Genotypic diversity in barley powdery mildew populations in northern France

Diversity in populations of Erysiphe graminis ( Blumeria graminis ) f.sp. hordei was studied with virulence and molecular markers. Isolates were sampled in two locations in northern France from a winter barley cultivar (Plaisant) and a spring cultivar (Caruso). Only a few pathotypes (determined by virulence markers) were common. The rest of the population was diverse. Diversity within common pathotypes, estimated by five RAPD and two SCAR markers, was generally high, except for one pathotype, which was frequent on Plaisant. This pathotype carried only one virulence, Va22, out of the 11 virulences tested. It appeared as a clonal lineage, which had occurred previously, at least in 1992, in northern France, demonstrating survival of asexual lineages in populations that often reproduce sexually.     

The effect of fungicide dose on the composition of laboratory populations of barley powdery mildew

The effect of the use of different doses of the fungicide fenpropimorph on populations of barley powdery mildew Blumeria ( Erysiphe ) graminis f. sp. hordei was investigated in a laboratory selection experiment. A sample from the Danish aerial population of powdery mildew was split into populations, and these were kept separately for 31 generations on susceptible barley seedlings treated with fungicide at two concentrations, as well as on a control. Samples from these populations were tested for their resistance to fenpropimorph and their virulence spectra. There was a large amount of environmental variation in the ED₅₀ values used to measure fungicide resistance. In both treated populations, the average level of fungicide resistance increased, this increase being faster and greater in the population treated with the high dose. The diversity of pathotypes of the treated populations decreased, with the decline being more rapid in the population treated with the high dose, where one pathotype dominated the population after 31 generations. This pathotype was apparently not the fittest in the population treated with the low dose. This implies that knowledge of ED₅₀ is not sufficient to predict pathotype evolution under different fungicide treatments. The dominant pathotype in the high-dose treatment may not have been clonal, as there was evidence of two levels of fungicide resistance. The large environmental variation observed in estimated ED₅₀ values for resistance towards fenpropimorph may help to explain why this resistance has evolved at a slower rate than resistance towards other fungicides.     

Virulence spectrum in populations of the barley powdery mildew pathogen, Erysiphe graminis f. sp. hordei in Tunisia and Morocco in 1992

The virulence of 57 single colonies of Erysiphe graminis f. sp. hordei from Tunisia and Morocco was investigated using the Pallas near-isogenic differential set. Virulence patterns in general were similar in both countries. Va8, Va10+Du2, V41/145, VLa and VRu2 appear to be common in the region. The resistance alleles Mla7 and Mla9, in combination with other resistances, were highly effective against the isolates tested. No virulence against mlo and Mla9 + k was detected in Tunisia. No virulence on Mla7 and Mla9 was detected in Morocco. The frequency of virulence against several resistances was significantly higher in Morocco than in Tunisia. On the other hand, virulence against other resistances was higher in Tunisia than in Morocco. Three isolates from Morocco infected mlo to an extent greater than previously described. All pathotypes were unique. An attempt is made to interpret the results by comparison with pathotype evolution in Europe.     

Polygenic powdery mildew disease resistance in Arabidopsis thaliana: quantitative trait analysis of the accession Warschau-1

To further the understanding of the natural genetic diversity for disease resistance to powdery mildew ( Erysiphe cichoracearum ) in Arabidopsis thaliana , quantitative trait loci analysis was undertaken on recombinant inbred lines derived from a cross between the resistant accession Warschau-1 and the susceptible Columbia-0. Powdery mildew grew less well on Warschau-1, but the resistance was not associated with a specific block in the infection sequence. Two potential powdery mildew disease-resistance loci were identified and mapped, one with a major effect and one with a minor effect on disease resistance. The two loci acted in an additive manner to confer resistance, and together they explained 65% of the variation in resistance. In addition, the major powdery mildew disease-resistance locus was genetically mapped to the bottom of chromosome III, a region containing the powdery mildew resistance loci RPW7 , RPW8 and RPW10 . Unlike resistance mediated by the RPW8 locus in the accession Moscow-1, resistance in Warschau-1 was not correlated with the hypersensitive response, highlighting the influence of genetic background or environmental factors on the expression of disease resistance. Together with the powdery mildew resistance loci described in other studies, these results suggest that A. thaliana is a useful source of natural powdery mildew disease resistance, which potentially can be utilized in fundamental studies and as a tool for applied studies.     

