Potato cyst nematodes: a critical review of the current pathotyping scheme1

The pathotype scheme proposed by Kort et al. (1977) is critically reviewed. It is proposed that many of their pathotypes are artifacts resulting from their using an arbitrary Pf/Pi value of ± 1 and differential clones with quantitatively based polygenic resistance to separate pathotypes. It is concluded that most European populations of potato cyst nematodes are heterogeneous for virulence genes and that Pf/Pi measurements are influenced too readily by several factors, including initial population density, the vigour of plant growth, and the viability of the nematode, to be used as either a measure of nematode virulence or plant resistance. Instead, reproduction should be related to that on appropriate non-resistant or partially resistant controls and the aim should be to quantify the proportion of virulent individuals in the population. For greatest accuracy it will be necessary to use procedures which minimize genotype × environmental interactions.     

Problems of potato cyst nematode pathotypes in the Czech Republic

During 1983–1989 about 1,200 infested plots in the Czech republic were investigated for the occurrence of new pathotypes of the potato cyst nematodes (PCN) Globoderu rostochiensis and G. pallida. Only eight populations of the pest were suspected of containing another pathotype than Ro1. Glasshouse pot trials were performed with those eight natural populations, two populations of Ro1, 10 potato cultivars resistant to Ro1 and 10 cultivars resistant to more pathotypes, G. pallida included. The multiplication rates (Pf/Pi) showed that four of the PCN populations were probably mixtures, with pathotype Ro1 prevailing. Three populations were suspected of containing G. pallida , but their morphological characters corresponded only with G. rostochiensis. One population corresponded with pathotype Ro5. The extent of the occurrence of pathotypes or virulence groups different from Ro1 can be estimated at 0.3–0.4% of infested plots.     

Species,subspecies, race and pathotype problems in nematodes1

By reference mainly to cyst nematodes, certain aspects concerning the species category and intraspecific categories are considered. Special attention is paid to the existence of cryptic or sibling species, the possibility of interspecific hybridization, the phenomenon of reduced reproductive compatibility of certain populations within a species and the loss of subspecies and race identity in mixtures. Both terms ‘biological or physiological race’ and ‘pathotype’ can be used to express differences in host specificity in nematology, entomology and plant pathology. A distinction is made between the two categories and differences among them are outlined in detail. Whereas ‘race’ is considered as a population concept, pathotypes are defined as virulence phenotypes. Races are genetically variable groups of related populations, while pathotypes are a priori‘artificial’ entities, established for practical reasons and characterized by constancy of their relevant characters.     

Potato cyst nematodes in the Canary Islands: an epidemiologic model for the Mediterranean region1

This work describes studies done on the Canary Islands in order to support the elaboration of quarantine and control legislation for potato cyst nematodes which take into account the special agroecological characteristics of the Mediterranean region. The interest of the islands for epidemiological studies is based on the occurrence of the oldest potato crops in the Old World and the use of several local cultivars and traditional management cropping systems. Also, the environmental conditions allow field experiments to be performed throughout the year. The phytonematological problems of the potato crop on the Canary Islands is reported and the absence of Ditylenchus destructor, D. dipsaci, Nacobbus aberrans, Rotylenchulus reniformis and trichodorids pointed out. The cyst nematodes, Globodera rostochiensis and G. pallida were found to cause severe damage to the crop and were studied in detail by field trials at Tenerife. Continuous growing of the cv. Cara, with the H1 gene conferring resistance to G. rostochiensis (pathotypes Ro1 and Ro4), exerted selection pressure for G. pallida. Under the edaphic and climatic conditions of Tenerife, nematicide application was found to be inefficient for controlling potato cyst nematodes. Alternative control techniques based on traditional management systems and on the use of environmentally adverse conditions were investigated.     

