Effects of Initial Nematode Density on Population Dynamics of Globodera rostochiensis on Resistant and Susceptible Potatoes

The influence of resistant and susceptible potato cultivars on Globodera rostochiensis population density changes was studied at different nematode inoculum levels (Pi) in the greenhouse and field. Soil in which one susceptible and two resistant cultivars were grown and fallow soil in pots was infested with cysts to result in densities of 0.04-75 eggs/cm³ soil. A resistant cultivar was grown in an infested field with Pi of 0.7-16.7 eggs/cm³ soil. Pi was positively correlated with decline of soil population densities due to hatch where resistant potatoes were grown in the greenhouse and in the field but not in fallow soil. However, Pi was not correlated with in vitro hatch of G. rostochiensis cysts in water or potato root diffusate. Under continuous culture o f a resistant cultivar, viable eggs per cyst declined 60-90% per plant growth cycle (4 weeks) and the number of cysts containing viable eggs had decreased by 77% after five cycles. The rate of G. rostochiensis reproduction on both resistant and susceptible cultivars was negatively correlated with Pi. These data were used to predict the effect of resistant and susceptible potato cultivars on G. rostochiensis soil population dynamics.     

Interactions between Heterodera avenae and Pratylenchus neglectus on Wheat

In a long-term field experiment, differential population densities of Heterodera avenae were produced by frequent cropping with resistant (cv. Panema) or susceptible (cv. Peniarth) oat. The two oat cultivars were equally good hosts of Pratylenchus neglectus in a glass house experiment with field soil. On wheat crops grown after oats in field experiments, P. neglectus population densities in roots were higher in plots where H. avenae had been controlled than in plots with moderate infestations (40 H. avenae eggs/g soil). The field observations indicated that the reduction in population densities of P. neglectus coincided with the development in roots of sedentary stages of the cyst nematode. Evidence for an indirect effect of H. avenae on P. neglectus was found in vitro in a split-root experiment. In the same field, grain yields of two wheat cultivars susceptible or resistant to H. avenae, but both susceptible to P. neglectus, was not reduced by P. neglectus. Alternation of H. avenae resistant and susceptible cultivars is a possible way of exploiting the inverse relationship between these nematodes, whilst controlling cyst nematode -populations in intensive cereal production systems.     

Impact of Planting 'Bell', a Soybean Cultivar Resistant to Heterodera glycines, in Wisconsin

Although the soybean cyst nematode (SCN), Heterodera glycines, has been known to exist in Wisconsin for at least 14 years, relatively few growers sample for SCN or use host resistance as a means to manage this nematode. The benefit of planting the SCN-resistant cultivar Bell on a sandy soil in Wisconsin was evaluated in 1992 and 1993. A range of SCN population densities was achieved by planting 11 crops with varying degrees of susceptibility for 1 or 2 years before the evaluation. Averaged over nematode population densities, yield of ''Bell'' was 30 to 43% greater than that of the susceptible cultivars, ''Corsoy 79'' and ''BSR 101''. Counts of cysts collected the fall preceding soybean were more predictive of yield than counts taken at planting. Yields of all three cultivars were negatively related (P < 0.001) to cyst populations. Fewer (P < 0.01) eggs were produced on ''Bell'' than on the susceptible cultivars. The annual (fall to fall) change in cyst population densities was dependent on initial nematode density for all cultivars in 1992 and for the susceptible cultivars in 1993. Yield reductions induced by the SCN under the conditions of this study indicate that planting a SCN-resistant cultivar in Wisconsin can be beneficial if any cysts are detected.     

Generalized Linear Dynamics of a Plant-Parasitic Nematode Population and the Economic Evaluation of Crop Rotations

In 1-year experiments, the final population density of nematodes is usually modeled as a function of initial density. Often, estimation of the parameters is precarious because nematode measurements, although laborious and expensive, are imprecise and the range in initial densities may be small. The estimation procedure can be improved by using orthogonal regression with a parameter for initial density on each experimental unit. In multi-year experiments parameters of a dynamic model can be estimated with optimization techniques like simulated annealing or Bayesian methods such as Markov chain Monte Carlo (MCMC). With these algorithms information from different experiments can be combined. In multi-year dynamic models, the stability of the steady states is an important issue. With chaotic dynamics, prediction of densities and associated economic loss will be possible only on a short timescale. In this study, a generic model was developed that describes population dynamics in crop rotations. Mathematical analysis showed stable steady states do exist for this dynamic model. Using the Metropolis algorithm, the model was fitted to data from a multi-year experiment on Pratylenchus penetrans dynamics with treatments that varied between years. For three crops, parameters for a yield loss assessment model were available and gross margin of the six possible rotations comprising these three crops and a fallow year were compared at the steady state of nematode density. Sensitivity of mean gross margin to changes in the parameter estimates was investigated. We discuss the general applicability of the dynamic rotation model and the opportunities arising from combination of the model with Bayesian calibration techniques for more efficient utilization and collection of data relevant for economic evaluation of crop rotations.     

