Vegetative Incompatibility Among Monoconidial Isolates of <Emphasis Type="Italic">Bipolaris sorokiniana</Emphasis>

Bipolaris sorokiniana is a phytopathogenic fungus that causes diseases of cereal crops, such as leaf-spot disease, common root rot, and black point of grain. Because of its great morphological, physiological, and genetic variability, this fungus is difficult to control. The aim of this investigation was to study the variability of isolates of B. sorokiniana by means of vegetative incompatibility. Thirty-five isolates of B. sorokiniana from different geographical regions in Brazil and other countries were used. The vegetative incompatibility between the isolates and the influences of different culture media on these reactions were evaluated. The total protein profile of the isolates was analyzed when the isolates were cultured separately, and in cultures of compatibility and incompatibility reactions. Eighteen of 31 confrontations showed vegetative incompatibility. The results obtained with different culture media for the vegetative compatibility/incompatibility genotypes suggested that the type of substratum influences these reactions. No differences in protein profiles among the isolates were observed. This result suggests that there is no induction of expression of different proteins in vegetative incompatibility reactions.     

Two Mycosphaerella graminicola French isolates differ in symptoms, in planta sporulation and cell wall degrading enzymes in vitro production

The pathogenicity level of two French Mycosphaerella grominicola field isolates (T0414 and T0251) was evaluated on Soissons wheat cultivar using two methods: detached wheat leafs assay in a climatic chamber and wheat seedlings assay in a greenhouse. Both methods revealed that chlorosis and necrosis caused by the T0414 isolate were larger than those caused by the T0251 isolate. Indeed, in the detached leaf assay, the first symptoms on leaves inoculated by T0414 were observed 12 days post inoculation and 50% of the leaves were infected on day 18, with a maximum of 84% of leaves on day 22. On the other hand, the first symptoms were observed on day 18 on leaves inoculated with T0251 with a maximum of only 10% of leaves that were infected on day 22. In addition, both methods showed that necrosis surfaces caused by T0414 were covered by large number of pycnidia, while no pycnidia were observed on the reduced necrosis caused by the T0251 isolate. To understand the pathogenicity variation between these two isolates, their ability to produce cell wall degrading enzymes, xylanases and polygalacturonases was investigated in vitro every 2 days for 20 days. The results showed similar time course for production of polygalacturonases for the two isolates, with non significant higher production for T0414. However, a peak of maximum production of xylanases by T0414 (343 +/- 52 mU mL(-1)) was observed on day 12 post inoculation, while the maximum production (265 +/- 72 mU mL(-1)) by T0251 was observed only on day 20 post inoculation. This result shows a relationship between the beginning of the appearance of symptoms on detached leaves inoculated by T0414 on day 12 and the maximum of xylanases production on the same day in enzymes assays. In conclusion, this study suggests pathogenicity variability between M. graminicola isolates and the role of xylanases in the pathogenicity of this fungus.     

Genetic diversity analysis of <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> causing stem rot in chickpea using RAPD,ITS-RFLP,ITS sequencing and mycelial compatibility grouping

Sclerotinia sclerotiorum is one of the most devastating soil-inhabiting fungal plant pathogens infecting various crop plants including chickpea. Genetic diversity of 24 isolates of S. sclerotiorum representing 10 different states of India was determined by different molecular markers and mycelial compatibility grouping (MCG). The majority of the isolates showed more than 90% genetic similarity. Unweighted paired group method with arithmetic average cluster analysis of DNA profiles generated by 21 RAPD primers grouped the isolates into seven categories showing high magnitude of genetic homogeneity and showed partial correlation with geographical origin of the isolates. Identical ITS-RFLP profiles were generated in all the isolates. Limited variability was observed among the nucleotide sequences of ITS region of the isolates. The phylogenetic tree generated from bootstrap neighbor-joining analysis indicated that 50% of Indian populations were distinct and grouped separately. The isolates were variable in mycelial compatibility and they were grouped into seven MCGs, namely, MCG A, MCG B, MCG C, MCG D, MCG E, MCG F and MCG G.     

