Quantitative Trait Loci (QTL) Analysis Reveals Both Broad-Spectrum and Isolate-Specific QTL for Scab Resistance in an Apple Progeny Challenged with Eight Isolates of Venturia inaequalis

ABSTRACT The major scab resistance gene Vf, extensively used in apple breeding programs, was recently overcome by the new races 6 and 7 of the fungal pathogen Venturia inaequalis. New, more durable, scab resistance genes are needed in apple breeding programs. F(1) progeny derived from the cross between partially resistant apple cv. Discovery and apple hybrid 'TN10-8' were inoculated in the greenhouse with eight isolates of V. inaequalis, including isolates able to overcome Vf. One major resistance gene, Vg, and seven quantitative trait loci (QTL) were identified for resistance to these isolates. Three QTL on linkage group (LG)12, LG13, and LG15 were clearly isolate-specific. Another QTL on LG5 was detected with two isolates. Three QTL on LG1, LG2, and LG17 were identified with most isolates tested, but not with every isolate. The QTL on LG2 displayed alleles conferring different specificities. This QTL co-localized with the major scab resistance genes Vr and Vh8, whereas the QTL on LG1 colocalized with Vf. These results contribute to a better understanding of the genetic basis of the V. inaequalis-Malus x domestica interaction.     

Molecular Identification of a Sexual Interloper: The Pear Pathogen, Venturia pirina, has Sex on Apple

ABSTRACT Venturia pirina (the pear scab pathogen) and V. inaequalis (the apple scab pathogen) were detected as ascospores discharged from apple leaf litter in New Zealand (spring 1998). Pseudothecia of both species were located on dead apple leaves; however, only those of V. inaequalis were associated with scab lesions. V. pirina was identified by rDNA sequence analyses, because morphological characters could not distinguish this fungus from V. asperata (a rare saprophyte on apple) and other Venturia spp. pathogenic on rosaceous fruit trees. Species-specific polymerase chain reaction primers designed to the 18S end of the internal transcribed spacer 1 region differentiated Venturia fruit tree pathogens reliably. V. pirina field isolates were pathogenic on pear, but only weak saprophytes on apple. In rare instances, when appressoria of V. pirina appeared to penetrate the cuticle of apple leaves, epidermal cells responded with a localized hypersensitive response (HR). To our knowledge, this is the first report of induction of HR-like events by V. pirina on its nonhost, apple, and also the first record of sexual reproduction of V. pirina on apple. It is assumed that V. pirina pseudothecia formed from saprophytic lesions in senescing apple leaves when active defense mechanisms such as HR were no longer induced.     

Characterization of Ribosomal DNA from Venturia inaequalis and Its Phylogenetic Relationship to rDNA from Other Tree-Fruit Venturia Species

ABSTRACT A portion of the 18S ribosomal DNA (rDNA) gene, the internal transcribed spacers (ITS1 and ITS2), and the 5.8S rDNA gene were polymerase chain reaction-amplified from strains and field populations of Venturia inaequalis and assessed for genetic variation. A previously reported optional group I intron in the 18S rDNA gene of V. inaequalis was detected in 75.0% of 92 strains collected worldwide and in 61.1 and 71.2% of 54 and 59 strains from two Michigan orchards, respectively. Sequence and restriction analysis of rDNA revealed four intron alleles, three of which were present both in worldwide strains and in each field population. Two ITS1 alleles were detected and found to be linked to specific intron alleles. The ITS1-5.8S-ITS2 sequences from V. asperata V. carpophila, V. cerasi, V. inaequalis, V. nashicola, V. pyrina, and Cladosporium caryigenum were compared using phylogenetic analysis. Strains of the Venturia species were placed in three distinct monophyletic groups in a phylogenetic tree. The first group comprised V. inaequalis; the second, V. pyrina and V. nashicola; and the third, V. cerasi, V. carpophila, and V. asperata. The described intron and ITS1 alleles in V. inaequalis provide genetic markers for subdividing populations of V. inaequalis, and the ITS1-5.8S-ITS2 sequences are valuable in determining the relationship of the species from tree-fruit crops with other Venturia species.     

