Role of perithecia as an inoculum source for stem rot type of pepper root rot caused by Fusarium solani f. sp. piperis (teleomorph: Nectria haematococca f. sp. piperis)

Pepper (Piper nigrum L.) root rot caused by Fusarium solani f. sp. piperis (FSP; teleomorph: Nectria haematococca f. sp. piperis) includes two symptom types called root rot (RR) type and stem rot (SR) type. In this study, the temporal and spatial associations between perithecial formation of FSP and development of SR were investigated in naturally infested fields to verify the hypothesis that ascospores from the perithecia are the major inoculum source of the SR type on vines in the field. In surveys of all vines in two neighboring pepper fields every month from December 2005 to November 2006, I mapped the locations of all vines with perithecia and all vines the SR type. The frequency of vines with perithecia increased during April and May, the late rainy season. In June, the early dry season, the number of vines with SR type greatly increased. The vertical range of perithecial formation on the vines extended to 200 cm in height, but was restricted to 30 cm in the dry season in both fields. The join-count statistics showed a significant spatial association between vines with perithecia and vines with SR type in one field (P = 0.042), while no significant spatial association was recognized in another field. The results suggested that ascospores from perithecia of FSP on pepper vines are likely to be one of the main inoculum sources of the SR type of the disease on adjacent vines, but they may not be the exclusive source.     

Effect of water potential on mycelial growth and perithecial production of Monosporascus cannonballus in vitro

The effects of osmotic water potential (Ψ_s) on mycelial growth and perithecial production of Monosporascus cannonballus , the cause of root rot and vine decline of melons, were examined at 25°C on potato dextrose agar (PDA) amended with KCl, NaCl or sucrose. Patterns of the growth responses of four isolates to decreasing Ψ_s were similar for each of the osmotica. Compared with growth on nonamended PDA (−0·3 MPa), growth of all isolates increased as Ψ_s was reduced to −0·8 MPa. Maximum growth occurred at Ψ_s values of −0·6 to −0·8 MPa. Growth was not reduced below that on nonamended PDA until Ψ_s was reduced to −1·8 MPa, and a 50% reduction in growth did not occur until Ψ_s was reduced to < −2·5 MPa. Reproduction was much more sensitive to reduced Ψ_s than was mycelial growth, and perithecia were produced only at Ψ_s ≥ −0·7 or −0·8 MPa on PDA amended with KCl or NaCl, respectively. Three isolates produced perithecia on PDA amended with sucrose only at Ψ_s ≥ −0·6 MPa, but the fourth isolate produced perithecia at ≥ −1·9 MPa. Colonization of the xylem early in disease development may provide an essential source of water for subsequent reproduction in the root cortex during plant senescence. Postharvest cultivation to expose and desiccate roots may prevent reproduction even when temperatures lethal to hyphae are not attained.     

Endophytic-Host Selectivity of Discula umbrinella on Quercus alba and Quercus rubra Characterized by Infection, Pathogenicity and Mycelial Compatibility

The fungal endophytic–host relationships of Discula umbrinella and two oak species, Quercus alba and Quercus rubra, were characterized on the basis of endophytic infection, pathogenicity, and mycelial compatibility. Isolates of D. umbrinella were cultured from leaves of Q. alba and Q. rubra collected from a hardwood forest located in Patuxent Wildlife Research Center in Laurel, Maryland, USA. Endophytic infection was observed on sterile leaf discs and living 2-month-old seedlings of Q. alba and Q. rubra. Fungal-host reciprocal inoculations revealed the presence of conidiomata on both hosts but conidiomata production was more abundant on Q. alba. Isolates from Q. rubra produced milder disease symptoms on both oak species. Mycelial compatibility studies identified seven different MCG groups. MCG groups 1–3 contained isolates from both oak species whereas MCG groups 4–7 contained host specific isolates. Field studies monitored the seasonal appearance of the sexual fruiting structures, perithecia, as a possible source of new genetic variation that might alter host specificity/pathogenicity of the D. umbrinella isolates on Q. alba and Q. rubra hosts. Only 1–2% of the leaves contained perithecia throughout the sampling period (April–September). Isolates collected from Q. alba differed from those collected from Q. rubra in endophytic infection, pathogenic response, and perithecia production. The results of this study suggest that the endophyte–host relationship is one of host selective preference for Q. alba, but that the endophyte has the ability to maintain the endophytic/pathogenic life cycle on the less preferred host species, Q. rubra.     