The Basis of Predictive Modelling for Estimating Yield Loss and Planning Potato Cyst Nematode Management

The basis of modelling yield loss and population dynamics relations of potato cyst nematodes is that both are strongly density-dependent. Potato cyst nematodes (PCN; Globodera pallida and G. rostochiensis) are particularly suitable for analysing such relationships because they have only one generation per year, potato is their only field host, the juvenile nematodes within the egg are very durable (up to 20 years persistence), and they hatch mainly in response to specific chemicals exuded from host roots. Small populations increase the most, up to 50-fold, when a potato crop is grown. Multiplication rates decrease as the population density increases because damage decreases root system size and increases competition so that very large populations may actually be decreased when potatoes are grown. The newly formed eggs have a ‘half-life’ of c.two years when non-host, rotational crops are being grown. Control is achieved largely by the use of rotation, the application of nematicidal chemicals, and growing resistant cultivars. As rotations are shortened, so PCN populations will be increased, and crop damage becomes more likely. Thresholds for damage vary with both soil type (greatest on sandy soils) and potato cultivar. Cultivars differ in their tolerance of PCN damage depending on how vigorously they grow, on their root sensitivity to damage from the PCN juveniles which invade close to root tips, on cultivar resistance which decreases the parasitic effect by reducing the numbers of developing PCN females, and on various environmental factors such as the amounts of fertiliser applied. The dependence of the yield-loss relationship on population density, soil type and cultivar effects has been described in a simple equation and assessed using field-trial data. Effects on yield are described in proportion to the PCN-free yield but the addition of information on expected yield (in tonnes ha⁻¹) in the absence of PCN renders this equation predictive. Nematicides are widely applied to infestations of potato cyst nematodes, both to prevent the crop from being damaged and to prevent population increase which could hazard the next potato crop in the rotation, but they are generally more effective at preventing the former than the latter. A complex equation has also been developed to model the population dynamics of PCN. This equation incorporates a factor for host-crop growth and tolerance (from the yield-loss equation) and also the effects of host resistance. This latter is particularly relevant to G. pallida, where all the resistance currently available is determined by minor genes and hence is ‘quantitative’ or ‘partial’. Effects of rotation and of nematicides can also be incorporated into this model. To provide a realistic prediction also requires accurate information on PCN population densities, species composition and distributions, and rates of PCN population decline between potato crops.     

Characterization of virulence reactions for Heterodera avenae populations from two localities in Algeria

Heterodera avenae is widely distributed in areas where most cereal crops are produced in Algeria. However, the virulence of the Algerian populations of this nematode on individual cereal species and cultivars has not been well documented. The virulence of H. avenae populations from Tiaret and from Oued Smar were tested under natural outdoor conditions and in an in vitro test to determine reactions of nine barley, oat and wheat cultivars selected from the International differential assortment for identifying pathotypes of H. avenae. All nine cereal differentials expressed the same reactions to both populations. The nematodes reproduced well on the barley cultivar ‘Emir’ and the wheat cultivar ‘Capa’. Resistant entries included the barley cultivars ‘Siri’, ‘Ortolan’ and ‘Morocco’, the oat cultivars ‘Nidar II’ and ‘A. sterilis I.376’, and the wheat cultivars ‘Loros’ and ‘AUS10894’. This matrix of reactions indicated that H. avenae populations from both locations were characterized as H. avenae Group 1 pathotypes but did not conclusively distinguish among pathotypes Ha21, Ha31 or Ha81. The Cre1 gene was identified as a potentially valuable source of resistance when developing wheat cultivars intended for release into these localities.     

Multiple Selection of Potato Cyst Nematode Globodera pallida Virulence on a Range of Potato Species. I. Serial Selection on Solanum-hybrids

A series of selection experiments on potato cyst nematode Globodera pallida, pathotype Pa1, tested the virulence response of the nematode to a range of resistant potato Solanum genotypes. Alleles conferring virulence against all four Solanum sources used in the study (i.e. Solanum vernei, S. multidissectum, S. sanctae-rosae and S. tuberosum ssp. andigena) were detected. Selection for multiple virulence against a combination of resistant sources resulted in the originally-selected virulence genes being retained or lost. The mechanism, or basis, of potato cyst nematode resistance differs between the Solanum species. The appropriate use of resistance cultivars produced from a range of Solanum-resistant species offers a management tool for controlling potato cyst nematode levels in infested land.     

