Evolution of a pathogen population in host mixtures: simple race–complex race competition

The evolution of the genetic structure of a pathogen population was studied in a varietal mixture with an epidemic simulator based on the model EPIMUL. The pathogen population was composed of simple races able to develop on only one component of the mixture and a complex race which developed on all mixture components. The effects on the simple race–complex race competition of a cost of virulence, of density dependence and of differential adaptation were studied. The selection for simple or complex races in the pathogen population did not depend on initial race frequencies. For a given multiplication rate, complex race frequency increased faster when the spore dispersal gradient was shallow, when distribution of initial disease was generalized, when amount of initial disease was reduced and when the number of mixture components was increased. This was attributed to a better efficacy of the mixture in controlling simple races, resulting in a higher relative fitness of the complex race. For measured values of density dependence or differential adaptation effects, the complex race was at a higher frequency after a mean number of pathogen cycles between 2.5 and 5. The effect of the cost of virulence was stronger and, in certain situations, could result in selection for simple races. In the conditions of our simulations and with the effects tested, stabilization of the pathogen population in host mixtures was unlikely to occur. However, more information is needed concerning the rate at which complex races could evolve and how quickly mixture resistance could be eroded.     

小麦条锈病菌致病类型G22-83及流行小种CYR32和CYR33在四川省小麦品种(系)上寄生适合度的测定及分析

为了解致病类型G22-83及流行小种CYR32和CYR33对四川省近年小麦生产品种和后备品种的寄生适合度,在田间分别将致病类型G22-83、流行小种CYR32和CYR33接种于四川省109个小麦品种(系),采用综合病情指数法测定3个供试菌株的相对寄生适合度。结果显示,致病类型G22-83、流行小种CYR32和CYR33的平均寄生适合度分别为0.1930、0.0560和0.0379,且前者显著高于后两者;流行小种CYR32和CYR33的平均相对寄生适合度有差异,但是差异不显著。致病类型G22-83、流行小种CYR32和CYR33在四川省109个小麦品种(系)上相对寄生适合度的中位数分别为0.0626、0.0000和0.0000,前者在四川省109个小麦品种(系)上相对寄生适合度的平均水平和波动程度均高于后两者。致病类型G22-83在四川省小麦生产品种和后备品种上的寄生适合度较高,在今后一段时间内可能成为该地区小麦条锈菌的优势菌系。     

Success and failure of invasive races of plant pathogens: The case of Puccinia striiformis f. sp. tritici in France

Invasions of new races can have contrasting consequences on populations of Puccinia striiformis f. sp. tritici, causing yellow rust of wheat. For example, the emergence of PstS7 (Warrior race) had major impacts in Europe and in France. By contrast, PstS2 had no impact in France, while it significantly affected other parts of the world. The objective of this study was to better understand factors that govern the success of an invasive race, taking the contrasting history of PstS7 and PstS2 in France as a case study. We compared these two races for three key factors driving invasive potential: (a) virulence against local cultivars, (b) aggressiveness in local environmental conditions, and (c) competitiveness against local races. During the period when PstS2 was detected, 70% of the grown wheat area was protected against this race by at least one known Yr resistance gene. By contrast, we found that only 15% of the wheat area had a low risk of infection by PstS7. In planta competition experiments suggested a higher competitiveness of PstS7 against local isolates compared to PstS2 in optimal thermal conditions. In silico experiments, based on thermal performance curves, suggested a high competitiveness of PstS7 considering infection efficiency. PstS2 was extremely competitive against local races in all considered environments (20 French sites × 15 years) due to its short latency period. Our findings highlight the importance of considering adaptation to environmental conditions, particularly temperature, in addition to virulence spectrum, in order to understand the evolutionary trajectories of emerging strains in pathogen populations.     

Selection on Erysiphe graminis in pure and mixed stands of barley

The response of populations of Erysiphe graminis f. sp. hordei to selection by pure and mixed stands of three spring barley cultivars was studied in two field trials. The range of virulence of the pathogen genotypes selected in mixed host stands was dependent on the relative fitness of each genotype over all hosts. Unnecessary virulences were rapidly selected against on some hosts, but were less deleterious or favoured on others. In general there was selection for widely adapted pathogen genotypes in mixed host populations but this selection for flexibility limited the abilitytions in the absolute size of the pathogen population in host mixtures reduced the absolute frequencies of pathogen genotypes with combined virulences in comparison with those in pure stands. It is argued that host mixtures are therefore unlikely to favour rapid pathogen evolution towards races which are both widely adapted and highly virulent on all component cultivars which they can infect his definition docs not conform with conventional usage in population genetics.     