Physiological characterization of the dandelion bioherbicide,Sclerotinia minor IMI 344141

The fungus Sclerotinia minor (IMI 344141) is being developed as a biological control for dandelion and other broadleaf weeds in turfgrass environments. Being a microbial pest control agent (MPCA), the S. minor strain must be characterized to show relatedness to like organisms and to distinguish the MPCA from related microorganisms. Phenotypic variation among 30 isolates of S. minor, collected from different regions and hosts, was studied on potato dextrose agar (PDA) and oatmeal agar (OMA). Isolates varied significantly in sclerotia shape (length/width ratio) and number, but did not vary in colony morphology or growth rates. There was high diversity (0.6) among the mycelial compatibility groups (MCG) as seven multi-member and 11 single member groups were recognized. Isolates were categorized into highly virulent, virulent, moderately virulent, and hypo virulent based on 48 h post mycelial growth on detached dandelion leaves. When assessed on dandelion plants in the greenhouse, isolate IMI 344141 ranked the highest in biocontrol efficacy, reduction of above- and below-ground biomass, and reduction in dandelion survival. Oxalic acid production was not correlated with isolate aggressiveness or growth rate and did not vary among isolates of the same MCG. IMI 344141 can be phenotypically distinguished from the other tested S. minor isolates by performing vegetative compatibility testing and counting sclerotia produced on standard 9-cm diameter PDA plates. IMI 344141 produces <100 sclerotia/plate.     

Comparisons of Isolates of Fusarium avenaceum from White Lupin and Other Crops by Pathogenicity Tests,DNA Analyses and Vegetative Compatibility Tests

Isolates of Fusarium avenaceum, mostly from crops of white lupin or wheat, were tested for pathogenicity on white lupin and wheat plants and compared by DNA tests and, in a limited study, vegetative compatibility. Most of the 80 isolates were pathogenic on both plant species after inoculation on shoot bases. Disease severity was greater at higher incubation temperatures that ranged from 15/10°C to 25/20°C (day/night temperatures). Isolates from lupin crops tended to be more pathogenic, on average, on lupins than on cereals. Polymerase chain reaction (PCR)‐restriction fragment length polymorphism (RFLP) analysis of the internal transcribed spacer region of the rDNA distinguished two groups of isolates that occurred in different proportions among isolates from lupins and cereal crops. Random amplified polymorphic DNA (RAPD)‐PCR analyses indicated considerable genetic variation among isolates, but there was some similarity among groups of isolates from populations in the same field. Genetic diversity was confirmed by a high degree of vegetative incompatibility among 20 isolates using nitrate nonutilizing mutants. There were no relationships among pathogenicity, RFLP group, RAPD group and vegetative compatibility group.     

Population structure ofSclerotium rolfsii in peanut fields

Sclerotium rolfsii isolates from peanut fields in Ibaraki were classified into mycelial compatibility groups (MCGs) based on the barrage zone formation. A total of 132 isolates collected from four fields within a 120 m radius in 1994 comprised four MCGs; MCG A (71 isolates), B (34 isolates), C (26 isolates) and D (one isolate). Fields 1 and 2 were occupied exclusively by MCG A. MCG A also predominated in field 3. In field 4, MCGs A, B and C were dominant. Population structure in 3 additional fields was determined in 1997. All 11 isolates from Field 5, which was 400 m distant from field 1, belonged to MCG C. A total of 42 isolates from fields 6 and 7, 2.5 km distant from other fields and 100 m distant from each other, were all MCG A. These results suggested that the population structure ofS. rolfsii was simple. RAPD fingerprintings showed that most isolates of the same MCG were clonal.     

Dynamics of double-stranded RNA segments in a Helicobasidium mompa clone from a tulip tree plantation

Eighty-three isolates of the violet root rot fungus, Helicobasidium mompa, were collected in a tulip tree plantation and analyzed for the dynamics of double-stranded (ds) RNA for five years. They were divided into eight mycelial compatibility groups (MCGs). Prevalent MCGs 60 and 68 included 61 and 11 isolates, respectively. Electrophoretic profiles of dsRNA in the first year collection of MCG 60 contained no or a single large dsRNA (more than 10 kb) with or without small dsRNAs (ca. 2.0-2.5 kb). Additional dsRNA fragments, i.e., a middle dsRNA (ca. 8.0 kb) or another type of small dsRNAs, became evident within MCG 60 isolates with time. Northern hybridization revealed the relatedness of all large and middle dsRNA fragments within MCG 60 but small fragments of dsRNA were variable. Large dsRNA fragment differed from that in other MCGs even in the same field. Correlation between specific dsRNA fragments and hypovirulence was not observed. Possible explanations for the accumulation of dsRNA fragments during the growth of disease patch by MCG 60 are discussed in terms of their internal changes such as evolution of novel dsRNA fragments from pre-existing viruses or fungal genomic DNA and horizontal transmissions.     

