Effect of wheat cultivar mixtures on populations of Puccinia striiformis races*

This study quantifies the frequency of simple and complex races (races that can infect two or more components) of Puccinia striiformis in mixtures of wheat cultivars possessing different race-specific resistance genes. Treatments were designed so that the complex race changed depending on the host mixture, thus enabling us to observe the influence of pathogen complexity in different genetic backgrounds. Six cultivar mixtures and one pure stand of winter wheat were inoculated with three races of P. striiformis at two locations for two seasons. Potted plants of three winter wheat cultivars, each susceptible to one of the three races of the pathogen, were used to sample the pathogen during the field epidemics. Disease incidence on the differential cultivars was used to calculate the proportion of the three races in each treatment. The specific cultivars included in the mixtures influenced the frequencies of the three races. Increasing the number of virulent races in a mixture reduced the frequency of the complex race relative to the other two races. The results suggest that genetic background of the pathogen race, host composition, and interaction among pathogen races may be as important as cost of virulence in determining race frequencies in mixtures.     

Qualitative and quantitative variation in pathogenicity of races of coffee leaf rust (Hemileia vastatrix) detected in the State of São Paulo,Brazil

Between 1977 and 1981, seven qualitatively distinct new races of coffee leaf rust (Hemileia vastatrix) were detected in breeding plots in the State of São Paulo, Brazil. Four races carry complex virulence against the resistance genes S_H1, S_H2, S_H4 and S_H5 ofCoffea arabica. Three races match unidentified resistance genes ofC. canephora. Two of these were isolated from cv. Kouillou and one from the hybrid population Icatu. Pending further identification, these races were indicated by the number of their type cultures (Is. 2, 10 and 11). Is. 2 and Is. 10 showed extra virulence to some coffee genotypes and decreased virulence to other coffee genotypes, suggesting stabilizing selection. Three rust isolates were detected which differed quantitatively from the common rust race II. Is. 1 was moderately virulent to the coffee differential for S_H3 in the laboratory but avirulent in the greenhouse, indicating a host x pathogen x environment interaction. Is. 3 and 12 showed levels of virulence intermediate between race II and Is. 2 and 10, respectively. The results show that incomplete resistance of coffee toH. vastatrix, at various levels, can be race-specific. The nature of race formation of coffee leaf rust in Brazil and breeding strategies for obtaining durable resistance are discussed.     

Stepwise Evolution of Races in Fusarium oxysporum f. sp. ciceris Inferred from Fingerprinting with Repetitive DNA Sequences

ABSTRACT Plant pathogens often exhibit variation in virulence, the ability to cause disease on host plants with specific resistance, evident from the diversity of races observed within pathogen species. The evolution of races in asexual fungal pathogens has been hypothesized to occur in a stepwise fashion, in which mutations to virulence accumulate sequentially in clonal lineages, resulting in races capable of overcoming multiple host plant resistance genes or multiple resistant cultivars. In this study, we demonstrate a simple stepwise pattern of race evolution in Fusarium oxysporum f. sp. ciceris, the fungus that causes Fusarium wilt of chickpeas. The inferred intraspecific phylogeny of races in this fungus, based on DNA fingerprinting with repetitive sequences, shows that each of the eight races forms a monophyletic lineage. By mapping virulence to each differential cultivar (used for defining races) onto the inferred phylogeny, we show that virulence has been acquired in a simple stepwise pattern, with few parallel gains or losses. Such a clear pattern of stepwise evolution of races, to our knowledge, has not been demonstrated previously for other pathogens based on analyses of field populations. We speculate that in other systems the stepwise pattern is obscured by parallel gains or losses of virulence caused by higher mutation rates and selection by widespread deployment of resistant cultivars. Although chickpea cultivars resistant to Fusarium wilt are available, their deployment has not been extensive and the stepwise acquisition of virulence is still clearly evident.     

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.     

