spain and the occurrence of unique virulences underscore the need to continue surveying and monitoring this disease.