小麦秆锈菌不同小种间相对生存能力研究

 将我国小麦秆锈菌优势小种21C3分别与历年来出现频率很低的34C1或116小种的夏孢子等量混合,在不同温度条件下,转接于八个来自次要越冬区长江中下游感病的生产品种上。经几个夏孢子世代的继代转接结果表明:21C3小种的毒力虽弱但与34C2或116种相比,其相对生存能力强;而34C2和116小种的毒力虽强,但相对生存能力却弱。小种间毒力强弱与其生存能力强弱呈负相关的现象,可能是我国小麦秆锈菌21C3小种在相当长的时期处于优势的原因之一。在一定高温条件下,虽能影响小种频率上升或下降的速度,但未能左右其总的趋势。试验应用的继代品种对不同小种的感病性没有显著差别。     

The development of a PCR-based method for detecting <Emphasis Type="Italic">Puccinia striiformis</Emphasis> latent infections in wheat leaves

Stripe rust of wheat caused by Puccinia striiformis f. sp. tritici is one of the most important diseases on wheat worldwide, especially in temperate regions with cool moist weather conditions. A rapid and reliable detection of the pathogen in latent infected wheat leaves during overwintering of the fungus in the dormant stage will contribute to determine the initial inoculum potential and thus to predict early outbreak and to improve effective management of the disease. To achieve this aim, a PCR-based method was developed for specific and sensitive detection of P. striiformis. Specific primers were designed according to a genome-specific sequence of P. striiformis. To evaluate the specificity of the primers, seven different isolates and races of P. striiformis as well as six other pathogens of wheat were tested. All isolates of P. striiformis yielded a distinct band of a fragment of 470 bp, while using DNA of the other wheat pathogens as a template no amplification product was detected. The sensitivity of the primers was tested using serial dilutions of total DNA from P. striiformis; the limit of detection was 10 pg of DNA. Using extracts from P. striiformis-infected wheat leaves, the fungus could be determined in the leaves before symptoms appeared. The stripe rust could also be detected in the dormant stage by the PCR assay in samples of wheat leaves taken during the winter season. The application of the PCR assay may be useful for rapid and reliable detection of P. striiformis in latent infected leaves of overwintering wheat plants.     

Competition and interactions among stripe rust pathotypes in wheat-cultivar mixtures

A series of experiments was conducted with wheat stripe rust to analyse competition between simple and complex pathotypes in host mixtures. Two different pathotype combinations were tested, with different host components. Each combination included a complex (able to infect two host components) and two simple pathotypes. For one of the combinations, induced resistance was tested in a separate experiment as a possible interaction among pathotypes. Disease severity and pathotype frequencies were measured three times during the epidemic, on each host component grown in pure stands and in mixtures. In one of the experiments, pathotype frequencies were also measured within secondary foci. One of the complex pathotypes appeared to have a low fitness on one of the host components and did not significantly increase in frequency in host mixtures relative to pure stands. The average frequency of the other complex pathotype increased during the first epidemic cycles, but remained stable afterwards, below expected values. The results suggest that the development of complex pathotypes in host mixtures may be influenced by differential aggressiveness on the host components, by induced resistance and by random effects resulting from the formation of disease foci, and depends on pathogen autoinfection rate and dispersal mechanisms.     

Replacement of the European wheat yellow rust population by new races from the centre of diversity in the near‐Himalayan region

Isolates of recently spreading races of yellow rust from wheat and triticale in Europe were analysed using virulence phenotypic data of 2605 isolates sampled in 12 countries between 2000 and 2014. A subset of 239 isolates was investigated by microsatellite markers. At least three races of non‐European origin, termed ‘Warrior’, ‘Kranich’ and ‘Triticale aggressive’, were identified in the post‐2011 population. The Warrior race was already present in high frequencies in the first year of detection in most European countries and to a large extent it replaced the pre‐2011 European population. In contrast, the two other exotic races were localized to certain regions and/or crop type. The presence already of at least six multilocus genotypes of the Warrior race and five genotypes of the Kranich race in the first year of detection and across large areas is consistent with a hypothesis of aerial spread from genetically diverse source populations. A comparison with reference isolates sampled from six continents suggested that the Warrior and Kranich races originated from sexually recombining populations in the centre of diversity of the yellow rust fungus in the near‐Himalayan region of Asia. However, the Triticale aggressive race was most similar to populations in the Middle East/Central Asia. The study illustrated the potential role of sexual Puccinia striiformis populations as a reservoir for new races replacing distant clonal populations.     