Comparison between Rosellinia necatrix isolates from soil and diseased roots in terms of hypovirulence

The white root rot fungus, Rosellinia necatrix, is a devastating soil-borne pathogen of many plant species. Biocontrol with the hypovirulence factor is promising, but disease symptoms, signs or culture morphology of the pathogen cannot be reliably used as markers for hypovirulence in this fungus. We attempted to obtain hypovirulent isolates from soil rather than from diseased roots, based on the hypothesis that hypovirulent isolates were more likely to persist in soil as saprobes. Sixteen isolates, belonging to eight mycelial compatibility groups (MCGs), were obtained from soil in two active and one abandoned Japanese pear orchards. Comparison of these isolates based on clonality revealed that six MCGs were commonly recovered from both diseased roots and soil and two MCGs exclusively from soil. No MCG was found in more than one orchard. With two exceptions, isolates within the same MCG were similar in virulence, competitive saprophytic ability (CSA) and mycelial growth rate whether or not they carried dsRNA. The two exceptional isolates recovered from soil had multiple dsRNA segments that caused hypovirulence, weakened CSA and restricted mycelial growth on nutrient-rich media. They belonged to different MCGs, each including dsRNA-free isolates. Isolates from soil contained various dsRNAs (44%), including the hypovirulence factor, more frequently than isolates from diseased roots in the same fields (25%), which is much higher than the proportion of isolates with dsRNA from diseased roots (19%) in a total of 424 isolates from Japan examined so far. These results suggest that isolation of R. necatrix from soil is an effective method to obtain isolates with dsRNAs, including the hypovirulence factor.     

Comparative Studies between Portuguese and Japanese Isolates of the Pinewood Nematode, Bursaphelenchus xylophilus

Comparative studies between Portuguese (T and HF) and Japanese (S10, T4, C14-5 and OKD-1) isolates of the pinewood nematode Bursaphelenchus xylophilus have been made in order to provide information to better understand the possible origin of the Portuguese isolates, recently introduced in the European Union. The main comparative aspects investigated were pathogenicity (seedling mortality ratio), sexual compatibility, and DNA sequences of the rDNA region. Four-year-old Japanese black pine (Pinus thunbergii) seedlings were used as host plants for pathogenicity tests. The Portuguese isolates, and in particular isolate “T,” propagated in higher numbers than the Japanese isolates within pine seedlings. All combinations of crossings produced viable progeny, with higher numbers obtained when crossings were made between Japanese and Portuguese isolates, a possible situation of heterosis and/or inbreeding depression. Reciprocal crossings yielded different values, which may reflect a sex effect (maternal inheritance, mtDNA). Regarding DNA sequencing, both Portuguese isolates displayed nearly identical ITS 1, ITS2, and 5.8S rDNA base sequences as the Japanese isolates. Although biologically very similar, and possibly reflecting a common origin, the Portuguese isolates may present a serious threat to Japanese black pine, due to their higher virulence.     

Microsatellite and Morphological Markers Reveal Genetic Variation within a Population of Sclerotinia sclerotiorum from Oilseed Rape in the Çanakkale Province of Turkey

The genetic variation among a population of Sclerotinia sclerotiorum collected from oilseed rape fields in the Çanakkale Province of Turkey was assessed using molecular and morphological markers. Seven microsatellite primer pairs (out of eight) revealed 32 clear polymorphic alleles among the 36 fungal isolates examined. An unweighted pair‐group mean analysis dendrogram was generated using the genetic distance matrix with the 32 microsatellite alleles. The level of similarity was as low as 15% between some isolates indicating a high level of genetic diversity within the fungal population; 23 distinct isolates were found (at a genotypic diversity level of 63%). Among the collection of 36 isolates, 19 mycelial compatibility groups (MCGs) were identified; 10 MCGs included at least two isolates. Molecular and morphological data suggest that most of the isolates within a single MCG were identical; however, the isolates belonging to the MCG2 and MCG4 had variable microsatellite haplotypes and were morphologically dissimilar. The data suggest that there is possibly a high rate of outcrossing as well as evolutionary potential within the population of the pathogen in oilseed rape fields. This is the first report demonstrating the genetic and morphological variation within a population of S. sclerotiorum in Turkey.     