Characterization of fungal pathogens (Diaporthe angelicae and D. eres) responsible for umbel browning and stem necrosis on carrot in France

A collection of 102 Diaporthe isolates was compiled from lesions on carrot, parsley and wild Apiaceae species in France from 2010 to 2014. Molecular typing based on ITS rDNA sequences resulted in the identification of 85 D. angelicae and 17 D. eres isolates. Based on sequences of the 3′ part of the IGS rDNA, intraspecific variability was analysed for 17 D. angelicae and 13 D. eres isolates from diverse plant species, locations in France, and plant tissues. The genetic diversity was greater for D. angelicae isolates than D. eres isolates. In vitro sensitivity of five D. angelicae and four D. eres isolates to each of nine fungicides was similar for isolates of both species, with a marked variation in fungicide sensitivity depending on the active ingredient. To assess the pathogenicity of D. angelicae and D. eres isolates on carrot, one isolate of each species was inoculated onto umbels in a controlled environment. Typical lesions were observed for both isolates. Carrot crop debris collected from a seed production field in France and placed in controlled conditions produced perithecia and ascospores typical of Diaporthe, that were further characterized molecularly as belonging to D. angelicae. Detection of Diaporthe species on seed lots from three carrot production fields in France was investigated. Both species were detected on seeds by conventional PCR assay, with a greater frequency for D. angelicae than D. eres (67% vs 33%, respectively). Overall, the results highlighted that umbel browning in carrot seed crops in France was mainly caused by D. angelicae.     

Assessing new SSR markers for utility and informativeness in genetic studies of brown rust fungi on wheat,triticale, and rye

The aim of the present study was to validate new simple-sequence repeat (SSR) markers and use them to assess genetic variability among 24 isolates of Puccinia triticina collected from wheat (Pt-wheat) and triticale (Pt-triticale), and 15 isolates of P. recondita f. sp. secalis (Prs) collected from rye. The Pt and Prs isolates were tested for virulence on a set of 35 Thatcher wheat near-isogenic lines, eight rye lines with known resistance genes, and 53 triticale cultivars with uncharacterized leaf rust resistance. Molecular genotypes were determined using a newly developed set of 34 SSR microsatellite primer pairs. All SSR markers tested on Pt isolates successfully amplified fragments of appropriate size. When tested on the Prs isolates, 21 out of the 34 Pt SSRs amplified expected fragments. Sixteen of these 21 SSRs were polymorphic, providing for the first time microsatellite markers to study genetic variation in Prs. Based on virulence data, variation among Prs isolates was low, probably due to the small number of rye differential lines available. Much higher variation for virulence was observed within the collection of Pt isolates from wheat and triticale, and two separate groups were established with mixed host origin. Substantial genetic variation was detected among the isolates studied with the SSR markers, assuming two different models of SSR evolution (infinite alleles model and stepwise mutation model). The newly developed set of SSR markers proved their effectiveness in detecting genetic variation and should be useful in further population genetics investigations of the two pathogens.     

