Low genetic variability in Sclerotinia sclerotiorum populations from common bean fields in Minas Gerais State,Brazil, at regional,local and micro‐scales

Sclerotinia sclerotiorum, causal agent of white mould, is the most destructive and widely distributed soilborne pathogen of common bean during the autumn–winter season in Brazil. Nevertheless, little is known about the genetic structure of the pathogen population. Microsatellite (SSR) markers and mycelial compatibility groups (MCGs) were used to characterize 118 isolates collected from 20 bean fields located in the most important growing regions of Minas Gerais State (MG). Additionally, the genetic variability among 10 isolates obtained from a single sclerotium was investigated in 10 different sclerotia. Seventy SSR haplotypes and 14 MCGs were identified among the 118 isolates. The genetic differences within bean growing areas accounted for most of the genetic variation (72%). Despite the relatively high genotypic diversity, the SSR loci were at linkage disequilibrium. Moreover, 70% of the isolates were assigned to only two MCGs, and haplotypes of a given MCG were closely related. The discriminant analysis of principal components revealed five groups. There was strong genetic differentiation between isolates collected in one municipality in southern MG when compared to other regions. Common bean resistance to white mould should be assessed with representative isolates of the five genetic groups and, if possible, of the different MCGs detected in the present study. One to five haplotypes were detected among the 10 isolates obtained from a single sclerotium. Therefore, in order to ensure genetic identity of an isolate, hyphal tip or monoascosporic isolates should be used.     

Genetic diversity of Phaeoisariopsis griseola in Argentina as revealed by pathogenic and molecular markers

Angular leaf spot, a disease of common bean produced by Phaeoisariopsis griseola, an imperfect (Deuteromycotina) fungus, causes significant yield losses in Argentina. The development of a strategy to control and/or reduce the impact of P. griseola requires a previous knowledge of the population structure. Therefore, the purpose of this work was to analyze diversity among 45 isolates of P. griseola collected within the production area of common bean in Northwestern Argentina. Pathotypes diversity was determined based on a set of bean differentials and genomic differences between isolates were determined by means of molecular markers. We confirmed that isolates belonging to Middle American and Andean groups coexist in Northwestern Argentina and the level of diversity between them was considerable and of similar level within each group. Even though the number of isolates analyzed was 45, among them 37 were Middle American and only eight were Andean. The number of haplotypes found based on ISSR and RAPD markers were 18 and as expected, they were unrelated with pathotypes. The wild bean species, Phaseolus vulgaris var. aborigineus, showed a high level of tolerance to most pathotypes of P. griseola except 63.63 and 63.23. This together with the fact that none of the bean differentials was resistant to all pathotypes led us to conclude that the range of pathogenic responses might be conditioned by multigenic interactions between the pathogen and the host. Our results not only provided basic information about the diversity of the causative agent of the disease but it will also help to develop cultivars with enhanced tolerance and/or resistance to the disease.     

Meta‐analytic modelling of the incidence–yield and incidence–sclerotial production relationships in soybean white mould epidemics

White mould (Sclerotinia sclerotiorum) is a destructive disease of soybean worldwide. However, little is known of its impact on soybean production in Brazil. A meta‐analytic approach was used to assess the relationship between disease incidence and soybean yield (35 trials) and between incidence and sclerotia production (29 trials) in experiments conducted in 14 locations across four seasons. Region, site elevation and season included as moderators in random‐effects and random‐coefficients models did not significantly explain the variability in the slopes of the incidence–yield relationship. The Pearson's r, obtained from back‐transforming the Fisher's Z estimated by an overall random‐effects model, showed that incidence of white mould was moderately and negatively correlated with yield (r = ?0.76, P < 0.0001). A random‐coefficients model estimated a slope of ?17.2 kg ha^?1%^?1, for a mean attainable yield of 3455 kg ha^?1, indicating that a 10% increase in white mould incidence would result in a mean yield reduction of 172 kg ha^?1. White mould incidence and production of sclerotia were strongly and positively correlated (r = 0.85, P < 0.0001). For every 10% increase in white mould incidence, 1 kg ha^?1 of sclerotia was produced. The relationship between disease incidence and production of sclerotia was stronger in southern regions and at higher elevation. In the absence of management, economic losses associated with white mould epidemics, assuming 43% incidence in 22% of the soybean area, were estimated at approximately US $1.47 billion annually within Brazil.     