Root Growth of Susceptible and Resistant Potato Cultivars and Population Dynamics of Globodera rostochiensis in the Field

Globodera rostochiensis population densities and potato root growth were measured in field plots of one susceptible and two resistant potato cultivars. Root growth and nematode densities were estimated from soil samples taken at three depths between plants within the rows, three depths 22.5 cm from the rows, and at two depths midway between rows (furrows). Four weeks after plant emergence (AE), nematode densities in the rows had declined 68% in plots of the susceptible cultivar and up to 75% in plots of both resistant cultivars. Significant decline in nematode densities in the furrows 4 weeks AE occurred only in plots of the susceptible cultivar. Total decline in nematode density in fallow soil was 50%, whereas in plots of the resistant cultivars, decline was more than 70% in the rows and more than 50% in the furrows. Nematode densities increased in the rows of the susceptible cultivar but declined in the furrows. We conclude that G. rostochiensis decline or increase is correlated with host resistance and the amount of roots present at any particular site.     

A Simulation Model of Heterodera schachtii Infecting Beta vulgaris

A simulation model of a single sugarbeet, Beta vulgaris L., plant infected by the sugarbeet cyst nematode, Heterodera schachtii Schmidt, was developed using published information. The model is an interactive computer simulation programmed in FORTRAN. Given initial population densities of the nematode at planting, the model simulates nematode population dynamics and the growth of plant tap and fibrous roots. The driving variable for nematode development and plant growth is temperature.     

Spatial Analysis of Heterodera glycines Populations in Field Plots

Spatial heterogeneity in nematode population densities presents an obstacle to the precise determination of infestation levels. Three field plots were intensively sampled for soybean cyst nematode (Heterodera glycines Ich.) cysts before and after spring cultivation to quantify the spatial attributes of the population. Population density strata were detected running parallel to plant rows. Highest population densities before cultivation were found in the plant row and the middle furrow, Population density in the plant row averaged 26% higher and 4% lower than the whole-plot mean before and after cultivation, respectively. Cysts containing fewer than 25 eggs were not stratified, indicating that most were produced before the previous season. Sample population counts were fit to the negative binomial distribution model before cultivation, but distributions differed among plots. The Neyman type A and negative binomial distributions both fit the data after cultivation disturbed the soil. Population clusters 1-3 m long were detected in plant beds before cultivation. Heterogeneity in population density increased with plant row length after cultivation. Optimum plot length for minimal spatial heterogeneity in four-row mechanically tended field plots was estimated at 6 m after trimming plot ends.     

Comparative Efficiency of the Fenwick Can and Schuiling Centrifuge in Extracting Nematode Cysts from Different Soil Types

The Fenwick can and Schuiling centrifuge are widely used to extract nematode cysts from soil samples. The comparative efficiencies of these two methods during cyst extraction have not been determined for different soil types under different cyst densities. Such information is vital for statutory laboratories that must choose a method for routine, high-throughput soil monitoring. In this study, samples of different soil types seeded with varying densities of potato cyst nematode (Globodera rostochiensis) cysts were processed using both methods. In one experiment, with 200 ml samples, recovery was similar between methods. In a second experiment with 500 ml samples, cyst recovery was higher using the Schuiling centrifuge. For each method and soil type, cyst extraction efficiency was similar across all densities tested. Extraction was efficient from pure sand (Fenwick 72%, Schuiling 84%) and naturally sandy soils (Fenwick 62%, Schuiling 73%), but was significantly less efficient from clay-soil (Fenwick 42%, Schuiling 44%) and peat-soil with high organic matter content (Fenwick 35%, Schuiling 33%). Residual moisture (<10% w/w) in samples prior to analyses reduced extraction efficiency, particularly for sand and sandy soils. For each soil type and method, there were significant linear relationships between the number of cysts extracted and the numbers of cysts in the samples. We discuss the advantages and disadvantages of each extraction method for cyst extraction in statutory soil laboratories.     