Taxonomic characterization of Pyricularia isolates from green foxtail and giant foxtail, wild foxtails in Japan

Twenty-eight Pyricularia isolates from two wild foxtails—green foxtail (Setaria viridis) and giant foxtail (S. faberii)—in Japan were taxonomically characterized by DNA analyses, mating tests, and pathogenicity assays. Although most of the isolates failed to produce perithecia in mating tests with Magnaporthe oryzae, a diagnostic polymerase chain reaction-restriction fragment length polymorphism phenotype of M. oryzae was detected in the beta-tubulin genomic region in all isolates. The pathogenicity assays revealed that host ranges of the isolates were similar to those of isolates from foxtail millet (S. italica), which were exclusively pathogenic on foxtail millet. In addition to the 28 isolates from wild foxtails, 22 Pyricularia isolates from 11 other grasses were analyzed by RFLP using single-copy sequences as probes. In a dendrogram constructed from the RFLP data, isolates that were previously identified as M. oryzae formed a single cluster. All the wild foxtail isolates formed a subcluster with foxtail millet isolates within the M. oryzae cluster. From these results, we conclude that Pyricularia isolates from the wild foxtails are closely related to isolates from foxtail millet and should be classified into the Setaria pathotype of M. oryzae.     

Sexual recombination of carbendazim resistance in Fusarium graminearum under field conditions

BACKGROUND: Carbendazim has been the major fungicide for control of Fusarium head blight (FHB) caused by Fusarium graminearum Schwade in China. However, the effectiveness of carbendazim has been threatened by the emergence of resistant pathogen populations in the field. RESULTS: Five isolates, representing three phenotypes of different carbendazim sensitivity levels (S, MR and HR), were randomly selected to study the inheritance of carbendazim resistance by three genetic crosses under field conditions. Each parent in all crosses was marked with a different class of nitrate non‐utilizing (nit) mutation. The presence of sexual recombinants indicated that outcrossing had occurred in the crosses. Over 100 putative self‐crossing or outcrossing perithecia for each cross were randomly sampled on the surface of the haulms of dead rice for each pair of parents. Results showed that 5.7–20.9% outcrossing frequency occurred in the three crosses and confirmed sexual recombination under field conditions. There were no significant differences in mycelial linear growth and pathogenicity between the selected recombinants and their parents, but they differed in sporulation ability and capacity to produce perithecia. Nevertheless, most of the sexual recombinants possessed fitness levels comparable with those of their parents. CONCLUSION: Outcrossing between carbendazim‐sensitive and ‐resistant isolates did occur under field conditions, and sexual recombination must play a role in the development of carbendazim resistance in the field. Copyright © 2009 Society of Chemical Industry     

Clonal population structure and introductions of the chestnut blight fungus, <Emphasis Type="Italic">Cryphonectria parasitica</Emphasis>, in Asturias,northern Spain

To understand the history of introductions of the chestnut blight fungus, Cryphonectria parasitica, in the Principality of Asturias in northern Spain, we conducted an extensive survey of chestnut blight and collected C. parasitica from 216 sites. All 778 isolates were assayed for vegetative compatibility (vc) type, whereas a subsample of 301 isolates was assayed for mating type, and 189 isolates were genotyped at 16 microsatellite markers. We found low diversity for all markers. Nearly all isolates (95%) were compatible with vc type EU-1 and had the same microsatellite multilocus haplotype, or differed from the most common type by mutation at one locus. Approximately 5% of the isolates were vegetatively compatible with EU-13 and only two isolates (< 1%) were compatible with EU-3; five different microsatellite haplotypes were found among isolates in these latter two vc types. The overall mating-type ratio was 218 MAT-1: 81 MAT-2, with both mating types represented in each of the three vc types. Microsatellite haplotypes based on ten markers used in France showed that most isolates in Asturias were either identical to or only one marker different from one of the seven most common genotypes in France, RE103. Based on these ten markers alone, the population of C. parasitica in Asturias, would appear to have been founded by a single genotype from the C1 lineage (to which RE103 belongs) found in eastern France and northern Italy. However, additional genotyping by vc types suggests the introduction of multiple genotypes, with different vc types. The exact source for introduction into Asturias cannot be determined without additional genotyping of isolates from other locations. Regardless of their origin, the low diversity of vc types makes this population ideal for deploying hypovirulence because there will be few barriers for virus transmission between individuals.     