Evaluation of Trichoderma spp. as biocontrol agents against avocado white root rot

Five isolates of Trichoderma atroviride and one isolate each of T. virens, T. harzianum and T. cerinum were tested for in vivo biological control of white root rot of avocado (Rosellinia necatrix). Five of these Trichoderma isolates were previously selected as possible biological control agents on the basis of their capacity to control the disease and high levels of colonization of the avocado rhizoplane. Combinations of the five selected isolates were evaluated on cellophane for compatibility with each other and T. virens CH 303 was eliminated because of a high incompatibility with other Trichoderma isolates. The four remaining isolates, all T. atroviride, were tested singly and in combination for their capacity to control avocado white root rot. Isolate CH 304.1 provided the highest levels of control when tested singly or in combination with isolate CH 101.     

Cytological Analysis of Anastomoses and Vegetative Incompatibility Reactions in <Emphasis Type="Italic">Helicobasidium monpa</Emphasis>

Our examination of the cytological characteristics of the vegetative incompatibility reaction in a filamentous basidiomycete, Helicobasidium monpa, by analyzing the fluorescence emitted by ethidium bromide and acridine orange stained nuclei is described. Hyphal anastomoses between strains belonging to different mycelium compatibility groups (MCG) were observed with cell death in fused hyphae, whose nuclei were intensified by ethidium bromide. In contrast, the nuclei in a living cell were not intensified by staining with ethidium bromide, but were intensified by staining with acridine orange. These results indicate that in H. monpa, ethidium bromide staining is a useful method for detecting dead cells. We also examined the relationships between the alternation of ploidy and hyphal anastomosis formation using the newly developed method on filamentous fungi. The tetraploid monokaryon strain derived from the original dikaryon strain by continuous subculture could not be fused to any wild type strains, but the original dikaryon strain could be fused without cell death to only the same MCG strain. In contrast, the haploid dikaryon strain derived from the original monokaryon strain fuses to several strains belonging to different MCGs without cell death. These results suggested that the cellular ploidy of this fungus is closely related to its mating system and, H. monpa may be a self-fertilizing fungus. Received: 13 June 2001 / Accepted: 8 August 2001     

High diversity of vegetative compatibility types in Cryphonectria parasitica in Japan and China

We found high diversity of vegetative compatibility (vc) types in native populations of the chestnut blight fungus, Cryphonectria parasitica, in Japan and China; almost every isolate was in a unique vc type. In Japan we found 71 vc types in a sample of 79 isolates pooled from six populations. Within two populations in China, all isolates (n=28 and 11) had unique vc types; we found 15 vc types among 25 isolates in a third Chinese population where multiple isolates were collected from some trees. None of the isolates from China and only three isolates in the 71 vc types from Japan were compatible with any of 64 vc type testers from Europe, which have known vegetative incompatibility genotypes. To our knowledge this is the first report of vc type diversity for C. parasitica in Japan or of any comparisons of vc types between Asia and Europe. The most significant result of this survey is the identification fungal isolates for expanding knowledge of the genetics of vegetative incompatibility.     

Culture-Independent Assessment of Rhizobiales-Related Alphaproteobacteria and the Diversity of <Emphasis Type="Italic">Methylobacterium</Emphasis> in the Rhizosphere and Rhizoplane of Transgenic Eucalyptus

The rhizosphere is an ecosystem exploited by a variety of organisms involved in plant health and environmental sustainability. Abiotic factors influence microorganism–plant interactions, but the microbial community is also affected by expression of heterologous genes from host plants. In the present work, we assessed the community shifts of Alphaproteobacteria phylogenetically related to the Rhizobiales order (Rhizobiales-like community) in rhizoplane and rhizosphere soils of wild-type and transgenic eucalyptus. A greenhouse experiment was performed and the bacterial communities associated with two wild-type (WT17 and WT18) and four transgenic (TR-9, TR-15, TR-22, and TR-23) eucalyptus plant lines were evaluated. The culture-independent approach consisted of the quantification, by real-time polymerase chain reaction (PCR), of a targeted subset of Alphaproteobacteria and the assessment of its diversity using PCR–denaturing gradient gel electrophoresis (DGGE) and 16S rRNA gene clone libraries. Real-time quantification revealed a lesser density of the targeted community in TR-9 and TR-15 plants and diversity analysis by principal components analysis, based on PCR–DGGE, revealed differences between bacterial communities, not only between transgenic and nontransgenic plants, but also among wild-type plants. The comparison between clone libraries obtained from the transgenic plant TR-15 and wild-type WT17 revealed distinct bacterial communities associated with these plants. In addition, a culturable approach was used to quantify the Methylobacterium spp. in the samples where the identification of isolates, based on 16S rRNA gene sequences, showed similarities to the species Methylobacterium nodulans, Methylobacterium isbiliense, Methylobacterium variable, Methylobacterium fujisawaense, and Methylobacterium radiotolerans. Colonies classified into this genus were not isolated from the rhizosphere but brought in culture from rhizoplane samples, except for one line of the transgenic plants (TR-15). In general, the data suggested that, in most cases, shifts in bacterial communities due to cultivation of transgenic plants are similar to those observed when different wild-type cultivars are compared, although shifts directly correlated to transgenic plant cultivation may be found.     