Molecular and morphological characterization of Colletotrichum gloeosporioides from native Mexican Stylosanthes species

A total of 264 Stylosanthes spp. plants collected from 78 Stylosanthes spp. populations in seven southern Mexican states were analysed for the presence of Colletotrichum spp. Isolates were obtained from 64 plants collected from 36 Stylosanthes populations; 198 isolates produced straight conidia, while 72 isolates produced falcate conidia. Molecular identification was performed to confirm the identity of C. gloeosporioides for the straight-spored isolates. PCR amplifications using the primer CgInt, synthesized from an ITS1 fragment specific to C. gloeosporioides , and the universal primer ITS4 generated the target fragment for 120 Mexican isolates with straight conidia. The endonucleases Ava II and Sma I were used for restriction of the entire amplified ITS1 region of these 120 isolates. The tree constructed from the restriction data grouped 118 Mexican C. gloeosporioides isolates into three clusters containing reference isolates from Africa and Australia, and generated two additional clusters for two Mexican isolates. Conidial shape and growth rate on solid medium were used as the major morphological criteria for distinguishing types A and B. On the basis of 32 other morphological characteristics, a phenogram grouped the colonies into three main clusters. These clusters were partially related to the Stylosanthes species from which they were isolated, and to the molecular groups.     

Molecular Characterization of Natural Hybrids of Phytophthora nicotianae and P. cactorum

ABSTRACT Hybrid isolates of Phytophthora nicotianae x P. cactorum from five different hosts (Cyclamen, Lavandula, Lewisia, Primula, and Spathiphyllum spp.) were identified by their atypical morphology and their well-defined heterozygous isozyme patterns. The hybrid nature of these isolates was tested by restriction fragment length polymorphism analysis of the internal transcribed spacer (ITS) region of rDNA, generating fragments typical for both P. nicotianae and P. cactorum. In hybrid isolates, polymerase chain reactions (PCR) with primers derived from unique parts of the ITS region (ITS-PCR) of both species yielded a combination of unique amplicons typical of both parental species. Eleven hybrid isolates, three isolates of each parental species and two atypical isolates from Rhododendron and Idesia spp. close to P. cactorum, were analyzed for amplified fragment length polymorphisms (AFLP). Consistent differences in AFLP patterns existed among the hybrid isolates, strongly indicating that these hybrids have arisen from independent hybridization events between P. nicotianae and P. cactorum. The two atypical isolates morphologically resembling P. cactorum were identical to the latter species in ITS-restriction fragment length polymorphism and response to the specific PCR primers but were intermediate between P. nicotianae x P. cactorum and P. cactorum in isozyme profiles and AFLP patterns. Since the introduction of hydroponic systems in greenhouses in the Netherlands, outbreaks of Phytophthora diseases are occurring in previously unaffected host species. This may be due to interspecific hybridization events resulting in novel pathogenic behavior.     

Effect of Fall Application of Fungal Antagonists on Spring Ascospore Production of the Apple Scab Pathogen, Venturia inaequalis

ABSTRACT The influences of Microsphaeropsis sp., M. arundinis, Ophiostoma sp., Diplodia sp., and Trichoderma sp., all antagonists of Venturia inaequalis, on ascospore production were evaluated under natural conditions and compared with urea and Athelia bombacina, a known antagonist. In the autumn, the fungi were applied to leaf disks artificially inoculated with V. inaequalis and to scabbed apple (Malus domestica) leaves incubated under controlled and natural conditions. In addition, large-scale trials were conducted with Microsphaeropsis sp. applied either as a foliar postharvest spray or as a ground application at 90% leaf fall. All fungal isolates, except Ophiostoma sp., were recovered from the leaf material that overwintered in the orchard. All treatments, except those with Ophiostoma sp., resulted in a significant reduction in V. inaequalis ascospore production on the leaf disks incubated under controlled conditions or in the orchard. In 1997, leaves with apple scab lesions treated with urea or Microsphaeropsis sp. produced significantly fewer ascospores of V. inaequalis than did nontreated leaves, with a reduction of 73.0 and 76.3%, respectively. In 1998, leaves treated with Microsphaeropsis sp., urea, Trichoderma sp., A. bombacina, and M. arundinis reduced ascospore production by 84.3, 96.6, 75.2, 96.6, and 52.2%, respectively. Based on all tests combined, the most efficient isolate was Microsphaeropsis sp. Postharvest applications of Microsphaeropsis sp. reduced the total amount of airborne ascospores trapped by 70.7 and 79.8% as compared with the nontreated plots in 1997 and 1998, respectively. Microsphaeropsis sp. provided a significant and consistent reduction in ascospore production in all tests.     