稻瘟病菌对稻瘟灵抗性遗传研究

在离体条件下就稻瘟病菌对稻瘟灵抗性的诱导、抗性水平和遗传进行了研究。结果表明,供试３个小种（ＺＡ_４９、ＺＦ_１和ＺＤ_１）４个菌株分别经稻瘟灵５０μｇ／ｍｌ、１００μｇ／ｍｌ和稻瘟灵１００μｇ／ｍｌ＋亚硝基胍０.５μｇ／ｍｌ的３种处理诱变,均得到了抗稻瘟灵突变株,在含稻瘟灵的培养基中加入诱变剂亚硝基胍可显著提高稻瘟病菌对稻瘟灵抗性的突变率。上述３个不同处理获得的突变株的抗性水平相似,为野生型亲本的２.８～８.８倍。突变株对稻瘟灵的抗性在单分生孢子无性系后代可以稳定遗传。     

Assessment of the antifungal activity of selected biocontrol agents and their secondary metabolites against <Emphasis Type="Italic">Fusarium graminearum</Emphasis>

Fusarium graminearum (teleomorph: Gibberella zeae) is the causal agent of several destructive diseases in cereal crops worldwide. In the present study we have evaluated the potential of two strains of Trichoderma sp. (T23, and T16), a strain of Paecilomyces sp. (PS1), and their secondary metabolites (SMs) in suppressing F. graminearum. Results from dual culture experiments show that in the presence of either Trichoderma sp., or Paecilomyces sp. mycelial growth of F. graminearum is considerably inhibited. Strain T23 causes the greatest inhibition (83.8%), followed by strain T16 (72.2%), and strain PS1 (61.9%). Likewise, mycelial growth of the pathogen is completely inhibited (≥ 98%) when grown under exposure to volatile metabolites excreted from Trichoderma cultures. Bioautographic analyses using culture filtrates revealed that several antifungal SMs are excreted. Among five metabolites tested, 6-pentyl-alpha-pyrone (6PAP) from strain T23, and PF3 from strain PS1 exhibit pronounced antifungal activity against F. graminearum. A new method for mass production of perithecia of F. graminearum which is simple and more effective than traditional methods was developed, which allows an increase in perithecial formation of more than 5-fold. Using this method, we found, that in the presence of SMs perithecial formation was negatively affected. Perithecial production was suppressed by 81.4% and 76.6% using 200 μg ml^?1 of either 6PAP or PF3, respectively. Moreover, ascospore discharge was significantly suppressed (67.0%) when perithecia were exposed to the metabolite F116 produced by T16. Including 6PAP or PF3 in conidial suspensions impeded germination of conidia completely. Similarly, both metabolites strongly inhibited ascospore germination (? 90%).     

Application of cycleave PCR to the detection of a point mutation (F167Y) in the β2‐tubulin gene of Fusarium graminearum

BACKGROUND: Resistance of Fusarium graminearum to the benzimidazole fungicide carbendazim is caused by point mutations in the β₂‐tubulin gene (FGSG_06611.3). The point mutation at codon 167 (TTT → TAT, F167Y) occurs in more than 90% of field isolates in China. It is important to find a suitable method for rapid detection and quantification of this point mutation in the F. graminearum populations. RESULTS: A pair of primers, Codon167F/Codon167R, were designed to amplify a fragment containing the mutation site, and two cycling probes labelled with different fluorescent reporters were used to detect whether the mutation was present. A cycleave real‐time PCR method was developed for rapid determination of the frequency of this point mutation in 282 F. graminearum perithecia collected from different fields in Jiangsu Province, China. The mutation frequency in ascospores from the perithecia to carbendazim by a spore germination assay was 6.0%, while the frequency of the point mutation at codon 167 by the cycleave real‐time PCR assay was 3.9%. CONCLUSION: The cycleave real‐time PCR method is suitable for accurate detection of the codon 167 point mutation. The frequency of this mutation in the β₂‐tubulin gene represents the resistance frequency in F. graminearum populations to carbendazim. Copyright © 2011 Society of Chemical Industry     