河南省小麦白粉病菌生理小种及其致病性测定

1990—1991年,于小麦生长季节从河南省14个县(市)采集了126个白粉菌株。根据9个鉴别寄主的抗感反应进行小种划分,并用毒性公式和八进制法编码。结果表明,15号小种的频率最高,达17.46%,与国内其它地区的结果基本一致。但对鉴别寄主毒力频率分析发现不同年份及地区间,小麦白粉菌群体有较大差异;而且同小种不同菌株对豫麦2号等10个推广品种的致病性存在明显差异。原因在于传统小种鉴定方法存在较大弊端,现行鉴别寄主中与生产基本无关的高抗品种较多,缺乏代表性的生产品种,不能准确反映当地小麦品种与当地白粉菌群体问的互作。提出应对鉴别寄主予以修改,或用病菌群体毒性基因结构测定代替优势小种分析,以提高研究结果的实用性。     

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。     

大豆疫霉根腐病菌生理小种鉴定及毒性分析

对采自黑龙江省、吉林省的42株大豆疫霉根腐病茵进行生理小种鉴定，其中11株可被分别鉴定为1号、3号和8号生理小种。1号小种为优势小种，3号、8号小种为国内首次报道。其余31株在鉴别寄主上出现中间类型反应无法鉴定小种，但可以划分为12个毒性类型，病原茵群体具有很高的毒性多态性，其中有些菌株毒性较强。     

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。     

Inter-isolate variation for virulence in Plasmopara halstedii (sunflower downy mildew) from Hungary

A total of 82 isolates of Plasmopara halstedii , collected from all production areas of Hungary between 1976 and 1993, were assessed for their virulence pattern on a standard set of sunflower differentials under glasshouse conditions. The isolates were classified into six pathogenic races each representing a particular virulence phenotype. From 1976 until 1988 all the isolates were found to be virulent only on sunflowers possessing no known resistance genes, thus classified as race 1. There was an apparent shift in the virulence of the P. halstedii population collected after 1988. Six races (1, 2, 3, 4, 8 and 9) were identified among the 45 samples collected between 1989 and 1993, with races 1 and 3 predominant, at a frequency of 35% each. While the increase in race virulence is undoubtedly due to selection imposed by resistant hybrids, the origin of the new races is unknown. Whether new races have arisen from the indigenous P. halstedii population, or whether they have been imported from abroad, can only be reliably determined by DNA techniques, such as fingerprinting.     

Pathogenic variability of Plasmopara halstedii infecting sunflower in the Czech Republic

Plasmopara halstedii was isolated from diseased sunflowers collected from eight locations in the Czech Republic from 2007 to 2014. Races of the pathogen were determined based on 84 isolates collected during the study. In total, eight races of P. halstedii were detected using a set of nine sunflower differential lines. Races 700, 704, 705, 710, 714 and 715 were proven by soil drench inoculation, and two additional races (730 and 770) proposed by the previously applied leaf disc inoculation method. Race 700 was the most dominant in the Czech P. halstedii populations, with race 710 being the second most frequent. Races 704 and 714 were found over three seasons, while other races were recorded only in one growing season (race 730 in 2010, and the new races 705 and 715 in 2014). A comprehensive study was further conducted for isolates collected in 2013–14 using an extended differential set consisting of 15 sunflower lines. According to the latter methodology which marks races with five‐digit virulence codes, races 70060, 70471, 70571, 71060, 71461 and 71571 were recorded. The growing complexity of P. halstedii pathogenicity exhibited by the ability to infect higher numbers of differential genotypes and resulting in determination of the new pathogen races (virulence profiles) 70571, 71461 and 71571 is alarming. Although the limited number of isolates studied cannot characterize the entire pathogen diversity in the Czech Republic, the trend towards more diverse virulence in P. halstedii populations is clearly demonstrated by the new records of races 704, 705, 714 and 715, all capable of overcoming the resistance gene Pl₆.     

Seasonal changes in pathotype complexity in French populations of barley powdery mildew

Airborne conidia of Erysiphe graminis f.sp. hordei were sampled in three regions and a single locality in the northern part of France for 2 years. Sampling was carried out in early spring, in late spring and in autumn, in order to separate the effects of winter barley cultivars, carrying few specific resistance alleles, and of spring barley cultivars, carrying diverse resistance alleles, on the structure of the pathogen population. Although complex pathotypes with three to 10 virulences were selected by spring cultivars, simple pathotypes, including a pathotype with the single unnecessary virulence allele Va22 , which formed a clear majority of the samples, remained dominant in early spring, when winter but not spring cultivars were growing. In early spring, simple pathotypes were more prevalent in the north, where the winter cultivars represented 90% of the barley acreage, than in the east, where winter cultivars represented 65%. In the west, the frequency of simple pathotypes was limited compared to the north, possibly because of the resistance allele Mlg in winter cultivars. The high frequency of simple pathotypes in early spring could be explained by a differential adaptation between simple and complex pathotypes or by delayed epidemics on spring cultivars compared to winter cultivars.     