Population structure of Sclerotinia sclerotiorum in crop and wild hosts in the UK

Sclerotinia sclerotiorum is an important pathogen of many crop plants which also infects wild hosts. The population structure of this fungus was studied for different crop plants and Ranunculus acris (meadow buttercup) in the UK using eight microsatellite markers and sequenced sections of the intergenic spacer (IGS) region of the rRNA gene and the elongation factor 1‐alpha (EF) gene. A total of 228 microsatellite haplotypes were identified within 384 isolates from 12 S. sclerotiorum populations sampled in England and Wales. One microsatellite haplotype was generally found at high frequency in each population and was distributed widely across different hosts, locations and years. Fourteen IGS and five EF haplotypes were found in the 12 populations, with six IGS haplotypes and one EF haplotype exclusive to buttercup. Analysis of published sequences for S. sclerotiorum populations from the USA, Canada, New Zealand and Norway showed that three of the IGS haplotypes and one EF haplotype were widely distributed, while eight IGS haplotypes were only found in the UK. Although common microsatellite and IGS/EF haplotypes were found on different hosts in the UK, there was evidence of differentiation, particularly for one isolated population on buttercup. However, overall there was no consistent differentiation of S. sclerotiorum populations from buttercup and crop hosts. Sclerotinia sclerotiorum therefore has a multiclonal population structure in the UK and the wide distribution of one microsatellite haplotype suggests spatial mixing at a national scale. The related species S. subarctica was also identified in one buttercup population.     

Assessing the phenotypic and genotypic diversity of <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> in France

White mould caused by the ascomycete Sclerotinia sclerotiorum affects the production of many economically important crops. The incidence of this disease has recently increased in France, especially in melon crops, which were not affected much in the past. One possible explanation for this situation is the emergence of strains with particular characteristics, including increased aggressiveness to melon. To test this hypothesis, 200 isolates of S. sclerotiorum were collected from six host crops (bean, brassica oilseed rape, carrot, lettuce, melon, witloof chicory) in different regions. They were genotyped with 16 microsatellites markers. A subsample of 96 isolates were assessed for their aggressiveness on melon leaves. Overall, the isolates from melon did not show higher aggressiveness on melon leaves than those which originated from other host plants. Moreover, the melon isolates did not present distinctive genetic characteristics in comparison with those from other crops and shared several of the 128 identified multilocus haplotypes with isolates collected from carrot, witloof chicory and oilseed rape. Furthermore the Bayesian analysis of the genetic structure indicated that the host plant is not a structuring factor of the three genetic clusters identified, and it suggested instead the occurrence of an isolation-by-distance process. Possible consequences of these results for the management of white mould and alternative hypotheses to explain the recent changes in disease incidence are presented.     

Population genetic structure of <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> on canola in Iran

The genetic structure of 276 Sclerotinia sclerotiorum isolates representing 37 field populations from four provinces in northern Iran were analysed with six polymorphic microsatellite loci. In total, 80 haplotypes were detected with 19 haplotypes (23.7%) shared amongst at least two regional populations. None of the haplotypes were shared among all four regional populations. Of the 80 haplotypes, 32 haplotypes (40%) occurred in low frequencies represented by only one isolate. Moderate levels of gene diversity (H = 0.51 to 0.61) and genotypic diversity (Ĝ = 12.0 to 22.0; clonal fraction = 0.39 to 0.67) for regional populations were observed. Genotypic diversities (Ĝ) did not differ significantly among populations. All regional populations were in linkage equilibrium indicating the occurrence of outcrossing. Low to moderate levels of population subdivision (0.03 to 0.07), were observed among regional populations. Only one large panmictic population was inferred by Structure, indicating no significant population structure. A Mantel test showed no significant isolation by distance (r = −0.43; P = 0.18), indicating anthropogenic movement of inoculum. The results demonstrated that S. sclerotiorum populations in northern Iran, are randomly mating and have a number of shared haplotypes among regional populations; this possibly represents recent founder populations and/or a high occurrence of anthropogenic migration of infected plant material among populations.     