Simple sequence repeat markers support the presence of a single genotype of Puccinia psidii in Australia

A non‐native rust of Myrtaceae was first detected in Australia in 2010, and was later identified as Puccinia psidii. The presence of many native species of Myrtaceae and a lack of understanding of genetic variability in P. psidii in Australia led to the current study. Low coverage genome sequencing of P. psidii suggested a genome size of c. 142 Mb. A set of 240 simple sequence repeat (SSR) primers was designed based on the genome sequence information generated. Seventeen isolates of P. psidii comprising 14 from Australia, two from Brazil and one from Hawaii were selected to study genetic variation in the pathogen. Out of 240 initially screened markers, 74% showed amplification among P. psidii isolates and 38% were polymorphic. Primers were fluorescently labelled and genotyping revealed three distinct genotypes among the isolates: one comprising Australian isolates and an isolate from Hawaii, and the second and third comprising two Brazilian isolates. Locus USYD_Pp151 produced a fourth genotype for the Hawaiian isolate of P. psidii. Markers revealed that all Australian isolates were genetically similar to the one from Hawaii. There was no genetic variation among the Australian isolates of P. psidii, supporting the hypothesis that only one genotype of P. psidii was introduced into Australia. The SSR markers developed in this study are highly specific to P. psidii and can be used confidently as a new profiling tool to monitor evolution of P. psidii in Australia and elsewhere.     

Development,characterization and application of genomic SSR markers for the oat stem rust pathogen Puccinia graminis f. sp. avenae

Oat stem rust, caused by Puccinia graminis f. sp. avenae (Pga), is one of the most severe diseases of oats worldwide. Population studies are scarce for this pathogen, mainly due to the lack of polymorphic molecular markers suitable for genetic analysis. In this study, an Australian Pga isolate was sequenced, the abundance of simple sequence repeats (SSRs) was determined and PCR‐based polymorphic markers suitable for genetic diversity analysis were developed. The amplification of 194 primer pairs was initially assessed using a set of 12 isolates of different cereal rust species and their formae speciales. A high frequency of cross‐species amplification was observed for most markers; however, 36 SSRs were diagnostic for P. graminis only. A subset of 19 genome‐derived SSRs were deemed useful for genetic diversity analysis of Pga and were assessed on 66 Pga isolates from Australia, Brazil and Sweden. Brazilian and Australian isolates were characterized by one and two predominant clonal lineages, respectively. In contrast, the Swedish isolates, previously shown to undergo sexual recombination, were highly diverse (nine distinct genotypes out of 10 isolates) and divided into two subpopulations. The genome‐derived SSR markers developed in this study were well suited to the population studies undertaken, and have diagnostic capabilities that should aid in the identification of unknown rust pathogen species.     

Toxin Profile, Fertility and AFLP Analysis of Fusarium verticillioides from Banana Fruits

Gibberella fujikuroi is composed of at least nine mating populations (MPs), corresponding to biological species and assigned letters (from A to I). Each MP possesses a specific toxicological profile and a preferential host. Members of Fusarium verticillioides and F. thapsinum, anamorphs respectively of MPs A (G. moniliformis) and F (G. thapsina), share identical morphological traits, but they have a different preferential hosts (maize and sorghum, respectively) and toxin profiles, beingable the only member of MP A to produce fumonisins and the only member of MP F to produce moniliformin. Isolates from banana fruits were identified morphologically as F. verticillioides. The isolates were analyzed for fumonisin and moniliformin production. While none of the isolates produced fumonisin, all the isolates produced moniliformin. The isolates were crossed with tester strains of MPs A and F, showing ability to produce fertile perithecia only when crossed by MP A tester strains isolated from maize. However, the time required for the formation of fertile perithecia and their size differed significantly from the usual fertile crosses of strains belonging to MP A. Pathogenicity tests using such isolates of F. verticillioides isolated from banana and a set of F. verticillioides isolates isolated from maize were also performed on banana fruits. The data showed that the isolates from banana were more pathogenic. Finally, isolates from banana and maize were compared using AFLP. The results revealed that isolates from banana and maize produced two distinctly different clusters. In conclusion, isolates of F. verticillioides from banana showed specific traits (toxin production, in vitro fertility, pathogenicity and molecular profiles), that were different to those of the same species from maize. This could reflect important differences in the ecology and natural history of the population from banana and should encourage further investigations into the mechanisms of toxin production and pathogenicity within the same MP.     