Occurrence and Identification of Phytophthora spp. Pathogenic to Pear Fruit in Irrigation Water in the Wenatchee River Valley of Washington State

ABSTRACT Seven hundred forty-nine isolates of Phytophthora spp. were obtained from irrigation canals in eastern Washington State during the 1992 to 1995 and 1999 growing seasons. Isolates were retrieved using pear baiting techniques. All isolates were pathogenic to pear and were present in irrigation water beginning early in fruit development. Over the course of the 5 year study, 10 and 5% of isolates were identified as P. cactorum and P. citricola, respectively, using morphological criteria. The remaining isolates could not be identified using morphological criteria. Colony morphology of these isolates was characterized during all years of the study. In 1999, more detailed studies utilizing polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis of entire internal transcribed spacer (ITS) regions (ITS1, 5.8S, and ITS2) of ribosomal DNA for 180 isolates, and sequence analysis of ITS2 for 50 isolates, were used to investigate genetic variation and phylogenetic relationships among isolates. Isolates were divided into 12 groups based on their growth type on corn meal agar. Restriction digestion of the entire ITS region with three enzymes revealed 11 restriction digestion patterns among 180 isolates. PCR-RFLP and sequence data were obtained for 12 reference Phytophthora spp. (two species in each of Waterhouse's six morphological groups). Phylogenetic analysis of ITS2 regions revealed nine clades, each with strong bootstrap support. Molecular analyses revealed 23 isolates that were in the P. gonapodyides clade, 9 in the P. parasitica clade, 1 in the P. cactorum clade, 7 in the P. citricola/capsici clade, and 4 in the P. cambivora/pseudotsugae clade. The three isolates comprising clade 5 were significantly distinct from all other Phytophthora spp. in the databases and may represent a new Phytophthora sp. Colony morphology was not consistently correlated to PCR-RFLP pattern or ITS2 phylogeny, suggesting that the former criterion is insufficient for species identification. The results of this study indicate that at least nine phylogenetically distinct taxa of Phytophthora pathogenic to pear are present in irrigation water in North Central Washington.     

The threat of wild habitat to scab resistant apple cultivars

Evaluations of plant resistance to pathogens are rarely made using isolates from wild habitats, although the heterogeneity of such habitats may generate pathogen diversity which could be a source of new virulence in cultivated habitats. The aim of this study was to investigate whether scab resistance factors, identified and characterized in apples using isolates of Venturia inaequalis from a cultivated habitat, remained effective against isolates from a wild habitat. Three V. inaequalis core collections originating from the cultivated apple Malus × domestica and from two wild species, M. sieversii and M. sylvestris, were established to maximize pathogen diversity. For each core collection, 10 isolates were inoculated in mixtures onto 51 genotypes from an apple progeny segregating for two qualitative resistance genes and six quantitative resistance loci (QRL). On each apple genotype, isolates that contributed to the scab symptoms were identified within the mixture using microsatellite markers. The most frequently detected isolates were inoculated singly to compare their aggressiveness according to their host origin. The results showed that isolates from a wild habitat were able to infect the susceptible apple genotypes. However, these isolates were never more aggressive than isolates from the cultivated habitat on the resistance factors tested. It can therefore be concluded that the resistance factors used in this study, identified with V. inaequalis isolates from a cultivated habitat, remained effective against isolates from M. sylvestris and M. sieversii.     

Accumulation of Pathogenesis-Related Proteins in the Apoplast of a Susceptible Cultivar of Apple (Malus domestica cv. Elstar) after Infection by Venturia inaequalis and Constitutive Expression of PR Genes in the Resistant Cultivar Remo

Leaves of apple (Malus domestica cv. Elstar) were infected with a cloned isolate of the apple scab Venturia inaequalis. The intercellular washing fluid (IWF) of these plants was collected and the variation in the composition of proteins in the IWF was analysed by SDS-PAGE and two-dimensional gel electrophoresis during and after the infection with V. inaequalis, the causal agent of apple scab. The subsequent analysis of induced proteins by electron spray ionization quadrupole time of flight mass spectroscopy revealed the presence of -1,3-glucanase, chitinase, thaumatin-like protein and a cysteine-like protease in M. domestica leaves infected by V. inaequalis. These results were confirmed by immunoblotting with antibodies against some of these proteins. Moreover, a non-specific lipid transfer protein was identified in uninfected leaves: the amount declined to a non-detectable level within the first week after infection by V. inaequalis. The analysis of the IWF of M. domestica cv. Remo, bearing resistances to apple scab, powdery mildew and fire blight, showed a protein pattern comparable to that of the IWF from V. inaequalis infected leaves from cultivar Elstar indicating the constitutive production at least of some of the pathogenesis-related proteins in the resistant cultivar.     