Genotypic Diversity among Brazilian Isolates of Sclerotium rolfsii

Thirty isolates of Sclerotium rolfsii Sacc. from different hosts and regions of Brazil were studied in relation to morphology, mycelial compatibility, analysis of genomic DNA through random amplified polymorphic DNA (RAPD), variation within the nuclear rDNA [internal transcribed spacers (ITS)] and sequencing of ITS fragments. There was considerable variability among isolates in relation to the number, size and location of sclerotia on the medium surface. Thirteen mycelial compatibility groups (MCG) were identified among 23 isolates. Seven isolates were only self‐compatible. With the exception of group 3, where all the isolates came from soybean, there was no apparent correlation between group and isolate origin. On the basis of RAPD profiles, 11 haplotypes (A to K) were identified. There was an association between the RAPD groups and MCG. Haplotypes A, B, D, G, I and K belonged to MCG groups 1, 2, 3, 4, 5 and 6, respectively. All other RAPD haplotypes contained incompatible isolates. Polymerase chain reaction (PCR) amplification with primers 4R and 5F amplified two fragments containing ITS1, ITS2 and 5.8 S rDNA sequences, that were present in all isolates, with molecular sizes of 739 and 715 bp. Restriction analysis of PCR products showed that the two fragments had sequence divergency which is referred to as ‘ITS types’. Four arbitrarily chosen soybean isolates (2, 6, 7 and 23) and two non‐soybean isolates (11 and 22) were used to investigate the variation within the ITS sequence and its role in the phylogeny. The strict consensus of nine most‐parsimonious trees inferred from the data set which included six isolates of S. rolfsii, four of which have two different ‘ITS types’, showed three well‐supported groupings. The neighbour‐joining tree inferred from the data set also showed three major clades as did the parsimony tree. The major difference was that in the neighbour‐joining tree the ‘ITS type’ 11 was resolved and grouped in one clade. These results show that the ‘ITS types’ within isolates are almost always phylogenetically distinct. There was no clear correlation between ITS‐based phylogeny and isolate origin.     

Mycelial interactions inSclerotinia sclerotiorum

Mycelial interactions were examined among 35 isolates ofSclerotinia sclerotiorum and two Asian species,Sclerotinia asari and an unnamed, Japanese species. Pairings were scored as compatible when strains merged to form one colony and incompatible when strains grew to form two distinct colonies. Incompatible mycelial pairings resulted in an interaction zone in which a distinct reaction line and abundant aerial mycelium or thin mycelium were observed with some variation among replicates. All pairings of a strain with itself were compatible. Of the 31 strains ofS. sclerotiorum tested, 21 were mycelially incompatible with all others. Among the remaining 10 strains ofS. sclerotiorum, there were four mycelial compatibility groups consisting of two or three strains each. Pairings ofS. asari with all other strains resulted in a unique incompatible reaction, a mycelium-free interaction zone. Two of three strains of the Japanese species were intercompatible, but pairings of each of the three strains with all other strains were incompatible. Microscopically, mycelial interactions in pairings of strains were complex. Anastomosis between paired strains was not always observed. This may be due in part to the conversion of many hyphal tips, in both compatible and incompatible interactions, to sites of microconidiogenesis no longer capable of hyphal fusion. Incompatible pairings were followed by hyphal deterioration in one or both strains; hyphal deterioration was not observed in compatible interactions. Of the 31 strains tested, 4 strains ofS. sclerotiorum produced apothecia. Pairings between single ascospore isolates within each strain were compatible, as were pairings with the parent isolate. Mycelial interactions of single ascospore isolates with other strains were identical to those of the parent isolate, indicating that the parent fruitbody was homozygous for any determinant(s) of mycelial incompatibility. The data from this study suggest that a high level of mycelial incompatibility exists among strains ofS. sclerotiorum, comparable to levels of vegetative incompatibility reported in other ascomycetes, that the extent of mycelial incompatibility indicates that genetic heterogeneity exists within the species, and that mycelial compatibility/incompatibility reactions may be an effective way of categorizing intraspecific heterogeneity.     