中国小麦条锈病菌CYR32和CYR33的毒性及基因型多样性

为明确我国小麦条锈病菌当前主要流行生理小种CYR32和CYR33的毒性及基因型特征,从全国11个省(区)随机选取29个CYR32菌株和39个CYR33菌株,利用近等基因系及辅助鉴别寄主对其进行毒性鉴定,利用SSR分子标记技术对其进行基因型分析,并对其进行聚类分析。结果显示,CYR32和CYR33菌系各有17种毒性表型,而且在抗病基因Yr2、Yr17、Yr27、Yr32、Yr43、Yr Sp、Yr Exp2、Yr28、Yr V23上都发生了毒性分化,CYR32和CYR33菌系的多样性指数分别为0.089、0.097;CYR32和CYR33菌系的香农信息指数均值分别为0.44和0.45;当相似性系数为0.93时,CYR32和CYR33菌系分别被聚为5个和8个毒性类群;当相似性系数为0.84时,CYR32和CYR33菌系分别被聚为10个和16个基因型类群。表明在中国鉴别寄主上具有相同毒性谱特征的CYR32和CYR33菌系在近等基因和SSR分子标记中发生了不同程度的毒性和基因型分化。     

Genetic Structure of the Population of Phytophthora infestans Attacking Solanum ochranthum in the Highlands of Ecuador

Thirty-nine isolates of Phytophthora infestans were collected from the wild host Solanum ochranthum in the highland tropics of Ecuador and characterized with a set of phenotypic and molecular markers (mating type, metalaxyl sensitivity, the allozyme loci Gpi, and Pep, mitochondrial DNA haplotype, RFLP, and SSR), as well as for pathogenicity on various hosts. Three groups of isolates (A, B, and C) were identified based on their multilocus genotypes and variable abilities to cause disease on different hosts. Group A had a combination of alleles for the Gpi (86/100), Pep (96/100) and mtDNA (Ia) loci, as well as an RFLP fingerprint, that have not been reported for P. infestans in Ecuador, or elsewhere. Group B shares many marker characteristics with the US-1 lineage described in Ecuador on tomato, pear melon (S. muricatum), and S. caripense, but has SSR alleles not present in typical US-1 isolates. Group C for all markers tested is identical to the EC-1 lineage described on cultivated and wild potatoes in Ecuador. All isolates from S. ochranthum were able to re-infect their host of origin in the detached leaf assay; however, we did not draw clear conclusions as to the relative aggressiveness of the three groups on this host. Isolates of group A were the most specialized and were generally non-pathogenic or weakly pathogenic on all hosts other than S. ochranthum. Groups B and C infected tuber-bearing hosts, including the cultivated potato but were generally non-pathogenic on other non-tuber bearing hosts. Solanum ochranthum was infected by isolates coming from tuber-bearing Solanum hosts (i.e., the EC-1 lineage of P. infestans) and some US-1 isolates from non-tuber bearing hosts. Thus, in nature this species might be a potential reservoir of inoculum of different pathogen populations able to infect the cultivated hosts potato, tomato and pear melon (S.␣muricatum). Unlike potato and tomato in Ecuador, each of which is primarily attacked by a highly specialized pathogen population, S. ochranthum appears to harbour at least three pathogen groups of␣different genetic make-up. The unresolved issue of potential host specificity in isolates found on S.␣ochranthum could complicate efforts to use this species in tomato improvement.     