成都地区黄秋葵蚜虫天敌种类及其类群动态研究

2014-2015年,调查研究了成都地区黄秋葵蚜虫发生特点、其天敌资源种类和优势天敌类群发生动态,发现成都地区为害黄秋葵的蚜虫主要是棉蚜,其天敌资源有5目8科21种,优势种有棉平突蚜茧蜂、异色瓢虫、龟纹瓢虫和大草蛉。棉蚜发生量在黄秋葵移栽40d即达到高峰期,后逐渐下降,棉平突蚜茧蜂发生动态与棉蚜的发生动态具有较强的跟随关系,瓢虫、草蛉和食蚜蝇的发生动态差异较大,与棉蚜的发生动态跟随关系不明显。     

The use of isoelectric focusing as a tool in the identification and management of potato cyst nematode populations1

Isoelectric focusing on thin layers of polyacrylamide or agarose gels may be used to separate proteins from the potato cyst nematodes Globodera rostochiensis and G. pallida. General protein patterns may be used to identify the two species, even from single cysts. Densitometry of species-specific protein bands may be used to assess the proportions of G. rostochiensis and G. pallida in potato cyst nematode samples. Staining for the enzymes phosphoglucomutase and phosphoglucose isomerase also revealed species-specific patterns, and variation among G. pallida populations was observed. Thus calculations of coefficients of similarity based on six enzymes, phosphoglucose isomerase, phosphoglucomutase, glycerol-3-phosphate dehydrogenase, enolase, hexokinase and malate dehydrogenase and the construction of a dendrogram for several Northern Ireland populations indicated clear separations between G. pallida (Pa1), G. pallida (Pa3) and G. rostochiensis (Ro1). The use of the technique of isoelectric focusing as a routine research and advisory tool in nematology, and as a means of further understanding the genetic basis of pathotype schemes, is discussed.     

Cereal and Grass Hosts of Some Gramineous Cyst Nematodes1

The host ranges of some British populations of cyst nematodes from cereals and grasses have been studied. The nematodes were isolated from agricultural and natural habitats. Their morphology corresponded to the Heterodera avenaeImani type, to ≪ pathotype 3 ≫, or to the H. hordecalisIlatipons type. The host series included barley, oats and wheat, some of which were known to be resistant to H. avenae pathotypes, and agricultural and wild grasses. Many of the H. avenae-resistant cereals were susceptible to pathotype 3 and to a population resembling H. avenae I mani, which had a very wide host range including grasses. The population of H. hordecalis, collected from grasses, also overcame the resistance of gene Rha2. Two populations of H. mani reproduced to a limited extent on cereals, and showed a clear preference for grass hosts. A third H. mani population reproduced only on grasses. The results draw attention to some problems in conducting and interpreting tests with many different hosts and nematodes. They reinforce the view that these gramineous cyst nematodes are a species complex, with variations in host range, as well as in morphology and biology.     

Distribution and frequency of occurrence of potato cyst nematode pathotypes in Northern Ireland

The distribution and frequency of occurrence of potato cyst nematodes, Globodera rostochiensis and G. pallida , were examined in Northern Ireland. Globodera rostochiensis (pathotype Rol) was the most abundant species, while Pal and Pa3 pathotype reactions could be discerned within G. pallida. Unlike other parts of the UK, species proportions appear not to have changed over the last 20 years, probably as a result of the legislative control currently practised combined with the limited use of Rol-resistant cultivars within the country. Differences in the geographical distributions of the three pathotypes were observed, and these are discussed in relation to original potato cyst nematode introductions and subsequent agricultural practices.     

A biotechnological strategy involving monoclonal antibodies for improvement of potato farming by identification and quantification of potato cyst nematodes in soil samples1

The two closely related nematode species Globodera rostochiensis and G. pallida are one of the major problems encountered in potato cultivation. There is a spectrum of potato plant genes known, which confer resistance to these species and their pathotypes. Potato growing in The Netherlands has to follow strict rules to control spread of the pests. Since distinction between the two nematode species is difficult, a rapid and reliable identification method is needed to allow better use of existing and forthcoming resistant potato cultivars. The aims of this project were: (1) identification and partial purification of species-specific proteins from the nematodes, (2) production of species-specific monoclonal antibodies, and (3) development of a screening test for qualitative and quantitative determination of Globodera spp. in soil samples.     

Is virulence phenotype evolution driven exclusively by Lr gene deployment in French Puccinia triticina populations?

