Genetic differentiation in the French population of Erysiphe cichoracearum, a causal agent of powdery mildew of cucurbits

The relative incidence of Erysiphe cichoracearum and Sphaerotheca fuliginea , both agents of powdery mildew of cucurbits, was determined from 275 samples of mildewed leaves of cucurbits collected in 1994 from five regions of France. E. cichoracearum was identified in 9 to 39% of the mildewed leaf samples from four of the regions but was not detected in samples from the Mediterranean island of Corsica. The genetic structure of the French population of E. cichoracearum was examined using RFLPs of the ribosomal internal transcribed spacers amplified by PCR, random amplified polymorphic DNA (RAPD) markers, pathogenicity and mating-type tests. Forty-one isolates, including one from England, were analysed. Cluster analysis from 147 RAPD fragments using 16 primers revealed the existence of three distinct genetic lineages corresponding to three rDNA haplotypes (designated groups A, B and C). Bootstrap, genetic diversity, gametic disequilibrium and private allele analyses supported this differentiation. The genetic differentiation observed in the French population was not related to the geographical origin of the isolates. Group A isolates may be more specialized on melon as, with one exception, they were of race 1 (growth on four of the five melon cultivars tested) in comparison with group B and C isolates, which were of race 0 (growth on IranH only). Thus, the genetic differentiation observed may indicate a host-specialized subdivision within the French population of E. cichoracearum from cucurbits. Gametic disequilibrium analysis among RAPD loci and biological observations suggest that the sexual stage is of minor importance for epidemics of E. cichoracearum on cucurbits.     

Genetic Structure and Variation in Aggressiveness in European and Australian Populations of the Grapevine Dieback Fungus, Eutypa lata

Eutypa lata is an ascomycete fungus causing a severe dieback in grapevine. The genetic structure of populations of E. lata from seven regions in Australia, France, Italy and Spain was examined using 20 random amplified polymorphic DNA (RAPD) markers. In some regions, populations were subdivided and a total of 14 samples were analysed. A total of 231 RAPD haplotypes were found among the 240 isolates. Vegetative compatibility testing further demonstrated that isolates of the same haplotype were genetically distinct. Gene diversity was the highest in the population from northern Italy and lowest in the Alsace region in France. Linkage disequilibrium between pairs of putative loci was very low and most of the multilocus analyses were consistent with the hypothesis of random association of the loci. This suggests that random mating occurred in every population and that the sexual stage shapes the genetic structure of E. lata populations in the regions sampled. Only 6% of the total variability was attributable to differences between populations. Nevertheless, significant differences in allele frequency appeared with respect to six RAPD markers indicating some genetic differentiation between populations. This differentiation appeared attributable to differences between the Italian and Spanish populations and the other populations. We thus hypothesize that a restriction of gene flow exists within Europe. The population from Australia was genetically closer to the French and Spanish populations than to that from Italy. Genetic diversity is associated with considerable variation in aggressiveness, which was assessed on cuttings in the greenhouse in six populations. All populations included a range of isolates differing in aggressiveness, but the Italian population seemed to have more isolates with low aggressiveness.     

Morphological and molecular characterization of Colletotrichum spp. from citrus orchards affected by postbloom fruit drop in Brazil

Brazilian isolates of Colletotrichum spp. from citrus orchards affected by postbloom fruit drop were examined for colony colour, mycelial growth, benomyl-resistance, pathogenicity, and genetic variability by random amplified polymorphic DNA (RAPD) analysis. All isolates were obtained from flowers and persistent calyxes from different citrus hosts from Sao Paulo, Brazil. DNA polymorphisms detected after amplification with random 10-mer primers were used to classify the isolates into two groups. Group I isolates grew rapidly on potato-dextrose agar (PDA) and were sensitive to benomyl, and group II isolates grew slowly on PDA and were benomyl-resistant. Colletotrichum acutatum was analyzed by RAPD and had high genetic similarity with group II isolates of Colletotrichum from citrus. Probably, the group I is C. gloeosporioides and group II is C. acutatum.     