ISSR markers detect high genetic variation among <Emphasis Type="Italic">Fusarium poae</Emphasis> isolates from Argentina and England

Fusarium poae is one of the Fusarium species isolated from cereal grains infected by Fusarium head blight (FHB), and in recent years it has been identified as a major FHB component. In this study, 97 F. poae isolates from Argentina (n = 62) and England (n = 35) were analysed by inter-simple sequence repeats (ISSR) to examine the genetic diversity and to determine whether intraspecific variation could be correlated with geographic and/or host origin. The molecular analysis showed high intraspecific variability within F. poae isolates, but did not reveal a clear relationship between variability and the host/geographic origin. Fusarium poae isolates from the same geographic region or host appeared in different subclusters. Conversely, isolates with the same haplotype were also collected from different geographic regions. However, we did observe subclusters consisting of isolates from Argentina only or from England only. Furthermore, a single seed sample was found to host different haplotypes. Analysis of molecular variance (AMOVA) indicated a high genetic variability in F. poae, with most of the genetic variability explained by differences within, rather than between Argentinean and English populations. This is the first report on genetic diversity of F. poae using ISSR markers. Moreover, ISSR fingerprinting generates highly polymorphic markers for F. poae and proved to be a useful and reliable assay for genetic variability studies.     

Inoculum potential of Sclerotinia sclerotiorum sclerotia depends on isolate and host plant

The soilborne fungus Sclerotinia sclerotiorum infects many important crop plants. Central to the success of this pathogen is the production of sclerotia, which enables survival in soil and constitutes the primary inoculum. This study aimed to determine how crop plant type and S. sclerotiorum isolate impact sclerotial production and germination and hence inoculum potential. Three S. sclerotiorum isolates (L6, L17, L44) were used to inoculate plants of bean, carrot, lettuce, oilseed rape (OSR) and potato, and the number and weight of sclerotia per plant quantified. Carpogenic germination of sclerotia collected from different hosts was also assessed for L6. Production of sclerotia was dependent on both crop plant type and S. sclerotiorum isolate, with OSR and lettuce supporting the greatest number (42–122) and weight (1.6–3.0 g) of sclerotia per plant. The largest sclerotia were produced on OSR (33–66 mg). The three S. sclerotiorum isolates exhibited a consistent pattern of sclerotial production irrespective of crop type; L6 produced large numbers of small sclerotia while L44 produced smaller numbers of large sclerotia, with L17 intermediate between the two. Germination rate and percentage was greatest for larger sclerotia (4.0–6.7 mm) and also varied between host plants. Combining sclerotial production data and typical field crop densities suggested that infected carrot and OSR could produce the greatest number (3944 m^?2) and weight (73 g m^?2) of S. sclerotiorum sclerotia, respectively, suggesting these crops potentially contribute a greater increase in inoculum. This information, once further validated in field trials, could be used to inform future crop rotation decisions.     