小麦条锈菌鉴别寄主和小种命名现状

 中国是世界上小麦条锈病最大的流行区之一,也是限制中国小麦生产的最严重病害之一,特别是当条件适于其发生时会造成巨大的产量损失.小麦条锈菌小种生理专化性研究及小种监测是掌握小麦条锈菌及其毒性变化动态、抗病基因有效性及病害流行预测预报的重要依据,是治理由此病原菌引起的小麦条锈病的基本环节之一.由于地域、条锈菌群体结构、研究水平和历史等的差异,目前国际上不同国家和地区采用相应的小麦条锈菌鉴别寄主和小种命名方法.中国、美国和印度均使用了各自不同的小麦条锈菌鉴别寄主,并采用了相应的小种命名体系,例如从1957至2002年,中国利用10多个鉴别寄主已鉴定和命名了32个小种和30多个致病类型,美国利用10多个鉴别寄主已鉴定和命名了80个小种;另一类是采用1972年Johnson提出的二进制国际小种命名法,这种体系包括了国际鉴别寄主和欧洲鉴别寄主,象一套鉴别寄主被整体使用,欧洲、中东、亚洲、非洲和南美国家及澳大利亚和新西兰均采用此体系监测小麦条锈菌变化,是一套被最为广泛接受和使用的小麦条锈菌小种鉴别方法和命名系统,但随着研究深入和各国实际情况差异等原因,不同研究人员加入了不同的辅助鉴别寄主,以更能准确和实际地反映本国和地区的现实情况,为生产服务,本文对此现状进行了分析和比较.由于此病菌具有长距离气传和扩散的特点(地区间、国家间和洲际间),因而也讨论了未来小麦条锈菌鉴别寄主组成、小种命名方法和研究结果的交流和比较等实际问题.     

Selection for increased spore efficacy by host genetic background in a wheat powdery mildew population

ABSTRACT A field experiment was designed to test the hypothesis of for increased reproductive ability on different host genetic backgrounds within a wheat powdery mildew population. Studies have suggested that, in host mixtures, such selection could increase the reproduction rate of simple patho-types that always develop on the same host genetic background, whereas complex pathotypes should not be affected because they infect different host genotypes. In our experiment, the Erysiphe graminis population reproduced for successive generations on cvs. Orkis and Etecho, either grown as pure stands or in a mixture. In an additional treatment, the host cultivar changed after each generation. Isolates were sampled in April and, after seven pathogen generations, in July. At the second sampling date and for pure stands only, mean spore efficacy was greater on the host from which isolates were sampled than on the other one. This was attributed to selection within the pathogen population for better spore efficacy on the host genetic background. This selection was independent of the virulence genes carried by the isolates. The possibility of a phenotypic plasticity effect was tested and rejected.     

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。     

Recombinant inbred line differential identifies race-specific resistance to phytophthora root rot in Capsicum annuum

A differential series is the normal method for identification of races within a plant pathogen and a host interaction. A host differential is extremely useful for phytopathological as well as breeding purposes. A set of recombinant inbred lines (RILs) were developed and characterized for race differentiation of Phytophthora root rot caused by Phytophthora capsici. The highly resistant Capsicum annuum accession Criollo de Morelos-334 was hybridized to a susceptible cultivar, Early Jalapeno, to generate the RIL population. The host differential characterized 17 isolates of P. capsici into 13 races. The establishment of a stable host differential for the P. capsici and C. annuum interaction will assist researchers in understanding the complex inheritance of resistance to Phytophthora root rot and to develop resistant cultivars.     