运用随机扩增多态性DNA标记检测中国东北大豆灰斑病菌株遗传变异

 运用致病力和DNA多态性检测中国东北地区的35个大豆灰斑病菌分离物的遗传变异、根据菌株在9个品种(系)上的致病力反应可将其分为7个组。利用13个随机引物扩增供试菌株共计产生105个RAPD标记,其中78.1%具有多态性。通过聚类分析计算了各菌株间的遗传距离,并产生树状图,发现同一地区内及不同地区间的病菌表现遗传变异、致病性和DNA多态性间具有一定相关性。     

四川省不同寄主立枯丝核菌的遗传分化和致病力研究

 在四川省生态条件下,从不同水稻和玉米植株上分别分离到来源不同的立枯丝核致病菌15株和7株。致病力、菌丝融合实验结果表明,菌株均属于AG-11A群,各菌株间致病力差异显著。对分离菌株进行RAPD分析,结果显示,相似系数为0.92处菌株可聚合为5类,聚类分组和寄主来源有一定的相关性,来自相同寄主菌株的亲缘关系较近,不同寄主对立枯丝核菌的遗传分化有一定的影响,与病原菌的致病力差异没有直接的相关性。     

Random amplified polymorphic DNA (RAPD) analysis of Phytophthora infestans isolates collected in Canada during 1994 to 1996

Mating type, glucose-6-phosphate isomerase ( Gpi ) allozyme banding patterns, response to the fungicide metalaxyl and random amplified polymorphic DNA (RAPD) markers were used to characterize genetic variability among 141 Canadian isolates of Phytophthora infestans collected between 1994 and 1996. Multiple correspondence analysis of RAPD profiles separated isolates into 21 groups that were not correlated to groups defined by mating type, Gpi allozyme banding patterns or response to metalaxyl. Population subdivision analysis showed that 97% of the total genetic variation was found among individuals within populations, compared with 3% among populations. The average similarity coefficient among isolates was 80%. No significant differences in haplotypic diversity were observed among the years under study, but levels of genetic diversity among local populations of P. infestans were high (0.76). All classes of response to the fungicide metalaxyl were observed, with 55% of isolates displaying moderate levels of insensitivity. The high level of genetic diversity detected within populations indicates that migration and sexual recombination probably play important roles in the population biology of P. infestans in Canada.     

Genetic diversity and pathogenic variability among isolates of colletotrichum species from strawberry

ABSTRACT Ninety-five isolates of Colletotrichum including 81 isolates of C. acutatum (62 from strawberry) and 14 isolates of C. gloeosporioides (13 from strawberry) were characterized by various molecular methods and pathogenicity tests. Results based on random amplified polymorphic DNA (RAPD) polymorphism and internal transcribed spacer (ITS) 2 sequence data provided clear genetic evidence of two subgroups in C. acutatum. The first subgroup, characterized as CA-clonal, included only isolates from strawberry and exhibited identical RAPD patterns and nearly identical ITS2 sequence analysis. A larger genetic group, CA-variable, included isolates from various hosts and exhibited variable RAPD patterns and divergent ITS2 sequence analysis. Within the C. acutatum population isolated from strawberry, the CA-clonal group is prevalent in Europe (54 isolates of 62). A subset of European C. acutatum isolates isolated from strawberry and representing the CA-clonal and CA-variable groups was assigned to two pathogenicity groups. No correlation could be drawn between genetic and pathogenicity groups. On the basis of molecular data, it is proposed that the CA-clonal subgroup contains closely related, highly virulent C. acutatum isolates that may have developed host specialization to strawberry. C. gloeosporioides isolates from Europe, which were rarely observed were either slightly or nonpathogenic on strawberry. The absence of correlation between genetic polymorphism and geographical origin in Colletotrichum spp. suggests a worldwide dissemination of isolates, probably through international plant exchanges.     