Population structure,genetic diversity,and sexual state of the rice brown spot pathogen Bipolaris oryzae from three Asian countries

Bipolaris oryzae causes brown spot in rice (Oryza sativa) inflicting substantial grain yield losses worldwide. Knowledge of the population structure, genetic diversity and sexual recombination of the fungal pathogen can help to implement effective disease management strategies. In this study, B. oryzae isolates sampled from Iran, the Philippines and Japan were analysed with 12 simple‐sequence repeat (SSR) markers, newly developed from the genome sequence of the fungus. Among the 288 B. oryzae isolates genotyped, 278 unique haplotypes were identified. High genotype numbers (richness) with even distribution (evenness) were found within the collection sites. Both mating types, MAT1‐1 and MAT1‐2, were present in each collection area, and the sexual state was induced under controlled conditions with production of viable ascospores. However, the tests of linkage disequilibrium rejected of the hypothesis of random mating. Discriminant analysis of principal components (DAPC) revealed that the B. oryzae collection formed three clusters, each consisting of isolates from different collection sites. Analysis of molecular variance (amova ) showed that genetic variation among clusters was 18.7%, with the rest of the variation distributed within clusters (R_ST = 0.187, P < 0.001). Statistically significant pairwise genetic differentiation was found between the clusters. These results show that Asian B. oryzae isolates are genetically diverse, and, overall, distributed in three groups. These findings will be helpful in managing the disease and guide the use of representative isolates needed for selection of resistant rice varieties.     

Genetic differentiation in populations of Exserohilum turcicum from maize and sorghum in South Africa

Exserohilum turcicum is the causal agent of northern leaf blight, a devastating foliar disease of maize and sorghum. Specificity of E. turcicum to either maize or sorghum has been observed previously, but molecular evidence supporting host specialization is lacking. The aim of this study was to compare the genetic structure of E. turcicum isolates collected from adjacent maize and sorghum fields in Delmas and Greytown in South Africa. In addition, the mode of reproduction of this pathogen was investigated. Isolates from maize (N = 62) and sorghum (N = 64) were screened with 12 microsatellite markers as well as a multiplex mating type PCR assay. No shared haplotypes were observed between isolates from different hosts, although shared haplotypes were detected between isolates from maize from Delmas and Greytown. Population structure and principal coordinate analyses revealed genetic differentiation between E. turcicum isolates from maize and sorghum. Analysis of molecular variance indicated higher among‐population variation when comparing populations from different hosts, than comparing populations from different locations. Lack of shared haplotypes, high proportion of private alleles, greater among‐population variance between hosts than locations and significant pairwise population differentiation indicates genetic separation between isolates from maize and sorghum. The high haplotypic diversity in combination with unequal mating type ratios and significant linkage equilibrium indicates that both sexual and asexual reproduction contributes to the population genetic structure of E. turcicum in South Africa.     

Resistance to a highly aggressive isolate of Sclerotinia sclerotiorum in a Brassica napus diversity set

Sclerotinia stem rot (SSR) of oilseed rape (OSR, Brassica napus), caused by Sclerotinia sclerotiorum, is a serious problem in the UK and worldwide. As fungicide‐based control approaches are not always reliable, identifying host resistance is a desirable and sustainable approach to disease management. This research initially examined the aggressiveness of 18 Sclerotinia isolates (17 S. sclerotiorum, one S. subarctica) on cultivated representatives of B. rapa, B. oleracea and B. napus using a young plant test. Significant differences were observed between isolates and susceptibility of the brassica crop types, with B. rapa being the most susceptible. Sclerotinia sclerotiorum isolates from crop hosts were more aggressive than those from wild buttercup (Ranunculus acris). Sclerotinia sclerotiorum isolates P7 (pea) and DG4 (buttercup), identified as ‘aggressive’ and ‘weakly aggressive’, respectively, were used to screen 96 B. napus lines for SSR resistance in a young plant test. A subset of 20 lines was further evaluated using the same test and also in a stem inoculation test on flowering plants. A high level of SSR resistance was observed for five lines and, although there was some variability between tests, one winter OSR (line 3, Czech Republic) and one rape kale (line 83, UK) demonstrated consistent resistance. Additionally, one swede (line 69, Norway) showed an outstanding level of resistance in the stem test. Resistant lines also had fewer sclerotia forming in stems. New pre‐breeding material for the production of SSR resistant OSR cultivars relevant to conditions in the UK and Europe has therefore been identified.     

