Influence of Heterodera avenae on winter wheat in France: experiments with resistant and susceptible varieties

A succession of oat crops resistant or susceptible to Heterodera avenue produced plots lightly or moderately infested with the pest with little risk of interaction with other soil pathogens in succeeding cereal crops. Using these plots, the effects of the nematode on growth and yield of winter wheat were studied. The nematode affected the crop throughout its development, with a marked effect on yield. The effects of H. avenue are similar to those of water stress and are accentuated to a greater or lesser extent by climatic conditions. In the field the efficacy and durability of resistant genes incorporated in wheat and oats were checked. After four-years' cultivation of oats (cv. Panema), no resistance-breaking pathotypes were detected. Some advanced wheat lines with nematode resistance were also tolerant of nematode attack.     

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.     

Differences in the tolerance of potato cultivars to potato cyst nematodes (Globodera rostochiensis and G. pallida) in field trials with and without nematicides

In field trials Cara, Brio, Maris Piper and Pentland Javelin were consistently more tolerant of damage by Globodera rostochiensis yielding more than Corsair, Pentland Dell, Maris Anchor and Maris Peer, in untreated, heavily infested soil and giving the smallest increases to nematicide treatment. No yield or growth differences were found between plants in untreated and nematicide treated plots at a nematode-free site. The most tolerant cultivars all had a gene (H₁) for resistance to G. rostochiensis derived from Solanum tuberosum ssp. andigena and in soil infested with G. pallida the tolerance of at least one resistant cultivar (Maris Piper) appeared to be lessened. However, some resistant cultivars were comparatively intolerant, even to G. rostochiensis. Early cultivars were generally less tolerant than late maturing cultivars but there were exceptions. Amongst cultivars with resistance derived from Solanum vernei the early cultivar Guardian was more tolerant than the main crop cultivar Corsair. The effect on the yield of several cultivars of a range of densities of G. rostochiensis, produced either by applying different rates of a nematicide or by cropping in the previous year, was examined at two sites. The results indicated that the slope of the regression for yield in relation to nematode density was less for tolerant than intolerant cultivars. At sites infested with G. rostochiensis Maris Piper was found to be consistently more tolerant than Pentland Crown.     

Control of potato pale cyst-nematode, Globodera pallida, with a granular nematicide and partially resistant potatoes

The effects of oxamyl applied to the seedbed and growing partially resistant potatoes in controlling potato pale cyst-nematode, Globodera pallida, were assessed in eleven field experiments on sandy, peaty and silty loam soils in England from 1986 to 1988. Standardised procedures allowed valid comparisons to be made between data from the three experiments in 1987 and from the seven experiments in 1988. In soil moderately or heavily infested with G. pallida, oxamyl frequently increased tuber yields of susceptible cv. Désirée and of partially resistant potatoes and lessened crude nematode increase (Pf/Pi) significantly at some sites but not at others. Although 2.8 kg oxamyl ha“¹ often increased tuber yields significantly, it was significantly less effective than 5.6 kg in controlling G. pallida at one site and at another site G. pallida increased more than in untreated plots. Potatoes partially resistant to G. pallida were more effective that oxamyl in controlling nematode increase at some sites but not at others. Combined use of oxamyl and partially resistant potatoes was generally more effective in controlling G. pallida than either measure alone. As measured by a ‘control coefficient’ (weight of tubers over 40mm diameter (t ha“¹) 4-crude nematode increase (Pf/Pi)), the most effective integrated control of G. pallida was obtained by growing cvs Glenna, Morag or Santé in soil treated with 5.6 kg oxamyl ha^-1.     

Genetic variation in natural populations of the cereal cyst nematode (Heterodera avenae Woll.) submitted to resistant and susceptible cultivars of cereals

The purpose of the present work was to study the genetic characteristics of cereal cyst nematode (Heterodera avenae) populations re-established after the long-term use of resistant oat cultivars in field conditions. Population features were analyzed through fitness components and variation in enzymatic polymorphism (esterase and malate dehydrogenase loci). The longest (6 year) use of the same resistance genes (oat cv Panema) at high frequency (Rotation IB) led to the selection of a resistance-breaking pathotype and to a decrease in viability which suggested either a founder effect or a lower reproductive potential for the new pathotype. Analysis of esterase allelic frequencies led to the conclusions that: (1) the genetic constitution of this pathotype was different from the reference population maintained on the susceptible host (oat cv Peniarth), and (2) that the esterase locus may develop a disequilibrium linkage with loci involved in virulence. Random mating was recorded at the whole-field level but not always at the single-plant level, suggesting that cultivation practices such as annual ploughing could play a major role in homogenizing subpopulations developed in the vicinity of a plant. These phenomena showed that the long-term use of highly effective resistance could provok marked genetic modifications in populations. These risks should be taken into account when deciding strategies for optimal use of resistance genes in nematode management programs.