Puccinia triticina is a highly damaging wheat pathogen. The efficacy of leaf rust control by genetic resistance is mitigated by the adaptive capacity of the pathogen, expressed as changes in its virulence combinations (pathotypes). An extensive P. triticina population survey has been carried out in France over the last 30 years, describing the evolutionary dynamics of this pathogen in response to cultivar deployment. We analysed the data set for the 2006–2016 period to determine the relationship between the Lr genes in the cultivars and virulence in the pathotypes. Rust populations were dominated by a small number of pathotypes, with variations in most of the virulence frequencies related to the corresponding Lr gene frequencies in the cultivated landscape. Furthermore, the emergence and spread of a new virulence matched the introduction and use of the corresponding Lr gene (Lr28), confirming that the deployment of qualitative resistance genes is an essential driver of evolution in P. triticina populations. However, principal component analysis (PCA) revealed that certain pathotype–cultivar associations cannot be explained solely by the distribution of Lr genes in the landscape. This conclusion is supported by the predominance of a few pathotypes on some cultivars, with the persistence of several other compatible pathotypes at low frequencies. Specific interactions are not, therefore, sufficient to explain the distribution of virulence in rust populations. The hypothesis that quantitative interactions between P. triticina populations and bread wheat cultivars—based on differences in aggressiveness—is also a driver of changes in pathotype frequencies deserves further investigation.     

Selection of nematodes by resistant plants has implications for local adaptation and cross‐virulence

The variability of resistance durability in different potato genotypes harbouring the same resistance QTL but differing by their genetic background was explored. The indirect consequences of the resistance adaptation in terms of local (i.e. genotype‐specific) adaptation and cross‐virulence was also investigated. Following the virulence of the potato cyst nematode Globodera pallida in a long‐term experimental evolution protocol, the results showed that nematode populations were able to adapt to the resistance of four potato genotypes carrying the QTL GpaV from Solanum vernei, and that the plant genetic background has an impact upon the durability of resistance. The pattern of local adaptation observed here indicates that divergent selection has occurred during the experimental evolution performed from the same initial nematode population, and revealed a trade‐off between the adaptation to a resistant potato genotype and the adaptation to another resistant genotype differing in its genetic background. In terms of cross‐virulence between potato genotypes derived from different resistance sources (S. sparsipilum and S. spegazzinii), this study shows that the adaptation to resistance QTL GpaV_vrn does not necessarily allow the adaptation to collinear GpaV loci. The results presented here could be useful for predicting evolution of nematode populations in natural agro‐ecosystems and identifying durable strategies for resistance deployment.     

Low pathotype diversity in a recombinant Puccinia striiformis population through convergent selection at the eastern Himalayan centre of diversity (Nepal)

Worldwide Puccinia striiformis f. sp. tritici (Pst) epidemics have been reported to be driven by few genetic lineages, while a high diversity is evident at the Pst Himalayan centre of diversity. This study investigated the relationship between pathotype diversity and genetic structure in Nepal, the eastern Himalayan region, which has been largely unexplored. Despite the high genetic diversity and recombinant structure detected through microsatellite genotyping, characterization of virulence phenotypes for 62 isolates identified only eight pathotypes, with two pathotypes predominant over all the populations. This is in contrast to the Pakistani and Chinese recombinant populations, where high pathotype diversity is associated with genetic diversity. The most prevalent Nepali pathotype was not a unique clonal lineage, but was represented by seven multilocus genotypes from four distinct genetic subgroups, suggesting strong directional selection on virulence genes, resulting in convergent pathotypes in distinct genetic groups. This convergent selection is discussed in comparison with clonal French and recombinant Pakistani populations. Additionally, the Nepali Pst population carried virulence to 17 out of 24 tested yellow rust resistance genes (Yr), with the absence of virulence to Victo and Early Premium and resistance genes Yr5, Yr10, Yr15, Yr24 and Yr26. Virulence to Yr2, Yr7, Yr27 and YrSu were fixed in all isolates, in line with the deployment of these resistance genes in Nepal. The results reflect the influence of resistance gene deployment on selection of virulence and pathotypes in a recombinant pathogen population, which must be considered in the context of durable resistance gene deployment.     

Changes in specific virulence in Polish populations ofPhytophthora infestans: 1985–1991