Genetic variation in populations of the cacao wilt pathogen, Ceratocystis cacaofunesta

Ceratocystis cacaofunesta (=  Ceratocystis fimbriata ) causes a lethal wilt disease of cacao ( Theobroma cacao ) in Latin America. Polymorphic microsatellite markers, (CAT)₅ nuclear DNA fingerprints and Hae III mitochondrial DNA fingerprints were used to compare genetic diversity among isolates of C. cacaofunesta collected from populations in western Ecuador, Costa Rica, Colombia, and Rondônia and Bahia in Brazil. Microsatellite markers and nuclear DNA fingerprints separated Ecuadorian isolates from isolates of the other four populations, and these two major groups correspond to genetic lineages already identified from ITS-rDNA sequences and intersterility groupings. Mitochondrial DNA fingerprints also demonstrated substantial diversity and split the Ecuadorian isolates into two groups. All marker types showed limited variation in the Colombian, Costa Rican and Bahian populations, as might be expected for introduced populations that have gone through recent genetic bottlenecks. In contrast, the Rondonian and western Ecuadorian populations showed gene diversity values similar to natural populations of other Ceratocystis species. The Rondonian population was the only sampled population in the native range of T. cacao (the Upper Amazon), and the putatively introduced populations were more closely related to the Rondonian population than to the western Ecuadorian population. The Ecuadorian population is in an area with other native Theobroma species, which may serve as natural hosts.     

Identification of eight effector candidate genes involved in early aggressiveness of the barley powdery mildew fungus

The barley powdery mildew fungus, Blumeria graminis f. sp. hordei (Bgh), expresses 491 candidate for secreted effector proteins (CSEPs), identified based on presence of a predicted signal peptide and relative sequence uniqueness. Virtually nothing is known about the mechanism of action of CSEPs. The present study used host‐induced gene silencing (HIGS) to identify genes involved in early fungal aggressiveness. Screening of 22 selected CSEP‐encoding genes suggested that CSEP0007, CSEP0025, CSEP0128, CSEP0211, CSEP0247, CSEP0345, CSEP0420 and CSEP0422 are individually required for normal levels of aggressiveness of Bgh. Expression data showed that these genes were up‐regulated at initial stages of infection, suggesting their involvement in early fungal aggressiveness. Altogether, the findings expand the current knowledge of genes contributing to Bgh pathogenesis and provide the basis for future functional characterization.     

Fungicide sensitivity of populations of wheat powdery mildew (Erysiphe graminis f.sp. Tritici) in Central Europe in 1993

In 1993 we observed the sensitivity of wheat powdery mildew populations (Erysiphe graminis DC f.sp. tritici Marchal) from the Czech Republic, Austria, Hungary and Slovakia to the fungicides triadimenol, tebuconazole, propiconazole, flutriafol and fenpropimorph. The highest resistance value was shown to triadimenol, which attained a mean resistance factor (MRF) of 29 (expressing how many times the population is more resistant than are standard sensitive isolates) in the mildew population from the Czech Republic. The mildew populations from eastern Slovakia and eastern Hungary, populations geographically isolated from the other populations, showed very high sensitivity to all fungicides tested. There was most sensitivity to fenpropimorph (smallest MRF values) compared with the other fungicides. Cross-resistance was established among all triazoles used, but not between triazoles and fenpropimorph. Sensitivity of wheat powdery mildew populations from Central Europe to these fungicides is considered adequate, and the development of resistance has shown a decreasing tendency in recent years.     

Effect of early infection on pathotype frequencies in barley powdery mildew (Blumeria graminis f.sp. hordei) populations in field plots

A field experiment with barley powdery mildew ( Blumeria graminis f.sp. hordei ) was designed in order to study how the time of arrival of inoculum in the field influenced pathotype frequencies in the resulting populations. Three isolates belonging to pathotypes that were absent or rare in the local aerial inoculum were used to inoculate field plots of winter barley cv. Plaisant. Two successive inoculations with different combinations of the three isolates were performed with an approximately two-generation delay, and frequencies of inoculated pathotypes were assessed four and nine generations after the first inoculation. Pathotypes of the first inoculated isolates generally persisted throughout the period of sampling; this is described as an 'early arrival' effect. During the epidemics the inoculated isolates were not replaced by isolates from the natural airborne inoculum. Pathotype frequencies depended mainly on the time of arrival of inoculum in the plot, but frequencies also depended on the isolate that had been inoculated. The most frequent isolate, GL1, belonged to the clonal lineage dominant in powdery mildew populations on winter barley in the north of France. These results confirmed that the composition of a powdery mildew population in a field is largely determined by the composition of the initial inoculum.     