Mapping Quantitative Field Resistance Against Apple Scab in a 'Fiesta' x 'Discovery' Progeny

ABSTRACT Breeding of resistant apple cultivars (Malus x domestica) as a disease management strategy relies on the knowledge and understanding of the underlying genetics. The availability of molecular markers and genetic linkage maps enables the detection and the analysis of major resistance genes as well as of quantitative trait loci (QTL) contributing to the resistance of a genotype. Such a genetic linkage map was constructed, based on a segregating population of the cross between apple cvs. Fiesta (syn. Red Pippin) and Discovery. The progeny was observed for 3 years at three different sites in Switzerland and field resistance against apple scab (Venturia inaequalis) was assessed. Only a weak correlation was detected between leaf scab and fruit scab. A QTL analysis was performed, based on the genetic linkage map consisting of 804 molecular markers and covering all 17 chromosomes of apple. With the maximum likelihood-based interval mapping method, eight genomic regions were identified, six conferring resistance against leaf scab and two conferring fruit scab resistance. Although cv. Discovery showed a much stronger resistance against scab in the field, most QTL identified were attributed to the more susceptible parent 'Fiesta'. This indicated a high degree of homozygosity at the scab resistance loci in 'Discovery', preventing their detection in the progeny due to the lack of segregation.     

Linkage of Vfa4 in Malus × domestica and Malus floribunda with Vf resistance to the apple scab pathogen Venturia inaequalis

The Vf locus from Malus floribunda clone 821 is an important source of resistance to apple scab disease caused by Venturia inaequalis , and has been introduced into numerous cultivars of domesticated apple, Malus  ×  domestica . Cloning of the putative Vf locus has revealed that it contains several receptor-like Vf candidate genes. In order to determine which of these genes is most closely linked to Vf resistance, primers were designed based on conserved regions in the Vf candidate genes adjacent to a variable portion of the leucine-rich repeat (LRR) domain, to yield PCR product length polymorphisms. PCR products were obtained from 31 cultivars of M.  ×  domestica , of which 19 are known to contain Vf resistance, and from 10 selections of M. floribunda . PCR products corresponding in size to Vfa1 and Vfa2 were found in all the plants tested. However, a PCR product with 100% predicted amino acid identity to Vfa4 was found only in M.  × domestica cultivars known to have Vf scab resistance. This PCR product was also found in most, but not all, selections of M. floribunda tested, including the original source of Vf resistance, M. floribunda 821. The PCR product matching Vfa4 appears to be the most closely linked to Vf resistance and should be a valuable tool for monitoring Vf inheritance in apple.     

Characterization of Venturia inaequalis pathogenicity on leaf discs of apple trees

Characterization of pathogenicity on whole plants is required to study host-pathogen interactions between Malus × domestica and Venturia inaequalis. We studied the reliability of an in vitro test of pathogenicity on leaf discs. Three strains of V. inaequalis (races 1, 6 and an English race) were inoculated in vitro onto a range of 16 Malus sp. clones including susceptible and resistant clones. The results were compared to those previously obtained in vivo. Resistant clones contained the main major known genes, i.e. Va, Vb, Vbj, Vf, Vg, Vm and Vr. Scab severity and the sporulation of the fungus were assessed 21 days after the inoculation date. The results indicated that it was possible to reproduce incompatible and compatible situations in vitro. A null severity corresponded to the avirulence of the strain for the clone considered. The resistance given by the Vb, Vbj, Vf, Vg, Vm and Vr genes were expressed in vitro. Only the clone carrying the Va gene and inoculated with the race 6 strain presented a compatible situation which was inconsistent with the observations on the whole plant. Improving this test will facilitate studies on the pathogenicity of V. inaequalis populations in relation to resistance genes of the host expressed in vitro as well as its genetic determinism.     