Ninety-five isolates ofPhytophthora infestans collected throughout Poland during 1985–1991 and characterized for multilocus genotypes based on mating type, allozymes and DNA fingerprint, were analyzed for specific virulence to differential potato cultivars carrying ten major resistance genes. The multilocus analysis led to three groupings. The first group contained 22 isolates of a clonal lineage (PO-1) that is postulated to have been present in Europe during most of the twentieth century, but PO-1 isolates were recovered in Poland only during 1985–1988. This group contained, on average, virulence to 5.5 specific resistance genes per isolate. The second group consisted of 30 isolates in a clonal lineage (PO-4) that had not been detected before 1988. PO-4 isolates had virulence to a mean of 6.5 resistance genes per isolate. The third group was composed of 43 isolates representing 38 multilocus genotypes also not detected before 1988. These diverse genotypes had virulence to an average of 6.7 specific resistance genes per isolate. More than half (53%) of the PO-4 isolates shared a single pathotype. The group of 43 isolates was dominated by two pathotypes: the most common one (47% of the isolates) was the same pathotype that dominated PO-4 isolates; the next most common one (21%) differed from the most common one by the absence of virulence to resistance gene R5. The recent immigrant isolates (not detected before 1988) generally had virulence to a greater number of specific resistance genes than did isolates in the previous population [detected before 1988 (PO-1)]. Recent immigrant populations were dominated by one or two pathotypes, so their pathotypic diversity values were somewhat less than that of the previous population.     

Identification of Pathotypes of the Potato Cyst Nematode1

Since the discovery of pathotypes of the potato cyst nematode (Heterodera rostochiensis Wollenweber, 1923), surveys on the distribution of these pathotypes have been carried out in different European countries. Test varieties of potato were selected according to findings resulting in the present situation of varying distinctive host-plants and of diverging programs for breeding potatoes resistant to the potato cyst nematode. With respect to rational selection and use of resistant potato varieties, there is a great need for standardization in pathotype identification (nomenclature), maintenance and supply of appropriate genotypes of Solanum species and methods for testing resistance in new potato varieties. Anticipating a cross-experiment with pathotypes and test varieties from Great Britain, the Federal Republic of Germany and the Netherlands in 1974, a preliminary test with all recognized or presumed pathotypes from these countries, but with only part of the potential differential host-plants was carried out by the Plant Protection Service at Wageningen. The results of this experiment are discussed.     

The effect of organophosphorus compounds and oxime carbamates on the potato cyst nematode Heterodera rostochiensis woll

The effect of organophosphate and oxime carbamates on the potato cyst nematode, Heterodera rostochiensis was investigated. Although aldicarb slightly delayed emergence of second stage larvae, none of the chemicals decreased total hatch. Aldicarb had a marked effect on second stage larvae in soil which was subsequently reflected in root invasion counts. Comparisons between the initial and final populations (multiplication rate) of the potato cyst nematode are very useful for determining the efficacy of treatment. The evidence from these experiments suggests that organophosphoros compounds and oxime carbamates, although “systemic”, act against plant parasitic nematodes in the soil phase of the life cycle and not in the plant.     

Resistance of Cereals to Heterodera avenae: Methods of Investigation, Sources and Inheritance of Resistance1

Techniques used in cereal breeding programmes for selecting for resistance to cereal cyst nematodes are described. Routine screening is done in field nurseries and in pots of infested soil in glasshouse and controlled environments. Other investigations of resistance use a laboratory technique which permits identification of the penetration sites of individual nematodes. Erosion of resistance, as indicated by 1) the development of only one or two females or 2) the occasional development of more females on plants with resistance genes, is discussed in relation to variation in nematode virulence and to environmental, especially temperature effects. Identification of tolerance is more difficult unless the differences are great, or the experimentation carefully controlled and replicated. Direct selection for tolerance in segregating generations is not presently practicable. Indirectly selecting for mechanisms of tolerance may eventually prove possible but has not yet been exploited. Available sources of resistance are described in relation to genetic studies and complementary studies of nematode pathotype and host range. There are three distinct resistances in barley which may have been responsible for selecting present pathotypes. Two, Rha1 and Rha2, are useful in breeding and linkage and allelism at these loci are described. In oats, as in wheat, only one locus has been used by breeders. There are probably additional loci in some of the other resistance sources but polyploidy of the host and different virulence gene frequencies in unselected nematode populations make it more difficult to identify genetic relationships in oats and wheat.     

Suggestions for Determination and Terminology of Pathotypes and Genes for Resistance in Cyst-forming Nematodes, especially Heterodera avenae1

A short review of differentiation into pathotypes is given. Use of the word ≪ pathotype ≫ is recommended when a very clear difference is established between virulence of nematode populations. Our present knowledge makes it possible to differentiate between 10 pathotypes of Heterodera avenae. It is suggested that the pathotypes are given numbers, and corresponding terms should be used for genes for resistance in plants, e.g. a gene Hal on the barley chromosome gives resistance to nematode pathotypes 11,21,31,41 etc. Some proposals for improvement of pathotype identification are given.