云南小麦条锈菌群体对18个抗条锈近等基因系的毒性分析

为监测云南省小麦条锈菌群体毒性及小麦抗条锈基因的有效性动态,2016年采用18个抗条锈近等基因系鉴别寄主对云南省9个州市的136个小麦条锈菌株进行毒性分析,并按八进制法对小种进行命名。结果表明,云南省小麦条锈菌群体毒性丰富,共鉴定出64个小种类型,其中居于前2位的小种是550273和550073,出现频率分别为28.68%和11.76%,是本年度优势小种;其它小种出现频率均在4.41%以下,为次要小种。条锈菌群体对Yr5、Yr10、Yr15、Yr32四个抗条锈基因的毒力频率均为0,对Yr24、Yr Tr1、Yr8、Yr17四个抗条锈基因的毒力频率在0.74%~11.76%之间,表明这8个基因是云南省当前有效的抗条锈基因;对Yr27的毒力频率为52.94%,对Yr1、Yr6、Yr7、Yr9、Yr43、Yr44、Yr SP、Yr Exp2、Yr Tye九个抗条锈基因的毒力频率为77.94%~91.91%,表明这10个抗条锈基因的抗性已减缓或丧失,说明这些基因在云南省已失效。     

芦笋茎枯病病菌鉴别寄主的构建

<正>芦笋(Asparagus officinalis Linn)又称石刁柏,百合科天门冬属,是世界十大名菜之一,在国际市场上被称为"蔬菜之王"。由于其营养价值高,并能润肺、镇咳、祛痰,具有抑制肿瘤生长的药理功能,深受人们的喜爱[1]。近年来,随着芦笋市场与栽培面积的扩大,病害的发生也逐年加重,尤其茎枯病的严重发生已影响芦笋的产量与质量,轻者生长发育不良,降低产量与品质,重者病株提前枯死,全田毁灭,严重影响芦笋生产与出口创汇[2]。目前该     

Effect of two-component cultivar mixtures and yellow rust on yield and yield components of wheat

Four winter wheat ( Triticum aestivum ) cultivars and three two-component cultivar mixtures were planted in a replacement series both inoculated with or protected from yellow rust ( Puccinia striiformis ) in three environments. Each cultivar was susceptible to one or two of the rust races used. Mixtures yielded, on average, 7 and 4% more than their component pure stand means under inoculated and rust-free conditions, respectively. Though all yield components were affected by yellow rust, seed weight was the component that was most consistently influenced. The component genotypes within mixtures varied considerably with respect to yield, and the yield of the same component cultivar included in different mixtures sometimes differed significantly. The correlation between yellow rust severity/tiller and grain yield/tiller in mixture differed among cultivars and depended on their companion cultivar. Variance component analysis indicated that yellow rust was the most important experimental variable influencing grain yield. There was no relationship between yield of the cultivars in pure stands and their yields or competitive abilities in mixture. Disease did not change the competitive ranking of cultivars in mixture. Mixtures with complementary, negative, and overcompensatory interactions were identified. On average, mixtures showed no greater yield stability than did pure stands.     

High Genetic Similarity Among Populations of Phaeosphaeria nodorum Across Wheat Cultivars and Regions in Switzerland

ABSTRACT Phaeosphaeria nodorum was sampled from nine wheat fields across a 30-km transect representing three geographical regions in Switzerland to determine the scale of genetic differentiation among subpopulations. Three different wheat cultivars were sampled three times to determine whether differences in host genotype correlated with differences among corresponding pathogen populations. Seven restriction fragment length polymorphism (RFLP) loci and one DNA fingerprint were assayed for each of the 432 isolates in the collection. DNA fingerprints differentiated 426 unique genotypes. Though absolute differences were small, five RFLP loci exhibited significant differences in allele frequencies across the nine sub-populations. Gene diversity within all subpopulations was high (H(T) = 0.51), but only 3% of the total genetic variation was distributed among the nine subpopulations. When subpopulations were grouped according to geographical region or host cultivar, less than 1% of the genetic variation was distributed among groups, suggesting widespread gene flow and the absence of pathogen adaptation to specific wheat cultivars. Tests for gametic equilibrium within subpopulations and across the entire Swiss population supported the hypothesis of random mating.     