Population Genetic Structure and Host Specificity of Alternaria spp. Causing Brown Spot of Minneola Tangelo and Rough Lemon in Florida

ABSTRACT Alternaria spp. were sampled from two rough lemon (RL) and two Minneola tangelo (MIN) groves in a limited geographic area in central Florida to test for host-specialized forms of the pathogen. Isolates of Alternaria spp. were scored for variation at 16 putative random amplified polymorphic DNA (RAPD) loci and for pathogenicity on both hosts. Subpopulations on each host were differentiated genetically and pathogenically, which was consistent with the hypothesis of host specialization. Highly significant genetic differentiation was detected among all four subpopulations (Nei's coefficient of gene differentiation [G(ST)] = 0.292, P = 0.000); most of the differentiation occurred between hosts (G(ST) = 0.278, P = 0.000). Phenograms of qualitative similarities among isolates within subpopulations revealed two or three distinct clusters of isolates within each subpopulation. The majority of isolates sampled from RL were pathogenic on RL and not on MIN, although a few RL isolates were able to induce disease on MIN, and 44% were nonpathogenic on either host. In contrast, isolates from MIN were pathogenic only on MIN, never on RL, and only 3% of the isolates were nonpathogenic. Overall, three genetically distinct clusters of isolates were detected on both hosts. One of the clusters (cluster A) sampled from RL was pathogenic on RL and not on MIN and consisted almost entirely of one RAPD genotype. This cluster also contained two isolates that were 93% similar to the majority genotype but were pathogenic on MIN and not RL. In isolates from MIN, two distinct clusters of isolates were found in one subpopulation (clusters B and C), and three distinct clusters were found in another subpopulation (clusters A, B, and C). Clusters A and B were found on both hosts, while cluster C was limited to MIN. Populations of Alternaria spp. sampled from RL and MIN showed a high degree of host specificity; however, the specificity obscured a high level of genetic variation within subpopulations.     

Distribution and characterization of Streptomyces species causing potato common scab in Germany

Field‐grown potatoes showing scab infections were sampled in two successive years and analysed for prevailing Streptomyces strains. In 2008 and 2009, 293 Streptomyces isolates were collected in Germany and analysed for morphology, pathogenicity and strain type. Isolates varied in mycelium colour, sporulation and pigmentation. Based on their morphology, no clear differentiation of species was possible. At the genetic level, sampled isolates, as well as a number of type strains from culture collections, were characterized by PCR using 16S rRNA‐specific primers and PCR‐RFLP of the 16S–23S internal transcribed spacer (ITS) region with Hpy99I. Using this fingerprinting approach, Streptomyces species could be differentiated genotypically. The data from this study show that diversity among scab‐causing species in Germany is much higher than previously thought. Isolates belonged to various Streptomyces spp. previously associated with common scab. This is apparently the first report of pathogenic strains of S. europaeiscabiei, S. stelliscabiei, S. acidiscabiei, S. turgidiscabiei and S. bottropensis within Germany. Streptomyces europaeiscabiei was the predominant species found. Other scab‐causing species were identified, but their local distribution was uneven. For most of the isolates, the presence of the txtAB gene was demonstrated, indicating pathogenicity. This analysis is one of the first reports to examine the distribution of common scab‐causing species in Germany.     

Monitoring of Venturia inaequalis harbouring the QoI resistance G143A mutation in French orchards as revealed by PCR assays

BACKGROUND: Genetic resistance to QoI fungicides may account for recent failures to control Venturia inaequalis (Cooke) Winter in French orchards. Two PCR-based assays were developed to detect the G143A point mutation in the fungal mitochondrial cytochrome b gene. The mutation is known to confer a high level of resistance to QoI fungicides. Occurrence of the G143A mutation in French field isolates collected from 2004 to 2007 was monitored. RESULTS: The QoI-resistant cytochrome b allele was specifically detected either following the cleavage of the amplified marker by a restriction endonuclease (CAPS assay) or its amplification using an allele-specific PCR primer. Using either method, the G143A mutation was found in 42% of the 291 field samples originating from French orchards in which apple scab proved difficult to be controlled. Monitoring of the G143A mutation in orchards located in 15 French administrative regions indicated that the mutation was detected at least once in nine of the regions, and its presence ranged from 33% to 64% of the orchards analysed in 2004 and in 2007 respectively. CONCLUSION: The PCR-based methods developed in this study efficiently reveal the presence of the G143A mutation in French V. inaequalis field populations.     