Populations of Phytophthora infestans in Israel Underwent Three Major Genetic Changes During 1983 to 2000

ABSTRACT In this survey, 799 isolates of Phytophthora infestans collected from potato crops in Israel during 1983 to 2000 were analyzed for mating type and sensitivity to metalaxyl, and 324 were analyzed for race structure. The A(2) mating type, first recorded in 1983, fully dominated the pathogen population from 1983 until 1991 (9 years). It was thereafter replaced by the A(1) mating type, which dominated the population during 1993 to 2000. Metalaxyl-resistant isolates were first recorded in 1982. During 1983 to 1991, the majority of the isolates were resistant. Isolates with intermediate sensitivity (I) to this fungicide were first observed in 1993, when both A(1) and A(2) mating types occurred in the population. The proportion of I isolates gradually increased, reaching 39 to 41% in 1997 to 1998, and then declined to approximately 15% in 1999 to 2000. Pathogenicity to nine potato differential cultivars was determined for 80 potato isolates collected in 1983 to 1991, to 11 potato differentials in 173 isolates collected in 1993 to 1998, and in 71 potato isolates collected in 1999 to 2000. The first population was composed of 5 races with race 1,3,4,7,8,10 predominating (76%), the second population was composed of 19 races with race 1,3,4,7,8,10,11 predominating (63%), and the third population exhibited 42 (34 new) races with no single predominating race. RG-57 DNA fingerprinting and allozymes loci assays of 23 isolates revealed that isolates collected during 1984 to 1986 belonged to the PO-57 lineage, whereas those collected during 1997 to 1999 belonged to the RFO-39 lineage. Among isolates collected during 1993 to 1995, two unreported DNA fingerprinting patterns were found. Severe late blight epidemics occurred in tomato crops during 1998 to 2000. Of 35 tomato isolates, 28 were A(1) and only 7 were A(2). Of these tomato isolates, 94% were sensitive to metalaxyl. Almost every isolate had a different race structure on the 11 potato differentials. When inoculated onto three tomato differential cultivars, tomato isolates showed a virulence much more enhanced than potato isolates. The data suggest the Israeli population of P. infestans has passed through three major genetic changes during the past 18 years: in 1983, 1993, and 1999. The recent change included host specialization to tomato.     

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.     

Pathogenic races,host resistance,and an analysis of pathogenicity

There are two kinds of pathogenic races: those that interact differentially with varieties of the host plant, and those that do not. There are two kinds of resistance in the host: vertical resistance, when varieties of the host interact differentially with races of the pathogen, and horizontal resistance, when differential interaction is absent. There are two kinds of pathogenicity, defined here as virulence and aggressiveness, differentially interacting races differing in virulence, other races in aggressiveness. In relation to strong resistance genes, unnecessary virulence in a race reduces its fitness to survive and, with obligate parasites, also its aggressiveness; i.e. with obligate parasites unnecessary virulence and aggressiveness appear to be negatively correlated.Samenvatting Er zijn twee soorten van pathogene fysio's: die welke interactie, vertonen met rassen van de waardplant, en die welke dat niet doen. Er zijn twee soorten resistentie in de waardplant: verticale resistentie, wanneer rassen van de waardplant verschillend reageren op fysio's van het pathogeen, en horizontale resistentie, wanneer dit niet het geval is. Er zijn twee soorten pathogeniteit, hier omschreven als virulentie en agressiviteit; de eerstgenoemde fysio's verschillen in virulentie, andere fysio's in agressiviteit. In verband met sterke resistentiegenen reduceert onnodige virulentie in een fysio de overlevingskans en, bij obligate parasieten, ook de agressiviteit; dit wil zeggen dat bij obligate parasieten onnodige virulentie en agressiviteit negatief gecorreleerd zijn.     

稻瘟病菌生理小种及其毒性

用我国一套稻瘟病菌生理小种鉴别品种对分离自云南省红河州个旧、石屏、建水不同种植模式的68个稻瘟病菌单孢菌株进行生理小种测定,同时以日本8个单基因鉴别品种对供试菌株进行基因分析。测定结果为B₉、B₁₃、B₁₅、B₂₇、B₂₉、B₃₁、C₉、C₁₁、C₁₅、D₁、E₃和G₁12个生理小种,其中粳型小种占60.29%,籼型小种占39.71%。此结果与以往云南省稻瘟病菌生理小种的变化趋势相吻合。供试的68个菌株进行毒性基因分析表明Av-K^s+和Av-i+频率毒性基因较高。