Persistence and spatial autocorrelation of clones of Erysiphe necator overwintering as mycelium in dormant buds in an isolated vineyard in northern Italy

The population structure of the grape powdery mildew fungus, Erysiphe necator (formerly Uncinula necator), has been hypothesized to vary from being clonal to highly diverse and recombining. We report here on the structure of an E. necator population sampled during a 4-year period from an isolated vineyard in northern Italy (Voghera, Pavia Province). We obtained 54 isolates of E. necator that overwintered asexually as mycelium in grapevine buds and caused severe symptoms on the emerging shoots, known as flag shoots. All isolates were genotyped for mating type, four multilocus polymerase chain reaction (PCR)-based markers (a total of 64 loci were scored), and two single-copy loci designed to identify genetic subgroups in E. necator. All isolates had the same mating type and single-locus alleles that correlate to isolates from flag shoots in other areas. Only 2 of the 64 loci scored from multilocus markers were polymorphic; 46 of the 54 isolates had the same multilocus haplotype. Seven isolates had a second haplotype that was recovered over 3 years, and only a single isolate was found with a third haplotype. Both variant haplotypes differed from the main clonal haplotype by single loci. Spatial autocorrelation analyses showed that vines with flag shoots were not aggregated within years, but they were aggregated between consecutive years. These results demonstrate that this subpopulation of E. necator on flag shoots is composed of a single clonal lineage that has persisted for at least 4 years. We speculate that the lack of diversity in the flag shoot subpopulation in this vineyard is the result of restricted immigration from surrounding areas and genetic drift operating through founder effects and periodic bottlenecks. We propose a model that integrates epidemiology and population genetics to explain the variation observed in genetic structure of E. necator flag shoot subpopulations from different vineyards or viticultural regions.     

Binucleate Rhizoctonia sp. AG G causing root rot in yacon (Smallanthus sonchifolius) in Brazil

A new rot caused by a binucleate Rhizoctonia sp. affecting the tuberous root cortex of the domesticated yacon ( Smallanthus sonchifolius ) has been observed in Brazil. Isolates of a binucleate Rhizoctonia sp. were collected from roots with rot symptoms and characterized by the number of nuclei per cell, hyphal anastomosis, RAPD molecular markers, ITS-5·8S rDNA sequence and pathogenicity tests. All isolates had a mean of 1·9–2·2 nuclei per cell and anastomosed with the binucleate Rhizoctonia sp. AG G-tester strain. RAPD analysis was carried out between 11 isolates recovered from yacon and 11 AG (A, Ba, Bb, Bo, C, D, F, G, O, P, Q) standard testers of binucleate Rhizoctonia sp. Genetic similarities of 94·8–100% were observed among isolates of the binucleate Rhizoctonia sp. from yacon and all isolates were genetically more closely related to the AG G tester than other strains according to upgma analysis using RAPD markers. Homologies of complete ITS nucleotide sequences were 100% between binucleate isolates of Rhizoctonia sp. from yacon and the AG G tester. According to pathogenicity tests, the isolates caused typical rot symptoms of yacon tubers 90 days after inoculation     

中国不同地区致病疫霉遗传多样性的RAPD分析

 本文应用RAPD技术检测了我国主要马铃薯产区致病疫霉的遗传分化情况及不同地区菌株间的亲缘关系。用筛选出的10个随机引物对1997-2001年间采自我国9省市的82株及3株来自日本的致病疫霉DNA进行了PCR扩增,获得了79条谱带,其中多态性标记75条,占95%。根据扩增结果,运用UPGMA分析,获得了表现菌株间亲缘关系的树状图。菌株间的最大遗传距离为0.5,以距离0.3为阈值,可将供试菌株划分为10个组(RG1-10)。结果发现:A1交配型菌株群体内的差异大于A1和A2菌株群体之间的;RAPD分组与菌株的地理来源、交配型及对甲霜灵的敏感性无明显相关性。研究结果显示,来自中国北方甘肃、内蒙、吉林、黑龙江地区的菌株与一些来自云南、四川等西南地区的菌株亲缘关系相近。病原菌随种薯的迁移可能是导致这种现象的原因之一。     

Phenotype Instability in Botrytis cinerea in the Absence of Benzimidazole and Dicarboximide Fungicides