Genetic Variation Among and Within Monilinia Species Causing Brown Rot of Stone and Pome Fruits

Nucleotide sequence analysis of the internal transcribed spacer (ITS) regions 1 and 2 of the ribosomal DNA (rDNA) divided the three brown rot pathogens Monilinia laxa, M. fructicola and M. fructigena into four distinct groups. Isolates of M. fructigena received from Japan, which varied by 5 base substitutions in the ITS region from the European M. fructigena isolates, formed the fourth group. Four of five Japanese isolates of M. fructicola tested varied from the New World isolates in that they did not possess a group-I intron in the small subunit (SSU) rDNA. RAPD-PCR data indicated that isolates of M. laxa varied but were randomly distributed worldwide; ITS data indicated no apparent distinction between those from Malus spp. and those from Prunus spp. M. fructigena similarly did not cluster according to geographic origin. In contrast, M. fructicola isolates tended to be clustered according to their origin; Japanese isolates of M. fructicola clustered together and showed similarity to some of the New Zealand isolates. Isolates from USA and Australia were more variable.     

昆虫病原线虫rDNA多态性分析

本文对国内外昆虫病原线虫斯氏属和异小杆属的47个品系进行rDNA—ITS PCR—RFLP分析，研究其DNA多态性，并构建了分子系统发育树状图。各品系的ITS区无明显的长度差异，PCR—RFLP分析将47个品系分为斯氏属和异小杆属两大类，两属线虫又分别分为11组和4组。所得结果丰富了ITS PCR—RFLP图谱库，为弄清我国的昆虫病原线虫与国外种类的分子系统发育关系及未定名线虫的鉴定提供重要依据，同时为筛选适合的线虫种类防治害虫奠定基础。     

Breakdown of the Scab Resistance Gene Vf in Apple Leads to a Founder Effect in Populations of the Fungal Pathogen Venturia inaequalis

ABSTRACT The recent breakdown of Vf, a major resistance gene to apple scab, provided an opportunity to analyze a population genetic process within the matching virulent subpopulation of the fungus Venturia inaequalis. We utilized the amplified fragment length polymorphism technique and allelic variation at four microsatellite loci to assess genetic structure of 133 isolates of V. inaequalis from a single commercial apple orchard sampled from one cultivar carrying the Vf gene (Judeline) and three cultivars devoid of the Vf gene. Both analyses indicated a strong decrease of the genetic diversity among isolates from the Vf cultivar compared with the high level of diversity among isolates from the three other cultivars. This leads to a high genetic differentiation between virVf and avrVf groups (F(ST) > 0.17). Analyses of the genetic distance between AFLP patterns based on the Jaccard index indicate that all virVf isolates could be assigned to a single clonal lineage. These results lead us to conclude that the clonal structure of the population isolated from the Vf cultivar is an example of a founder effect in response to a resistance gene breakdown and it is likely that this event occurred in the orchard during the sampling year.     

Host Range of Tropical and Sub-tropical Isolates of Polymyxa graminis

The host range of Polymyxa graminis isolates originating from peanut clump-infested areas in India (Andhra Pradesh and Rajasthan), Pakistan and Senegal was studied on monocotyledonous and dicotyledonous cultivated species, using known quantities of sporosori as inoculum. Profuse multiplication occurred only on some graminaceous species, but the various isolates showed different host specificity. All the isolates produced high infection on sorghum and pearl millet, and all but one isolate from Rajasthan infected maize. Wheat, rye and barley were susceptible to some of the tested isolates. The isolates from Rajasthan and Pakistan produced moderate to severe infection on at least one of these species. On rice, groundnut and sugar beet, only traces of infection by some isolates were detected, whereas no infection was observed on mustard and sunflower. Differences of susceptibility in Pennisetum spp. and Sorghum spp. were demonstrated. The variations in host specificity among isolates from peanut clump-infested areas may result from an adaptation of P. graminis populations to various biotopes. The implications of these results for the management of peanut clump disease are discussed. A comparison of the host ranges of isolates of P. graminis and P. betae from temperate areas demonstrated that distinct types of Polymyxa might be identified based on their relative ability to multiply on susceptible species. Nevertheless, overlapping in the host ranges among the different Polymyxa types, characterised by distinct ecological and genomic features, raises doubts about the host range as a classification criterion for the Polymyxa genus.