Genetic diversity of sulfonylurea-resistant and -susceptible Monochoria vaginalis populations in Japan

Monochoria vaginalis is one of the most serious weeds of rice fields in Asia. The species is predominantly selfing. To reveal the potential for multiple mutational events, outcrossing and gene flow in the sulfonylurea‐resistant (SU‐R) M. vaginalis populations, we investigated (i) if each SU‐R population was a single SU‐R biotype or a mixture of several SU‐R biotypes using restriction analysis or direct sequencing of acetolacatate synthase (ALS) genes and (ii) genetic diversity of SU‐R and ‐susceptible (S) populations using amplified fragment length polymorphism (AFLP) analysis. Nineteen or 20 individuals were sampled from four SU‐R and five SU‐S populations respectively. Amino acid substitutions conferring resistance in the SU‐R populations were Pro₁₉₇Ser in the ALS1 or ALS3, or Asp₃₇₆Glu in the ALS1 and each SU‐R population was composed of a single SU‐R biotype. In cluster analysis each SU‐R individual formed a cluster, whereas the individuals from a SU‐S population belonged to different clusters. Some SU‐R populations showed polymorphic AFLP loci. The results indicated that these SU‐R biotypes emerged from a single mutational event and any gene flow of SU‐R genes from adjacent populations did not occur. A low level of outcrossing and recombinations of SU‐R genes occurred within some SU‐R populations of M. vaginalis.     

香蕉枯萎病菌群体多样性研究进展

香蕉枯萎病严重威胁世界香蕉产业,而目前尚无有效防治药剂。开发快速诊断技术以加强检疫,控制其传播速度,同时加快选育抗病品种是有效控制该病的根本策略。然而,无论是研发快速诊断的分子技术还是进行抗病品种的选育,都需要深入了解病原菌的群体结构及其基因多样性背景。香蕉枯萎病菌经过一个多世纪的变异与进化,已分化出4个生理小种,23个营养亲合群和多个基因多样性类群。本文从香蕉枯萎病的起源及其病原菌培养性状、生理小种、致病性、营养亲合群及基因多样性等方面研究进展进行梳理和分析,以期为下一步的研究提供思路和启发。     

Genetic diversity and interfertility among highly differentiated populations of Ceratocystis fimbriata in Brazil

Mating studies showed that isolates of the insect‐associated wilt pathogen Ceratocystis fimbriata from Eucalyptus spp., mango, fig, inhame (Colocasia esculenta), Gmelina arborea and sweet potato were interfertile, and progeny from those crosses showed normal segregation for microsatellite markers. Genetic diversity was compared among 13 populations of C. fimbriata collected from six states in Brazil using 15 highly polymorphic microsatellite markers. The gene diversity values of most eucalyptus and mango populations from Minas Gerais, Bahia, Rio de Janeiro and São Paulo states were similar to putatively native populations of Ceratocystis platani and C. cacaofunesta, two other species in the C. fimbriata complex that are homothallic. Index of association values indicated substantial asexual reproduction or selfing in populations on mango and eucalyptus. Most of these eucalyptus and mango populations were not highly differentiated from each other, and these populations and genotypes appeared to be more closely related to each other than to other populations by upgma analyses. By contrast, the G. arborea population from Pará and the fig and inhame populations from São Paulo had relatively low levels of diversity and were highly differentiated from each other and all other studied populations, suggesting that they were from different origins and had gone through genetic bottlenecks. One of the eucalyptus populations in Bahia consisted of a single genotype and may have been introduced to the site in infected cuttings from another Bahia location. Similarly, a mango population from Mato Grosso do Sul consisted of a single genotype, which was identical to one of the genotypes found on mango in São Paulo. Aside from introductions by humans, mating studies and genetic analyses suggest that limited dispersal distance and a high degree of selfing or asexual reproduction lead to local populations of C. fimbriata that have limited diversity but are highly differentiated from other populations.     