ABSTRACT Stability of phenotypes of isolates of Botrytis cinerea that were sensitive or resistant to benzimidazole and dicarboximide fungicides was examined in the absence of fungicides in laboratory and growth room experiments. Twelve greenhouse isolates of B. cinerea were subcultured on potato dextrose agar (PDA) for 20 generations and on geranium seedlings for 15 generations. Three isolates of each of the following four phenotypes were used: sensitive to the fungicides thiophanate-methy1 (a benzimidazole) and vinclozolin (a dicarboximide) (S(T)S(V)), resistant to both fungicides (R(T)R(V)), resistant to thiophanate-methy1 and sensitive to vinclozolin (R(T)S(V)), and sensitive to thiophanate-methy1 and resistant to vinclozolin (S(T)R(V)). In three trials on PDA, 36 populations were subcultured; 8 populations changed phenotypes by the end of 20 generations, as determined by conidium germination on fungicide-amended medium. Five of the eight initially were S(T)R(V); the resulting phenotypes were S(T)S(V), R(T)S(V), and R(T)R(V). Populations from eight other isolates exhibited temporary changes in phenotype during intermediate generations on PDA but reverted to initial phenotypes by the twentieth generation; five of these populations changed to phenotype R(T)R(V). In two geranium seedling trials, each of the 12 greenhouse isolates was inoculated onto a set of three seedlings for each generation, and diseased tissue that developed was used to initiate the next generation. Therefore, a total of 72 populations of B. cinerea were subcultured in the two trials; 5 of these populations changed phenotype at the end of 15 generations. Three of the five initially were S(T)R(V); these changed to phenotypes S(T)S(V) or R(T)R(V). In each of the two trials on geranium seedlings, a population subcultured from one S(T)S(V) isolate changed phenotype one to phenotype R(T)R(V) and one to phenotype R(T)S(V). In all trials, no population resistant to thiophanate-methy1 changed to a thiophanate-methy1-sensitive phenotype, and no population changed to phenotype S(T)R(V). Random amplified polymorphic DNA (RAPD) fingerprints were generated with the 12 initial isolates and 49 isolates subcultured on PDA or geranium seedlings. Cluster analyses of RAPD markers showed that subcultured isolates exhibiting the same phenotype clustered together and that subcultured isolates derived from a common greenhouse isolate but with different phenotypes were in different clusters. Some populations that did not change phenotype exhibited considerable differences in RAPD marker patterns. The results of this study indicate that, in the absence of fungicides, sensitive populations of B. cinerea can develop resistance to thiophanate-methy1 and vinclozolin, and this resistance can be maintained in populations through multiple generations. Populations resistant only to vinclozolin (S(T)R(V)) exhibited a high frequency of phenotype change, and populations resistant to both fungicides (R(T)R(V)) were stable.     

Variation in the Fusarium section Liseola: pathogenicity and genetic studies of isolates of Fusarium moniliforme Sheldon from different hosts in Ghana

Fusarium moniliforme , the imperfect stage of the ascomycete Gibberella fujikuroi , is an economically important pathogen with a very wide host range. The genetic characteristics of isolates of the fungus collected from different regions of Ghana from maize, rice and sorghum were determined using the random amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) techniques. The pathogenicity of the isolates was also compared on maize and rice. DNA fingerprints detected as RFLPs of ribosomal DNA and RAPDs separated the isolates into discrete groups which were generally host-related. The possibility of a sub-structuring of the maize population of the fungus into tissue-related subgroups was suggested by the results. A dendrogram of the relatedness of the isolates is presented. However, the pathogenicity of the isolates on rice, measured by their ability to cause 'bakanae' symptoms, did not resolve the isolates into the clearly defined groups suggested by the genetic studies, and maize isolates of the fungus could cause 'bakanae' symptoms to the same extent as rice isolates. Similarly, some isolates identified as rice-type isolates caused as much shoot stunting in maize as maize isolates. However, the effects of the isolates on root growth of maize seedlings showed a broad correlation with the defined genetic groups, with maize isolates of the fungus showing the greatest tendency to cause root stunting     

Genetic variability of Colletotrichum lindemuthianum in wild populations of common bean