Fusarium solani f. sp. passiflorae: a new forma specialis causing collar rot in yellow passion fruit

The aim of this study was to characterize a Fusarium population obtained from yellow passion fruit (YPF) with collar rot using pathogenicity, morphocultural characteristics and molecular tests. Pathogenicity and disease severity were assessed in six plant species: YPF, zucchini, tomato, bean, soya bean and cucumber. Potato dextrose agar medium (PDA) was used to determine mycelial growth at five temperatures (15–35°C). The colour produced by isolates was also determined on PDA at 25°C. Synthetic nutrient agar medium was used to evaluate: (i) type of mycelium and phialides; (ii) size, shape and number of septa from conidia; and (iii) production of chlamydospores and perithecia. Molecular tests consisted of sequencing the ITS–5·8S rDNA region and elongation factor 1α (EF‐1α) gene. The isolates caused large lesions on YPF, zucchini and tomato, with YPF having the highest mean disease severity and being the only one that showed wilt symptoms and death of the plant. Thus the isolates showed host specificity. Maximum mycelial growth occurred at 25°C and the predominant colour was bluish‐white. The isolates produced long phialides, dense aerial mycelium, oval microconidia with a mean size of 9·5 × 2·6 μm, macroconidia of 32·7 × 3·4 μm with 3·3 septa, and chlamydospores; only one isolate lacked perithecia. Phylogenetic trees of the ITS region and EF‐1α gene showed that isolates from YPF formed a distinct group within the F. solani group and the formae speciales of F. solani. It is proposed to name all isolates from YPF as F. solani f. sp. passiflorae.     

Development of sequence tagged site markers to identify races of Fusarium oxysporum f. sp. lactucae

By random amplified polymorphic DNA (RAPD) analysis of the representative isolates of each race of Fusarium oxysporum f. sp. lactucae, RAPD fragments of 0.6, 1.6, and 2.9kb were obtained. The 0.6-kb RAPD fragment was common to the representative isolates of all three races. Amplification of the 1.6- and 2.9-kb fragments were unique to the isolates of races 1 and 2, respectively. Sequence tagged site (STS) marker FLA0001, FLA0101, and FLA0201 were generated from the 0.6-, 1.6-, and 2.9-kb RAPD fragments, respectively. Polymerase chain reaction (PCR) analysis showed that FLA0001 was common to all 49 isolates of F. oxysporum f. sp. lactucae. FLA0101 was specifically generated from all 23 isolates of race 1 but not from races 2 or 3. FLA0201 was specifically amplified from all 12 isolates of race 2 but not from races 1 or 3. In two isolates of F. oxysporum f. sp. lactucum, PCR amplified FLA0001 and FLA0101 but not FLA0201. On the other hand, these STS markers were not detected from isolates of five other formae speciales. Because these STS markers were not generated from isolates of other plant pathogenic fungi, bacteria, or plant materials examined in this study, PCR analysis combined with the three STS markers should be a useful means for rapid identification of races of F. oxysporum f. sp. lactucae.     

Population dynamics and pathogenic races of rice blast fungus, <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis> in the Mekong Delta in Vietnam

In a study of the population dynamics of Magnaporthe oryzae in the Mekong Delta in Vietnam, 226 isolates were collected from various sites in December 2001, September and December 2004. The pathogenic races of isolates were determined using international differential rice varieties. Isolates with the same race number were not found, not even in the same field or on the same seedling, suggesting that the fungus in the Mekong Delta was dynamically changing. But focusing on the known major resistance genes in the Japanese differential varieties, some isolates in the area were found to be the same race. Phylogenetic analyses based on the transposable elements Pot2 and MGR586 in the genomes supported that the pathogenic races were critically variable in comparison with the genomic diversity. Isolates with MAT1-1 predominated in the Mekong Delta, but in some provinces those with MAT1-2 coexisted at low frequency with MAT1-1. However, no isolates produced perithecia and ascospores. Isolates in the Mekong Delta probably had hot spots in their genomes that are easily altered and associated with some avirulence genes.