White mould of sweet basil: conditions influencing its development in greenhouses and cultural measures for disease management

During the winter season, Sclerotinia sclerotiorum infects the stem bases of greenhouse‐grown sweet basil plants and, in severe epidemics, it may also infect the shoots. Sclerotia of S. sclerotiorum, and the ascospores that are released from the apothecia that form on them, serve as the inoculum for white mould epidemics. This research aimed to identify cropping parameters associated with lower incidence of white mould in a survey of the main basil‐growing region in Israel, and study cultural methods that might suppress the disease. The survey revealed that this mould, in the main growing area in Israel, has one cycle of infection. Factors associated with increased moisture in the greenhouse were found to be associated with increased levels of the disease. The use of a lower planting density reduced the incidence of white mould in semi‐commercial experimental plots, as well as the severity of the disease on shoots infected by S. sclerotiorum after harvest, in comparison to the commonly used higher planting density, with no negative effect on yield. Mulching the beds with polyethylene effectively reduced disease, and a combination of polyethylene mulch and increased plant spacing reduced disease severity on cut shoots in a synergistic manner. In conclusion, cultural control methods reduced disease incidence under field conditions and severity of the disease on cut shoots.     

Pathogenic and molecular variability among isolates of Pyrenophora tritici-repentis, causal agent of tan spot of wheat in Argentina

Tan spot, caused by Pyrenophora tritici-repentis, is a common disease of wheat (Triticum aestivum) responsible for economic losses in some wheat growing areas worldwide. In this study the pathogenic and genetic diversity of 51 P. tritici-repentis isolates collected from different ecological regions of Argentina were analyzed. Virulence tests were conducted on 10 selected wheat cultivars: Buck Halcón, Chris, Gabo, Glenlea, Klein Dragón, Klein Sendero, Max, ND 495, ProInta Guazú and ProInta Imperial. Data revealed significant differences between all main factors evaluated and the interactions for 19 of the isolates analyzed. Based on the reaction type of each isolate/cultivar combination, 48 different pathogenic patterns were detected. The molecular analysis using Inter-Simple Sequence Repeats (ISSR) revealed the existence of 36 different haplotypes among 37 isolates of P. tritici-repentis originally selected for this study. These results indicate that P. tritici-repentis on wheat in Argentina is a heterogeneous fungus, implying that screening wheat germoplasm for resistance for tan spot disease requires a wide range of pathogen isolates.     

HRM analysis provides insights on the reproduction mode and the population structure of Gnomoniopsis castaneae in Europe

Gnomoniopsis castaneae is an emergent nut rot agent of chestnut in southern Europe. To elucidate its population genetics, three simple sequence repeat (SSR) and two hypervariable markers were developed and assessed through high‐resolution melting (HRM) analysis on 132 isolates collected from 10 sites in Italy, France and Switzerland. High allele diversity (ranging from 0.23 to 0.40 depending on site) and number of haplotypes (49) were observed. More than 70% of the molecular variance could be accounted among isolates within sites. Multilocus analysis showed absence of linkage disequilibrium, suggesting a predominant role played by sexual reproduction and random mating. Data analyses indicated the presence of at least two putative distinct subpopulations and this was confirmed by several approaches, including analysis of shared haplotypes, multivariate and Bayesian analyses. Based on data of allelic diversity, the possibility that the pathogen could have been introduced is discussed. This work assessed the genetic variability and the sexual strategies of G. castaneae in Europe, adding useful information on the epidemiology of this fungal plant pathogen.     