Genetic diversity of Striga hermonthica and Striga asiatica populations in Kenya

The parasitic angiosperms, Striga hermonthica and Striga asiatica, severely constrain cereal production in sub-Saharan Africa by causing huge losses in grain yield. Understanding the diversity of Striga populations is important because it allows identification of races or biotypes thus improving chances of breeding success. Amplified fragment length polymorphism (AFLP) analysis was used to study genetic diversity among 17 populations of S. asiatica and 24 populations of S. hermonthica from Kenya. A total of 349 DNA fragments ranging from 51 to 500 bp were obtained from four EcoRI and MseI primer combinations. Genetic distances for S. asiatica populations ranged from 0.009 to 0.116 with a mean of 0.032. S. hermonthica populations had a genetic distance that ranged from 0.007 to 0.025 with a mean of 0.015. Only two clusters were found in S. asiatica populations whereas no apparent structure was evident in S. hermonthica populations. There was no evidence of isolation by distance for the two species. Although the low genetic diversity suggests Striga is relatively uniform across the populations studied, it is possible that pathogenicity and virulence genes may be located in genomic regions that were not sampled. The data, however, does not provide evidence to support diversification of both Striga species in the region where the study was conducted.     

克拉玛依植物群落物种多样性研究

根据30个样地102个样方的调查资料,采用Simpson多样性指数、Shannon-Wiener多样性指数、Gleason丰富度指数[17]、Simpson优势度指数与Pielou均匀度指数[16]对克拉玛依植物群落进行研究。研究结果表明:克拉玛依植物群落结构简单、物种稀少,群落物种多样性水平较低,群落多样性指数的顺序为:胡杨群落﹥柽柳群落﹥梭梭群落﹥琵琶柴群落﹥芦苇群落﹥假木贼群落﹥猪毛菜群落﹥盐节木群落﹥碱蓬+麻黄群落﹥盐穗木群落。随着水分与土壤盐碱度的变化,群落物种多样性出现相应的变化趋势:乔木群落﹥灌木群落﹥草本群落。     

A comparative study of resistance to powdery mildew in wild emmer wheat in the seedling and adult plant stage

An isometric virus was isolated from cucumber plants growing in a plastic house in Crete and showing stunting and bright yellow mosaic of the leaves. Based on host range, properties in crude sap, behaviour during purification, electron microscopy and serology, the virus was identified as an isolate of artichoke yellow ringspot nepovirus. Ecological data corroborate transmission of the virus via the soil.Samenvatting Uit komkommerplanten in plastic-foliekassen op Kreta werd een bolvormig virus geïsoleerd; de aangetaste komkommerplanten vertoonden dwerggroei en helder geel mozaïek op de bladeren. Gebaseerd op de resultaten verkregen uit onderzoek met het virus naar de waardplantenreeks, de eigenschappen in perssap, zuivering, elektronen-microscopie en serologie kon het virus worden geïdentificeerd als een strain van het artichoke yellow ringspot nepovirus. Waarnemingen op het gebied van de ecologie wijzen op overdracht van het virus via de grond.     

Genetic diversity of herbicide-resistant and -susceptible Avena fatua populations in North Dakota and Minnesota

Genetic diversity within and among 20 herbicide-resistant (HR) and 16 herbicide-susceptible (HS) Avena fatua multi-field populations was determined using 82 polymorphic loci resulting from two intersimple sequence repeat (ISSR) primers and one long-primer random amplified polymorphic DNA (LP-RAPD) primer. Collections from the Red River Valley of North Dakota and Minnesota, sampled in 1964 and 2000, represented A. fatua populations before and after intensive exposure to herbicides. A 1995 collection from south-west North Dakota represented A. fatua exposed to low herbicide selection. Despite differences in years of herbicide exposure among collections, both HR and HS populations from every collection maintained nearly similar levels of ISSR and RAPD diversity. Genetic differentiation among populations (G_ST) varied from 11% to 13% among HR populations and from 9% to 16% among HS populations, indicating that 84–91% of total variation remained within HS or within HR populations. Minimal difference in gene diversity between HR and HS is consistent with multiple origins of resistance, where HR A. fatua most likely evolved from diverse founding individuals.     

Genetic diversity in the coconut lethal yellowing disease phytoplasmas of East Africa

DNA primers, based on the ribosomal sequences of lethal yellowing-type disease (LYD) phytoplasmas, were used to analyse genetic variation within the lethal yellowing-type diseases of coconut in East Africa. Samples were collected from palms in Kenya, Mozambique and high, medium and low disease incidence areas of Tanzania. The mollicute-specific primer pair P1 and P6 amplified a 1.5 kbp product from all diseased palms and no product from symptomless palms, indicating that phytoplasmas were associated with all of these diseases. However, the Rohde forward and Rohde reverse primers (a second rRNA primer pair designed to detect East African LYD-associated phytoplasmas) only amplified products from Tanzanian and Kenyan diseased palms and not from those of Mozambique. Conversely, primers Ghana 813 and AK-SR, designed for specific detection of coconut-associated phytoplasmas in West Africa, amplified products only from the Mozambique palms, indicating that the phytoplasma associated with LYD in Mozambique is more closely related to those from West Africa. This was supported by restriction enzyme digestion of PCR products. DNA sequencing of PCR products from phytoplasmas within Tanzania revealed no detectable differences in the rDNA sequences of isolates from high, medium and low incidence areas.     