The genetic structure of wild populations of Colletotrichum lindemuthianum was evaluated for virulence and molecular markers. Forty-five isolates were collected from five wild common bean populations located in their South-Andean centre of origin. The five pathogen populations were monomorphic in their ITS regions, but 45 polymorphic markers were identified using RAPDs. Polymorphism for virulence was also observed; 15 pathotypes were characterized on an international set of 12 differentials. A molecular variance analysis ( AMOVA ) showed that a very large part of the total genetic variation was within populations. Statistical analysis showed that there was a weak though significant differentiation among the five populations for the RAPD and virulence markers. A positive and significant correlation was found between geographic distance and the distances from RAPD and virulence data, suggesting migration between adjacent populations along the Argentinian transect. Our results suggest that the Andean wild isolates of C. lindemuthianum do not reflect all the putative diversity found in the isolates from cultivated common bean.     

Host Range Specificity in Verticillium dahliae

ABSTRACT Verticillium dahliae isolates from artichoke, bell pepper, cabbage, cauliflower, chili pepper, cotton, eggplant, lettuce, mint, potato, strawberry, tomato, and watermelon and V. albo-atrum from alfalfa were evaluated for their pathogenicity on all 14 hosts. One-month-old seedlings were inoculated with a spore suspension of about 10(7) conidia per ml using a root-dip technique and incubated in the greenhouse. Disease incidence and severity, plant height, and root and shoot dry weights were recorded 6 weeks after inoculation. Bell pepper, cabbage, cauliflower, cotton, eggplant, and mint isolates exhibited host specificity and differential pathogenicity on other hosts, whereas isolates from artichoke, lettuce, potato, strawberry, tomato, and watermelon did not. Bell pepper was resistant to all Verticillium isolates except isolates from bell pepper and eggplant. Thus, host specificity exists in some isolates of V. dahliae. The same isolates were characterized for vegetative compatibility groups (VCGs) through complementation of nitrate nonutilizing (nit) mutants. Cabbage and cauliflower isolates did not produce nit mutants. The isolate from cotton belonged to VCG 1; isolates from bell pepper, eggplant, potato, and tomato, to VCG 4; and the remaining isolates, to VCG 2. These isolates were also analyzed using the random amplified polymorphic DNA (RAPD) method. Forty random primers were screened, and eighteen of them amplified DNA from Verticillium. Based on RAPD banding patterns, cabbage and cauliflower isolates formed a unique group, distinct from other V. dahliae and V. albo-atrum groups. Minor genetic variations were observed among V. dahliae isolates from other hosts, regardless of whether they were host specific or not. There was no correlation among pathogenicity, VCGs, and RAPD banding patterns. Even though the isolates belonged to different VCGs, they shared similar RAPD profiles. These results suggest that management of Verticillium wilt in some crops through crop rotation is a distinct possibility.     

Rapid emergence of hybrids between the two subspecies of Ophiostoma novo-ulmi with a high level of pathogenic fitness

During the 1970s Europe was invaded by two subspecies of the Dutch elm disease pathogen Ophiostoma novo-ulmi : subsp . americana from the west and subsp. novo-ulmi from the east. As a result their geographic ranges began to overlap in several areas. Only a weak prezygotic barrier to hybridization exists between the subspecies and in 1980 two hybrids were detected in the Netherlands. A subset of 107 O. novo-ulmi isolates collected in a subspecies overlap zone in Limburg, Netherlands in 1983 was characterized for three phenotypic markers and seven RAPD PCR markers. By phenotype, 33% were shown to be hybrid whereas by RAPD markers 69% were shown to be hybrid. Some isolates shown to be hybrid by phenotype were not revealed to be hybrid by PCR and vice versa. Combining the phenotype and RAPD data the estimated hybrid frequency was ∼78%. The mean growth rate of Limburg hybrid isolates was significantly faster than that of the Limburg subsp. novo-ulmi isolates but not significantly different from Limburg subsp. americana isolates. The Limburg hybrid isolates were just as pathogenic as the parent subspecies on both clonal Ulmus procera and on U.  × Commelin. A subset of 100 isolates collected in another subspecies overlap zone at Orvieto, Italy in 1986 was also assessed with RAPD markers and ∼ 72% were shown to be hybrids. When 20 isolates of a 'pure' subsp. novo-ulmi population in the Baltic Ports area of Poland collected in 1980 were assessed by RAPD markers three isolates exhibited early introgression of subsp. americana DNA. This study therefore demonstrates very rapid emergence of O. novo-ulmi subspecies hybrids and introgressants in Europe in the early 1980s. In terms of two major fitness characters, growth rate and pathogenicity, these early hybrids were as fit as their parent subspecies. It is likely that complex hybrid swarms are now expanding across the continent.     