Expression of Pectinase Activity Among Aspergillus Flavus Isolates from Southwestern and Southeastern United States

Aspergillus flavus is a widely distributed filamentous fungus that contaminates crops with the potent carcinogen aflatoxin. This species can be divided into S and L strains on the basis of sclerotial morphology. During crop infection, A. flavus can secrete a large array of hydrolytic enzymes. These include pectinase, which aids fungal spread through plant tissues. A survey of pectinase expression by soil isolates derived from different regions of the United States revealed geographic polymorphisms. Strain L isolates from Arizona produced moderate to high levels of a specific pectinase P2c, while S strain isolates produced variable amounts of P2c. In contrast, L strain isolates from southeastern U.S. yielded variable P2c production, while S strain isolates consistently expressed high P2c levels. These results were corroborated by pectinase surveys of additional collections of A. flavus from soil and cottonseed. Expression patterns for P2c and pectinmethylesterase were evaluated for a select number of isolates using an isoelectric focusing technique. Clear zone reactions from the pectinase plate assay corresponded to the presence of P2c, while red ring reactions corresponded to the lack of P2c. Commercial cottonseed infected by S strain isolates frequently contained aflatoxin, even when infected by S strain isolates that did not produce pectinase P2c. Thus, although P2c-lacking isolates have reduced invasiveness, these isolates still have sufficient pathogenicity to cause aflatoxin contamination.     

Molecular genotyping of Sclerotinia sclerotiorum isolates from different regions and host plants in Iran

Genetic variation among Sclerotinia sclerotiorum isolates from different regions and host plants were investigated using pathogenicity test, mycelial compatibility groups (MCGs) and molecular markers. Six MCGs were identified and significant differences of virulence variability were observed within and among MCGs. Cluster analysis of combined repetitive sequence-based polymerase chain reaction and randomly amplified polymorphic DNA data discriminated 12 isolates into 11 genotypes, indicating high level of genetic polymorphism among tested isolates. Twelve isolates clustered into four major groups corresponding to their hosts andgeographical region. The variability found within closely related isolates of S.sclerotiorum indicated that such morphological and molecular markers are useful in population studies of this pathogen.     

Genetic Variation of Sclerotinia sclerotiorum from Multiple Crops in the North Central United States

Sclerotinia sclerotiorum is an important pathogen of numerous crops in the North Central region of the United States. The objective of this study was to examine the genetic diversity of 145 isolates of the pathogen from multiple hosts in the region. Mycelial compatibility groups (MCG) and microsatellite haplotypes were determined and analyzed for standard estimates of population genetic diversity and the importance of host and distance for genetic variation was examined. MCG tests indicated there were 49 different MCGs in the population and 52 unique microsatellite haplotypes were identified. There was an association between MCG and haplotype such that isolates belonging to the same MCG either shared identical haplotypes or differed at no more than 2 of the 12 polymorphic loci. For the majority of isolates, there was a one-to-one correspondence between MCG and haplotype. Eleven MCGs shared haplotypes. A single haplotype was found to be prevalent throughout the region. The majority of genetic variation in the isolate collection was found within rather than among host crops, suggesting little genetic divergence of S. sclerotiorum among hosts. There was only weak evidence of isolation by distance. Pairwise population comparisons among isolates from canola, dry bean, soybean and sunflower suggested that gene flow between host-populations is more common for some crops than others. Analysis of linkage disequilibrium in the isolates from the four major crops indicated primarily clonal reproduction, but also evidence of genetic recombination for isolates from canola and sunflower. Accordingly, genetic diversity was highest for populations from canola and sunflower. Distribution of microsatellite haplotypes across the study region strongly suggest that specific haplotypes of S. sclerotiorum are often found on multiple crops, movement of individual haplotypes among crops is common and host identity is not a barrier to gene flow for S. sclerotiorum in the north central United States.