Morphological and Molecular Identification of Globodera pallida Associated with Potato in Idaho

The identity of a newly discovered population of pale potato cyst nematode Globodera pallida associated with potato in eastern Idaho was established by morphological and molecular methods. Morphometrics of cysts and second-stage juveniles were generally within the expected ranges for G. pallida with some variations noted. The Idaho population and paratype material from Epworth, Lincolnshire, England, both showed variations in tail shape, with bluntly rounded to finely pointed tail termini. Compared to literature values for the paratypes, second-stage juveniles of the Idaho population had a somewhat shorter mean body length, and cysts had a slightly higher mean distance from the anus to the nearest edge of the fenestra. PCR-RFLP of the rDNA ITS region, sequence-specific multiplex PCR and DNA sequence comparisons all confirmed the identity of the Idaho population as G. pallida. The ITS rDNA sequence of the Idaho isolate was identical to those from York, England, and the Netherlands. Species-specific primers that can positively identify the tobacco cyst nematode Globodera tabacum were also developed, providing a new assay for distinguishing this species from G. pallida and the golden potato cyst nematode Globodera rostochiensis.     

Population Dynamics of Dactylella oviparasitica and Heterodera schachtii: Toward a Decision Model for Sugar Beet Planting

A series of experiments were performed to examine the population dynamics of the sugarbeet cyst nematode, Heterodera schachtii, and the nematophagus fungus Dactylella oviparasitica. After two nematode generations, the population densities of H. schachtii were measured in relation to various initial infestation densities of both D. oviparasitica and H. schachtii. In general, higher initial population densities of D. oviparasitica were associated with lower final population densities of H. schachtii. Regression models showed that the initial densities of D. oviparasitica were only significant when predicting the final densities of H. schachtii J2 and eggs as well as fungal egg parasitism, while the initial densities of J2 were significant for all final H. schachtii population density measurements. We also showed that the densities of H. schachtii-associated D. oviparasitica fluctuate greatly, with rRNA gene numbers going from zero in most field-soil-collected cysts to an average of 4.24 x 10⁸ in mature females isolated directly from root surfaces. Finally, phylogenetic analysis of rRNA genes suggested that D. oviparasitica belongs to a clade of nematophagous fungi that includes Arkansas Fungus strain L (ARF-L) and that these fungi are widely distributed. We anticipate that these findings will provide foundational data facilitating the development of more effective decision models for sugar beet planting.     

Control of potato cyst nematode (Globodera pallida) by host plant resistance and nematicide

Seven trials conducted over four years on sites naturally infested with the white potato cyst nematode established that potato clones bred for resistance to Globodera pallida allowed significantly less nematode multiplication than conventional cultivars under field conditions. Nematode multiplication was inversely related to initial infestation level. The nematicide, aldicarb, significantly reduced nematode multiplication. However, nematode multiplication on nematicide treated susceptible cultivars was greater than on untreated partially resistant clones, indicating that resistance may offer more effective control of G. pallida than chemical treatment. Integration of host plant resistance and nematicide treatment is discussed.     

Competition between the Plant-parasitic Nematodes Pratylenchus neglectus and Meloidogyne chitwoodi

In experiments on competition between Pratylenchus neglectus and Meloidogyne chitwoodi in barley, the species that parasitized the roots first inhibited penetration by the latter species. Prior presence of P. neglectus impeded the development of M. chitwoodi. Pratylenchus neglectus reduced egg production, final population levels, and reproductive index of M. chitwoodi. The reduction was linearly related to initial population densities of P. neglectus. Initial population densities of M. chitwoodi had no effect on final population levels of P. neglectus. Carbon assimilation by barley plants was reduced when either nematode species was present alone, but not when both were present together. Both nematode species assimilated lower amounts of carbon when present together than when present alone. A split-root experiment demonstrated that translocatable chemicals were not involved in the competition between the two species.     

A field study of the relationship between pre-planting density of Globodera pallida and the growth and yield of two potato cultivars of differing tolerance

Non-resistant but tolerant cv. Cara and non-resistant but relatively intolerant cv. Pentland Dell were grown in split plots encompassing a range of population densities of potato cyst nematode, Globodera pallida. Light interception and its efficiency of conversion were estimated by regular ground cover measurements and plant harvests. It was concluded that increasing levels of infestation with G. pallida only slightly decreased the efficiency of utilisation of intercepted radiation. Heavy infestation of G. pallida initially decreased the top growth and light interception of both cultivars by similar proportions, but in later harvests, this adverse effect markedly decreased for Cara whereas it slightly increased for Pentland Dell. This difference was due to the heavily infested Cara eventually achieving and maintaining 100% ground cover whereas the equivalent Pentland Dell never exceeded 75% ground cover. Consequently, final tuber yields were decreased much more for Pentland Dell than for Cara though the decreases in tuber yield were less than those for top growth. The importance of nematode effects on top growth, and hence on light interception, with regard to both yield losses and tolerance differences, were clearly demonstrated. Both linear and logarithmic models were used to describe the relationship between the initial population density of G. pallida and yield, and the implications of differences in tolerance on the parameters in the logarithmic model are discussed.     