Contrasting species boundaries between sections Alternaria and Porri of the genus Alternaria

The fungal genus Alternaria comprises a large number of asexual taxa with diverse ecological, morphological and biological modes ranging from saprophytes to plant pathogens. Understanding the speciation processes affecting asexual fungi is important for estimating biological diversity, which in turn affects plant disease management and quarantine enforcement. This study included 106 isolates of Alternaria representing five phylogenetically defined clades in two sister sub‐generic groups: section Porri (A. dauci, A. solani and A. limicola) and section Alternaria (A. alternata/tenuissima and A. arborescens). Species in section Porri are host‐specific while species in section Alternaria have wider host ranges. For each isolate, DNA sequences of three genes (Alt a1, ATPase, Calmodulin) were used to estimate phylogenies at the population and species levels. Three multilocus haplotypes were distinguished among A. dauci isolates and only one haplotype among A. solani and A. limicola isolates, revealing low or no differentiation within each taxon and strong clonal structure for taxa in this section. In contrast, 37 multilocus haplotypes were found among A. alternata/tenuissima isolates and 21 multilocus haplotypes among A. arborescens isolates, revealing much higher genotypic diversity and multiple clonal lineages within taxa, which is not typical of asexual reproducing lineages. A species tree was inferred using a Yule Speciation model and a strict molecular clock assumption. Species boundaries were well defined within section Porri. However, species boundaries within section Alternaria were only partially resolved with no well‐defined species boundaries, possibly due to incomplete lineage sorting. Significant association with host specificity seems a driving force for speciation.     

Nursery‐linked plantation outbreaks and evidence for multiple introductions of the pitch canker pathogen Fusarium circinatum into South Africa

In recent years, Pinus plantation forestry has been significantly hampered by outbreaks of pitch canker caused by the fungus Fusarium circinatum. This study investigated the role of Pinus host, geographic origin and reproductive mode in structuring the F. circinatum populations in plantations. For this purpose, 159 isolates originating from diseased plantation trees in the Western and Eastern Cape Provinces of South Africa were genotyped using 10 microsatellite markers. Analyses of these data revealed 30 multilocus haplotypes and that the populations were distinct based on geographic origin as well as host. However, shared haplotypes were observed between populations, showing that these populations are connected, possibly through the movement of haplotypes. A second aim was to determine whether the genetic variation found in these populations of the fungus could be attributed to outbreaks of the seedling disease caused by this pathogen in Pinus nurseries. To achieve this goal, an additional set of 43 isolates originating from pine seedling nurseries was genotyped and analysed. The results showed that the populations of F. circinatum in plantations most probably originated from the nursery outbreaks that occurred prior to the plantation outbreak. Inferences regarding reproductive mode further showed that sexual reproduction has little impact on the genetic makeup of the F. circinatum populations and that they primarily reproduce asexually. Overall, the results of this study showed that the F. circinatum diversity in South Africa has arisen due to multiple introductions of the pathogen and is not due to sexual reproduction.     

Host‐induced gene silencing in the necrotrophic fungal pathogen Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic fungus that causes a devastating disease called white mould, infecting more than 450 plant species worldwide. Control of this disease with fungicides is limited, so host plant resistance is the preferred alternative for disease management. However, due to the nature of the disease, breeding programmes have had limited success. A potential alternative to developing necrotrophic fungal resistance is the use of host‐induced gene silencing (HIGS) methods, which involves host expression of dsRNA‐generating constructs directed against genes in the pathogen. In this study, the target gene chosen was chitin synthase (chs), which commands the synthesis of chitin, the polysaccharide that is a crucial structural component of the cell walls of many fungi. Tobacco plants were transformed with an interfering intron‐containing hairpin RNA construct for silencing the fungal chs gene. Seventy‐two hours after inoculation, five transgenic lines showed a reduction in disease severity ranging from 55·5 to 86·7% compared with the non‐transgenic lines. The lesion area did not show extensive progress over this time (up to 120 h). Disease resistance and silencing of the fungal chs gene was positively correlated with the presence of detectable siRNA in the transgenic lines. It was demonstrated that expression of endogenous genes from the very aggressive necrotrophic fungus S. sclerotiorum could be prevented by host induced silencing. HIGS of the fungal chitin synthase gene can generate white mould‐tolerant plants. From a biotechnological perspective, these results open new prospects for the development of transgenic plants resistant to necrotrophic fungal pathogens.     