Genetic variation within and between southern Ontario populations of Sclerotinia homoeocarpa

Restriction fragment length polymorphisms (RFLP) of the intergenic spacer region (IGS) of rDNA and random amplified polymorphic DNA (RAPD) markers were used to survey genetic variability among 181 isolates of Sclerotinia homoeocarpa from Ontario and 10 isolates from Japan. RAPD and IGS-RFLP analyses revealed polymorphisms within and between populations of S . homoeocarpa , distinguishing 151 genotypes. Both types of markers gave similar results in phenetic analysis of genetic distances between populations. Cluster analysis showed that Japanese isolates of S. homoeocarpa were genetically distinct from Ontario isolates, demonstrating significant intraspecific differentiation. An average genetic similarity of 0.66 was found between Japanese isolates. Among Ontario isolates, average genetic similarity was 0.86, and genotypic diversity analysis showed that 49.3% of the total genetic variation observed within Ontario populations occurred among individuals within populations compared to 50.7% between populations. Gametic linkage disequilibrium analysis within Ontario populations revealed an average 15.6% significant nonrandom associations between putative RAPD loci, and that half of the populations showed signs of significant linkage disequilibrium. These results suggest that both clonal propagation and recombination events occurred in local populations of S. homoeocarpa . The high level of genetic similarity between populations and the low levels of intraspecific genetic variation may reflect a small founding population for southern Ontario isolates of S. homoeocarpa .     

Genetic and Pathogenic Analyses of Colletotrichum gloeosporioides Isolates from Strawberry and Noncultivated Hosts

ABSTRACT Colletotrichum crown rot of strawberry in Florida is caused primarily by Colletotrichum gloeosporioides. To determine potential inoculum sources, isolates of Colletotrichum spp. from strawberry and various noncultivated plants growing in the areas adjacent to strawberry fields were collected from different sites. Species-specific internal transcribed spacer primers for C. gloeosporioides and C. acutatum were used to identify isolates to species. Random amplified polymorphic DNA (RAPD) markers were used to determine genetic relationships among isolates recovered from noncultivated hosts and diseased strawberry plants. Selected isolates also were tested for pathogenicity on strawberry plants in the greenhouse. In all, 39 C. gloeosporioides and 3 C. acutatum isolates were recovered from diseased strawberry crowns, and 52 C. gloeosporioides and 1 C. acutatum isolate were recovered from noncultivated hosts. In crown inoculation tests, 18 of the 52 C. gloeosporioides isolates recovered from noncultivated hosts were pathogenic to strawberry. Phylogenetic analysis using RAPD marker data divided isolates of C. gloeosporioides from noncultivated hosts into two separate clusters. One cluster contained 50 of the 52 isolates and a second cluster contained 2 isolates that were homothallic in culture. Isolates from strawberry were interspersed within the cluster containing the 50 isolates that were recovered from noncultivated hosts. The results are not inconsistent with the hypothesis that C. gloeosporioides isolates obtained from strawberry and noncultivated hosts adjacent to strawberry fields are from the same population and that noncultivated hosts can serve as potential inoculum sources for Colletotrichum crown rot of strawberry.     