Reproduction of Pratylenchus penetrans on Potato and Crops Grown in Rotation with Potato

The relative suitability of potato and crops frequently grown in rotation with potato as hosts for Pratylenchus penetrans was evaluated. Suitability of rye, wheat, corn, oat, sorgho-sudangrass, and potato were compared in pot studies based on ratios of final population : initial population density and densities of nematodes in roots at harvest. Population densities increased more on potato, oat, and corn than on rye, wheat, and sorgho-sudangrass. There were no differences among the four rye cultivars or between the two oat cultivars in host suitability. Population increases were not related to root weight or consistently to nematode densities in roots. Although rye and wheat were equally suitable hosts in pot studies, P. penetrans increased more on wheat than on rye in a field study, indicating that reproduction was reduced or mortality was increased on rye under field conditions.     

Effect of Initial Nematode Density on Managing Globodera rostochiensis with Resistant Cultivars and Nonhosts

Cropping systems in which resistant potato cultivars were grown at different frequencies in rotation with susceptible cultivars and a nonhost (oats) were evaluated at four initial nematode population densities (Pi) for their ability to maintain Globodera rostochiensis at a target level of <0.2 egg/cm³ of soil. At a Pi of 0.1 to 1 egg/cm³ of soil, cropping systems with 2 successive years of a resistant cultivar every 3 years of potato production reduced and maintained G. rostochiensis at <0.2 egg/cm³ of soil. At a Pi of 1 to 4 eggs/cm³ of soil, 2 successive years of a resistant cultivar followed by 1 year of oats for every 4 years of production were necessary to reduce and maintain G. rostochiensis populations at <0.2 egg/cm³ of soil. At a Pi greater than 4 eggs/cm³ of soil, 2 successive years of a resistant cultivar plus 1 year of oats reduced G. rostochiensis densities to <0.2 egg/cm³ of soil, but the population increased above that density after cropping 1 year to a susceptible cultivar. The numbers of cysts and eggs per cyst in the final population (Pf) of G. rostochiensis were influenced by initial density and the frequency of growing a susceptible cultivar in a cropping system. The lowest number of cysts and eggs per cyst in the final G. rostochiensis population occurred with a cropping system consisting of 2 successive years of a resistant cultivar followed by oats with a susceptible cultivar grown the fourth year of production.     

Pratylenchus penetrans Population Dynamics on Three Potato Cultivars

Reproduction of Pratylenchus penetrans on the potato cultivars Hudson, Katahdin, and Superior was determined in greenhouse and field microplot experiments. Although all three cultivars were good hosts for P. penetrans, differences in reproductive rate were found. In one greenhouse experiment, Katahdin plants inoculated with 1,500 or 15,000 P. penetrans per pot had larger population densities at harvest than did Superior; however differences between these cultivars were not significant in three other greenhouse experiments. In another experiment, population densities of P. penetrans on Hudson did not differ from those on Katahdin and Superior when inoculated with 270 and 5,080 nematodes per pot after 45 days in the greenhouse. However, population densities were usually higher on Hudson and Katahdin than on Superior in field microplots at four initial population densities during two seasons. Higher population densities on Hudson were detectable 304 days after planting in one of the two microplot studies. The juvenile:female and the male:female ratios were sometimes larger on Katahdin than on Superior, but differences were inconsistent. There was no evidence of resistance in the three cultivars evaluated, but reproduction was generally highest on Hudson and lowest on Superior.     