石羊河上游不同海拔天然草地群落多样性与生产力的关系

生态系统的结构和功能、生物多样性与生产力的关系问题是近年来群落生态学中研究的中心问题,其中,生态系统生产力水平是其功能的重要表现形式,用10种不同海拔梯度草地群落探讨自然群落的物种多样性与生产力关系。结果显示:丰富度、多样性指数、均匀度指数随海拔升高先增大后减少;生态优势度指数随海拔升高先减少后增大;群落物种丰富度与初级生产力之间趋势模拟呈极显著的线性相关,相关系数为0.8678;物种多样性与初级生产力之间趋势模拟呈极显著的二次函数关系,相关系数为0.7396;均匀度指数和生态优势度与初级生产力之间趋势模拟均呈显著的二次函数关系,相关系数分别为0.6671、0.7287。不同的环境资源和环境异质性是形成群落结构特征、物种多样性分布格局差异的主要原因之一。     

Priming of the oxidative burst in rice and wheat cell cultures by ulvan,a polysaccharide from green macroalgae,and enhanced resistance against powdery mildew in wheat and barley plants

The capacity of ulvan, a sulphated heteropolysaccharide, to prime the chitin‐ and chitosan‐elicited oxidative burst in wheat and rice cells was tested. Gas‐liquid chromatography showed that ulvan was composed of rhamnose, xylose, glucuronic acid, glucose and galactose. It contained very low amounts of proteins and ca. 19% sulphate groups. The polymer did not elicit the production of hydrogen peroxide in suspension‐cultured wheat or rice cells. Furthermore, in both cell cultures, the simultaneous addition of ulvan and chitin hexamer or chitosan polymer did not significantly alter the intensity of the oxidative burst caused by the elicitors alone. However, pretreatment of wheat cells with ulvan increased the chitin‐elicited oxidative burst about five‐ to sixfold, and that elicited by chitosan about twofold. In rice cells, the production of H₂O₂ elicited by chitin or chitosan was strongly primed by pretreatment with the same concentration of ulvan, increasing the burst triggered by the elicitors alone by 150 and 80 times, respectively. Pretreatment of whole plants with ulvan significantly reduced the symptom severity of Blumeria graminis infection, by 45% in wheat and by 80% in barley. Thus, the priming activity of ulvan on the oxidative burst correlates with a decrease of disease symptoms in infected plants. This is apparently the first report on priming activity of a natural algal polysaccharide.     

Analysis of molecular diversity in <Emphasis Type="Italic">Crinipellis perniciosa</Emphasis> with AFLP markers

Crinipellis perniciosa causes a serious disease of cacao known as witches broom (WB). Heritable resistance to witches broom has been used in cacao improvement programs. SCA6 and SCA12 are highly resistant and are the most commonly used parents in the breeding schemes. However, SCA hybrids are not resistant to witches broom in all production areas. Presumably, different populations of C. perniciosa cause these variable responses. Amplified fragment length polymorphism (AFLP) markers were used to assess variation and population structure in this pathogen. We examined 40 isolates of C. perniciosa and one isolate of Melanotus subcuneiformis. Nine of 64 primer pairs produced consistent and informative DNA amplification, and were used to screen all isolates. Fifteen haplotypes (AFLP fingerprints) were detected with 186 polymorphic markers. Cluster analysis grouped isolates of the C biotype (pathogenic on cacao) from Bolivia, Brazil, Ecuador and Trinidad together in a major cluster that was distinct from isolates of the S biotype (pathogenic on solanaceous hosts) and M. subcuneiformis. Isolates of the C biotype were divided further into well supported, country-specific groups. Segregation of AFLP alleles was not observed among basidiospore isolates from the same basidiome, broom, tree or field, supporting previous reports that the fungus did not outcross. The results corroborated prior conclusions that C. perniciosa was probably introduced into the Bahia state of Brazil from the Amazon basin. Representative isolates from the genetically distinct groups that were revealed will be used to examine pathogenic specialization in C. perniciosa and differential responses that have been reported in SCA6-derived germplasm.