Characterization of <Emphasis Type="Italic">Phytophthora clandestina</Emphasis> races on <Emphasis Type="Italic">Trifolium subterraneum</Emphasis> in Western Australia

Phytophthora clandestina is a causal agent of root rot disease of subterranean clover in Western Australia (W.A). As a significant number of isolates of P. clandestina from W.A. could not previously be designated using existing differentials, a comprehensive set of subterranean clover (Trifolium subterraneum) cultivars was used as differentials to delineate a broader range of races of the pathogen. One hundred and one isolates of the pathogen collected from W.A. were screened on nine subterranean clover cultivars, of which seven were found to be useful as host differentials. A total of 10 races (in contrast to the five recognized previously) were defined and differentiated using octal nomenclature, presenting a clearer picture of the racial distribution of P. clandestina among W.A. isolates. Differences were found in the race populations between Australian states and are therefore important to the selection/breeding of cultivars for specific regions of Australia to counter the predominant race populations and for enforcing quarantine measures in relation to seed movements within and outside Australia. The octal nomenclature used provides a sound basis for follow-up studies and future race designations. Races 173 and 177 in this study were widely distributed and were the most common races in W.A., and together constitute 80% of the isolates characterized. While six of the seven host differentials were resistant to isolates belonging to race 001 and all were resistant to 000, it is of concern that only one differential was resistant to 157 and 173 and that none of the host differentials were resistant to 177. Our approach to P. clandestina race delineation is clearly conservative and is different from previous studies. The octal nomenclature we applied in this study is not only scientifically sound but also will facilitate rapid recognition and characterization of the races.     

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。     

Intrapathotype diversity for aggressiveness and pathogen evolution in cultivar mixtures

ABSTRACT A model was developed and used to study the consequences of diversity for aggressiveness within pathotypes on pathogen evolution in two-component and four-component cultivar mixtures. It was assumed that, within a pathotype, a proportion of the isolates would have higher or lower spore efficacy than the average on a given host genetic background. Two situations were examined in which the pathogen can have either independent or negatively correlated values for spore efficacy on different cultivars. In the latter case, a pathogen genotype more aggressive than the average on a host genotype was always less aggressive on other host genotypes. In the simulations, isolates with greater aggressiveness relative to a host genotype were selected for and increased in frequency. However, because simple pathotypes always reproduced on the same host genotype whereas complex pathotypes were able to grow on several hosts, selection was faster for simple pathotypes. Pathotypes with two different levels of diversity for aggressiveness were compared with nondiversified pathotypes. In order to make comparisons, the effect of a 5 and 10% cost of virulence on the development of complex pathotypes was simulated. In general, increased diversity within pathotypes reduced the rate of increase of complex pathotypes in host mixtures, and this effect was stronger with greater frequencies of autodeposition of pathogen spores.     

水稻稻瘟病菌小种间相对生存力研究

选用水稻稻瘟病菌ZA_(49)、ZD_3和ZE_3 3个小种,各2个小种为一组合,以等孢子量混合,于相同条件下在秀水620和秀水48两个感病品种上进行继代接种,经5个世代的转接结果表明:在秀水620上ZA_(49)小种较ZD_3和ZE_3小种具更强的致病力,在秀水48上,ZE_3小种较ZA_(49)和ZD_3小种具更强的致病力,同样其相对生存力也强,两者呈正相关。病菌小种间相对生存力的强弱与病菌小种在品种上单独接种时产生的病斑数、病斑大小和产孢量有关。相对生存力强的小种在病菌群体中易取得优势,从而导致寄主品种的病害量增加。因此认为小种间相对生存力差异也是影响水稻稻瘟病菌群体小种组成发生变化的因素之一。     

Genetic analysis and molecular mapping of resistance to Puccinia striiformis f. sp. pseudo‐hordei in common wheat