Hierarchical models for white mould in snap bean

Limitations on the acceptable proportion (incidence) of pods with white mould may lead to the rejection of entire fields of processing snap bean. The low tolerances (no more than 2% to 6% of pods with white mould) are difficult to estimate with sufficient precision in the field when time is limited. These constraints motivated this study of white mould across three spatial hierarchical levels: pods, plants and quadrats consisting of two adjacent plants within rows. Hierarchical relationships are required when designing formal rules for estimating the incidence of pods with white mould from units higher in the spatial hierarchy. Disease assessments were made on all pods from 38 within‐row transects of 40 plants each of the snap bean cultivar Hystyle. Using probability‐based principles, equations were derived and fitted to data on the incidence of white mould on pods (i_pod), plants (i_pad) and quadrats (i_q(2)), which led to a function directly linking i_pod to i_q(2). The variance of i_pod increased with i_q(2), but that may be of little consequence at the lower values of i_q(2) likely to be associated with i_pod values at processor‐set tolerances. For example, at i_q(2) = 0.1 there was a 92% probability that i_pod was less than 0.02. Assessing i_q(2) may be more efficient than directly estimating i_pod because i_q(2) was about an order of magnitude higher than i_pod. Results suggest it may be feasible to design sampling plans for estimating the proportion of pods with white mould from an assessment of the proportion of diseased quadrats.     

Lack of host specificity of Colletotrichum spp. isolates associated with anthracnose symptoms on mango in Brazil

The aim of the present study was to analyse the genetic and pathogenic variability of Colletotrichum spp. isolates from various organs and cultivars of mango with anthracnose symptoms, collected from different municipalities of São Paulo State, Brazil. Colletotrichum gloeosporioides isolates from symptomless citrus leaves and C. acutatum isolates from citrus flowers with post‐bloom fruit drop symptoms were included as controls. Sequencing of the ITS region allowed the identification of 183 C. gloeosporioides isolates from mango; only one isolate was identified as C. acutatum. amova analysis of ITS sequences showed larger genetic variability among isolates from the same municipality than among those from different populations. fAFLP markers indicated high levels of genetic variability among the C. gloeosporioides isolates from mango and no correlation between genetic variability and isolate source. Only one C. gloeosporioides mango isolate had the same genotype as the C. gloeosporioides isolates from citrus leaves, as determined by ITS sequencing and fAFLP analysis. Pathogenicity tests revealed that C. gloeosporioides and C. acutatum isolates from either mango or citrus can cause anthracnose symptoms on leaves of mango cvs Palmer and Tommy Atkins and blossom blight symptoms in citrus flowers. These outcomes indicate a lack of host specificity of the Colletotrichum species and suggest the possibility of host migration.     

Population genetic analysis of Phytophthora infestans in northwestern China

Late blight caused by Phytophthora infestans is the most devastating disease of potato worldwide. To understand the P. infestans population structure and dynamics in northwestern China, 959 single‐lesion isolates were purified in three consecutive years (2009–2011) and were characterized for mating type, pathotype, mtDNA haplotype and molecular variation at eight SSR loci. The results showed that the distribution of mating types changed significantly over years, with self‐fertile isolates dominant in 2010 and 2011. SSR genotyping distinguished 959 isolates into 151 genotypes, and association analysis indicated that P. infestans populations in 2010 and 2011 were strictly asexual while in 2009 they showed signs of sexual reproduction. Population analysis showed that the majority of genetic variation was within P. infestans populations. Isolates sharing identical SSR genotypes were detected in distant regions, indicating that migration of P. infestans could have occurred between regions. Pathogenicity assays on a set of potato differential lines containing R1 to R11 resistance genes detected four pathotypes from 74 selected isolates, with the pathotype virulent against all 11 R genes being dominant. Three mtDNA haplotypes (Ia, IIa, IIb) were detected with Ia being dominant among 507 isolates examined. Phylogenetic analysis indicated that P. infestans populations in northwestern China are distant from European lineages including 13‐A2 (blue‐13) at the time of this survey. The results have implications for the trade of healthy seed tubers as a means of managing late blight.