Effect of Initial Nematode Population Density on the Interaction of Pratylenchus penetrans and Verticillium dahliae on 'Russet Burbank' Potato

Four similar growth chamber experiments were conducted to test the hypothesis that the initial population density (Pi) of Pratylenchus penetrans influences the severity of interactive effects of P. penetrans and Verticillium dahliae on shoot growth, photosynthesis, and tuber yield of Russet Burbank potato. In each experiment, three population densities of P. penetrans with and without concomitant inoculation with V. dahliae were compared with nematode-free controls. The three specific Pi of JR penetrans tested varied from experiment to experiment but fell in the ranges 0.8-2.5, 1.8-3.9, 2.1-8.8, and 7.5-32.4 nematodes/cm³ soil. Inoculum of V. dahliaewas mixed into soil, and the assayed density was 5.4 propagules/gram dry soil. Plants were grown 60 to 80 days in a controlled environment. Plant growth parameters in two experiments indicated significant interactions between P. penetrans and V. dahliae. In the absence of V. dahliae, P. penetrans did not reduce plant growth and tuber yield below that of the nematode-free control or did so only at the highest one or two population densities tested. In the presence of K dahliae, the lowest population density significantly reduced shoot weight and photosynthesis in three and four experiments, respectively. Higher densities had no additional effect on shoot weight and caused additional reductions in photosynthesis in only one experiment. Population densities of 0.8 and 7.5 nematodes/cm³ soil reduced tuber yield by 51% and 45%, whereas higher densities had no effect or a 15% additional effect, respectively. These data indicate that interactive effects between P. penetrans and V. dahliae on Russet Burbank potato are manifested at P. penetrans population densities less than 1 nematode/cm³ soil and that the nematode population density must be substantially higher before additional effects are apparent.     

Field trials using single potato plants as whole plots, with and without nematicide treatment, to assess tolerance of attack by Globodera rostochiensis

It is important to assess the tolerance of new potato cultivars with partial resistance to potato cyst nematode attack. The experiments described here are intended to quantify tolerance adequately using a minimum of resources. When test plants were spaced widely in the field trials it was also possible to estimate their resistance levels but standard plant spacing allowed roots of neighbouring plants to encroach on a test plant's root space and to interfere with attempts to assay resistance. Standard plant spacing did, however, give more realistic yields per plant and competition from neighbouring plants was standardised by surrounding each test plant by four identical guard plants (cv. Désirée). Regressions of yield on severity of nematode attack were drawn when nematode infestation varied naturally across the trial site or when different levels of attack were created by applying nematicide at a range of dose rates. Cara was the most tolerant cultivar tested, with a complete spectrum of tolerance shown by other cultivars/clones. Some clones with a high degree of resistance were intolerant and it is precisely these that this type of test seeks to identify.     

Effects of three rates of aldicarb and of different degrees of resistance and tolerance in potato cultivars on yield and post-harvest population densities of the potato cyst nematodes Globodera rostochiensis and G. pallida

Three field experiments were made to determine the effectiveness of small-plot trials in detecting differences between potato cultivars/clones in their tolerance of damage by potato cyst-nematodes. A nematicide (aldicarb) was applied at three rates to decrease nematode damage. The largest rate of aldicarb increased tuber yields most but the relationship between yield response and nematicide rate was not linear. The yield increases of the cultivars and clones differed, indicating that they have different degrees of tolerance of potato cyst nematodes. The results were analysed in several ways and the untreated yield as a proportion of the treated provided the best means of expressing and comparing tolerance; but whichever method was used the tolerance rankings of the cultivars and clones were similar. At two sites infested with Globodera rostochiensis, the rankings of the 10 cultivars and clones were similar but at a third site, heavily infested with G. pallida, they were different. Aldicarb decreased the nematode population density after harvest at the G. pallida site but was less effective at the G. rostochiensis sites, which were less heavily infested. Growing resistant or partially resistant potatoes usually prevented nematode increase, and the more resistant cultivars and clones decreased population densities markedly.     

Influence of Alfalfa Plant Growth on the Multiplication Rates and Ceiling Population Density of Meloidogyne hapla

The rates of reproduction and multiplication of Meloidogyne hapla decreased as a result of self-regulatory, density-dependent processes with time and nematode population increase in the soil and roots of Medicago sativa cv. Cuf 101. Juvenile, egg, and mature female population densities increased at a maximum rate until damage to the host resulted in alfalfa yield reductions. Temporal differences in multiplication and reproduction rates of M. hapla were observed to be a function of initial population density (Pi), host damage, and root biomass, indicating increased levels of competition for a constant but limited number of feeding sites. Over time, a log linear relationship emerged between multiplication rate of M. hapla and Pi. Slopes of -0.90953 for combined eggs and juveniles and -0.71349 for mature females indicated a gradual approach to ceiling densities. Reproductive rates decreased exponentially from an initial maximal value of 200 to a relatively constant rate of 53 eggs per female.