Genetic Diversity in South American Colletotrichum gloeosporioides Isolates from Stylosanthes guianensis, a Tropical Forage Legume

The degree of genetic diversity of 127 Colletotrichum gloeosporioides isolates from Stylosanthes guianensis genotypes in South America was measured at the molecular level by random amplified polymorphic DNA (RAPD) with nine arbitrary primers of 10 bases, and by restriction fragment length polymorphism (RFLP) with a non-LTR (long terminal repeats) retrotransposon DNA sequence. The RAPD products revealed scorable polymorphism among the isolates, and a total of 80 band positions were scored. Sixty-three of the 127 isolates were clustered into 13 distinct lineages usually correlating with geographic origin. Where isolates from various regions were clustered together, most had identical host genotype origin. The pathogen population sampled from Carimagua, Colombia, a long-time Stylosanthes breeding and selection site, with a savanna ecosystem, was highly diverse. A set of 12 S. guianensis genotype differentials was used to characterize pathogenic variability of 104 isolates and their virulence patterns were grouped into 57 pathotypes. However, when they were tested on four Australian differentials, they grouped into 11 pathotypes. As shown in previous studies, no strict correlations existed between genetic diversity measured by RAPD or RFLP, and pathotype defined by pathogenicity pattern on the differentials. Southern blot analysis of the 127 isolates revealed 23 hybridizing fragments, resulting in 41 fingerprint patterns among the 127 isolates. Relationships between RFLP and RAPD variables were examined using Spearman's Rank Correlation Coefficient, which showed that the two measures of genotypic variation are in agreement.     

Intraspecific diversity within avocado field isolates of <Emphasis Type="Italic">Rosellinia necatrix</Emphasis> from south-east Spain

Fifty-five isolates of Rosellinia necatrix, the cause of common avocado white root rot disease, were collected from south-east Spain and characterised according to their virulence behaviour and their molecular patterns to assess broader levels of genetic diversity. Virulence properties were revealed by in vitro inoculation on avocado plants. Differences in reaction types showed variability among these isolates. No sequence differences were observed when the internal transcribed spacer 1 (ITS1) and ITS2 regions and DNA fragments of the β-tubulin, adenosine triphosphatase and translation elongation factor 1 genes were explored in representive isolates from five virulence groups. Random amplified polymorphic DNA (RAPD) amplifications were also performed for each isolate using 19 random primers. Four of these primers revealed polymorphism among isolates and repetitive and discriminative bands were used to build an unweighted pair group with arithmetic mean tree. However, RAPD clustering showed low stability, and no correlation between RAPD and virulence groups was observed, possibly indicating high levels of sexual recombination.     

Molecular analysis of Spanish populations of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">dianthi</Emphasis> demonstrates a high genetic diversity and identifies virulence groups in races 1 and 2 of the pathogen

Fusarium wilt, caused by Fusarium oxysporum f. sp. dianthi (Fod), is the most important carnation disease worldwide. The knowledge of the diversity of the soil population of the pathogen is essential for the choice of suitable resistant cultivars. We examined the genetic diversity of Fod isolates collected during the period 1998–2008, originating from soils and carnation plants in the most important growing areas in Spain. Additionally, we have included some Fod isolates from Italy as a reference. Random amplified polymorphic DNA (RAPD) fragments generated by single-primer PCR were used to compare the relationship between isolates. UPGMA analysis of the RAPD data separated Fod isolates into three clusters (A, B, and C), and this distribution was more related to aggressiveness than to the race of the isolates. The results obtained in PCR amplifications using specific primers for race 1 and race 2, and SCAR primers developed in this work, correlated with the molecular groups previously determined from the RAPD analysis, and provided new molecular markers for the precise identification of the isolates. Results from successive pathogenicity tests showed that molecular differences between isolates of the same race corresponded with differences in aggressiveness. Isolates of races 1 and 2 in cluster A (R1I and R2I isolates) and cluster C (R1-type isolates) were all highly aggressive, whereas isolates of races 1 and 2 in cluster B (R1II and R2II isolates) showed a low aggressiveness profile. The usefulness of the molecular markers described in this study has been proved in double-blind tests with Fod isolates collected in 2008. Results from this work indicate a change in the composition of the Spanish Fod population over time, and this temporal variation could be related to the continuous change in the commercial carnation cultivars used by growers. This is the first report of genetic diversity among Fod isolates in the same race.