This is the first genetic study reporting on the interaction and molecular mapping of resistance to the barley grass stripe rust pathogen (Puccinia striiformis f. sp. pseudo‐hordei, Psph) in common wheat. Seedlings of 638 wheat accessions were tested and it was determined that wheat is a near‐nonhost to Psph based on rare susceptibility observed in <2% of commercial cultivars and <5% of wheat landraces. As previously observed for P. striiformis f. sp. tritici (Pst), the Australian cultivar Teal was highly susceptible to Psph. In contrast, a selection of cv. Avocet carrying complementary resistance genes Yr73 and Yr74 (Avocet R; AvR) was resistant. The Teal × AvR (T/A) doubled haploid (DH) population was used to map resistance in AvR to Psph. Infection types on the T/A DH lines inoculated with Psph and Pst indicated that all DH lines carrying both Yr73 and Yr74 were also resistant to Psph; however, fewer DH lines were susceptible to Psph than expected, suggesting the resistance was more complex. QTL analysis using 9053 DArT‐Seq markers determined that resistance to Psph was polygenically inherited and mapped to chromosomes 3A, 3D, 4A and 5B. The 3DL and 5BL markers co‐located with Yr73 and Yr74, suggesting an overlap between host and non‐host resistance mechanisms.     

Reaction of selected winter wheat cultivars from Europe and United States to Karnal bunt

A study was conducted to determine the susceptibility of popular United States (U.S.) and European winter wheat cultivars to the fungal pathogen Tilletia indica. Historically, the disease has been limited to autumn-sown spring-habit wheat areas and not associated with winter wheat. In 1997, Karnal bunt was observed on winter wheat in limited regions of Texas. This region marks the southern end of the contiguous U.S. central winter wheat belt, which extends north into Canada. The aim of this study was to assess the levels of disease resistance in winter wheat. Fifty U.S. and European winter wheat cultivars were tested using two different greenhouse inoculation procedures. For each cultivar, 12 spikes in boot were inoculated by boot-injection with a sporidial suspension (1.0 ml/boot, 10,000 spores ml⁻¹), and 12 other emerged spikes were spray-inoculated with the same concentration. The experiment was repeated for three seasons. Among cultivars, mean seed infection ranged from 2.1 to 87.2% and 0 to 15.6% for boot-injected and spray-inoculated treatments, respectively. Results showed that the majority of winter wheat cultivars tested were susceptible to Karnal bunt.     

小麦抗病遗传多样性对条锈病的调控效应

选用小麦抗条锈病近等基因系的5个品种,进行不同混播处理,研究了混播群体中存在各不同抗病基因时对小麦条锈病的控制效果。试验结果表明,两个单基因品种混播多不能显著降低条锈病的发生,而3个以上单基因品种混播多可显著降低条锈病的发生,而且4~5个品种混播的REM值也说明多个品种的混播可减少条锈病的发生。但是,防效并非完全与混播基因数目有关,而与基因的类型也有关。不同抗病基因对条锈菌生理小种的选择存在差异,小麦群体中所含的丰富抗病基因可减缓优势小种的出现。千粒重数据显示,品种混播的千粒重高于单个品种。     

小麦品种多样性对白粉病及产量和蛋白质的影响

 对品种多样性控制小麦白粉病的效果以及对产量和蛋白质的影响进行了研究,为利用品种多样性控制白粉病提供理论依据和技术支持。采用SSR技术分析5个小麦品种的亲缘关系;2008年和2009年按组合所含品种数为2、3、4和5个进行组合,在田间种植混种组合和单播小区,人工接种白粉菌,比较各小区中小麦白粉病的AUDPC值、产量和蛋白质变化。结果表明,5个品种亲缘关系相对较近;2个品种混合时以抗感搭配较好,随着抗病品种比例增加控病效果增强;2008年混合种植中有防治效果的组合占54.55%,相对防效为10.02%~47.58%,2009年有防治效果的组合占23.08%,相对防效在1.85% 到18.96 % 之间;在适当的组合中,如京冬8号/轮选987、京双16/京411/京冬8号对小麦白粉病有控制效果,混种不会影响产量和蛋白质含量;当有大量能克服本地品种抗性的外来菌源时混合种植的控病效果降低。品种多样性可以用做防治小麦白粉病的一项措施。