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Integrated control of potato cyst nematodes using small amounts of nematicide and potatoes with partial resistance

The control of potato cyst nematode (PCN) by less than approved amounts of nematicide combined with partially resistant potato clones was studied in a series of field experiments. On a site heavily infested with Globodera pallida only the most resistant clone (12380ac2) decreased the population density in untreated soil. With aldicarb at its full approved rate (3·36 kg ha^-1) numbers of PCN were decreased under all the genotypes, including the non-resistant Maris Piper. Aldicarb at 1·68 kg ha^-1significantly decreased populations on all clones except 12380ac2. Aldicarb at 0·84 kg ha^-1still significantly decreased population densities and multiplication rates of G. pallida on two clones with intermediate resistance (12243acl and 11233ab22). At two G. rostochiensis sites with light infestations nematode multiplication rates were greater and the control given by aldicarb and partially resistant genotypes of potato was not as great as that at the site with G. pallida. Tuber yields were not increased by the application of aldicarb at the G. rostochiensis infested sites. However, at the site heavily infested with G. pallida the yield of the most intolerant genotype (12380ac2) was increased seven-fold by the full rate of aldicarb (3·36 kg ha^-1) and four-fold by the quarter rate (0·84 kg ha^-1)-Yield of the most tolerant genotype (12243acl) was unaffected by the application of aldicarb.     

Effects of rotation length and oxamyl on potato yield and the potato cyst-nematode, Globodera rostochiensis, in a sandy loam soil

In sandy loam infested with golden potato cyst-nematode, Globodera rostochiensis, oxamyl at 5.6 kg a.i. ha^-1 incorporated in the top 15 cm of the soil just before planting potatoes greatly reduced nematode population increase on susceptible cv. Désirée grown six, seven or eight years after the last susceptible potato crop, but did not significantly increase tuber yields. In four-course and two-course rotations, oxamyl also controlled increase of G. rostochiensis and greatly increased yields of both cv. Désirée and resistant cv. Maris Piper. Oxamyl maintained tuber yields in a four-course rotation at the same level as in a six to eight-course rotation. Decline of G. rostochiensis in the soil was much faster under barley in some two-course rotations than under barley in four-course rotations.     

Effects of potato genotype, oxamyl and the numbers of potato cyst-nematodes, Globodera rostochiensis or G. pallida on tuber yields and nematode increase

A new technique is described for establishing different numbers of the potato cyst-nematode Globodera rostochiensis in field soil, which leaves the soil homogeneous in nutrient status. Field plots established in this way were used to compare yield losses in four potato cultivars (Maris Piper, Pentland Crown, Pentland Dell and Désirée) associated with different numbers of G. rostochiensis. Over the range of 7.4 to 148.4 eggs g^-1 soil at planting, yield losses were 18.7% (Maris Piper), 53.2% (Désirée), 55.7% (Pentland Crown) and 63.5% (Pentland Dell). Similar results were obtained in another experiment on the same field in a different year using only lightly and heavily infested plots. Treating the seedbed soil with oxamyl before planting prevented significant injury to potatoes by G. rostochiensis but increased the yield of Pentland Dell and perhaps Désirée (but not Maris Piper or Pentland Crown) more than expected from nematode control alone. Treating heavily infested soil with such a nematicide cannot therefore be recommended as part of a valid procedure for establishing lightly and heavily infested plots for comparing tolerances of attack by potato cyst-nematodes in a range of potato genotypes. In peaty loam soils moderately or heavily infested with G. pallida, oxamyl at 5.6 kg a.i. ha^-1 incorporated into the seedbed before potatoes were planted generally increased tuber yields, though the effects varied considerably with the cultivar grown. Increase of G. pallida in these soils was controlled better by growing potatoes bred for resistance to it (ZB 35 – 29, Caxton, Santé, Morag, 11233 ab 22, Fingal, A27/23, Cromwell). Increase of G. pallida on susceptible cultivars varied greatly and Romano increased G. pallida no more than the resistant Morag. G. pallida is probably controlled best in peaty loam by growing a resistant cultivar in soil treated with a granular (non-fumigant) nematicide.     

Sources of resistance to stem nematode, Ditylenchus dipsaci (Kühn) Filipjev, in species of Medicago and Trifolium

The putatively resistant lucerne cv. Euver was as susceptible to 11 English and 11 French populations of ‘lucerne race’ stem nematode, Ditylenchus dipsaci, as the susceptible cv. Europe. The ‘resistant’ cv. Vertus showed some resistance to five of the English and four of the French populations but was just as susceptible as Europe to three English and four French populations. Twelve annual species of Medicago were very resistant to a mixture of two English and two French populations of the nematode. None of 13 perennial spp. of Medicago was very resistant to the same mixture of nematodes but resistant plants were identified in M. borealis, M. dzhawakhetica, M. glutinosa and M. romanica. Twenty six spp. of Trifolium were resistant to a mixture of two English and two French populations of ‘red clover race’ stem nematode.     

Differential control of potato cyst-nematodes, Globodera rostochiensis and G. pallida by oxamyl and the yields of resistant and susceptible potatoes in treated and untreated soils

In sandy loam infested with Globodera rostochiensis (2–95 eggs g^-1 soil) the yield of Desiree potatoes was decreased by 8·2 t ha^-1 for every increment of 20 eggs g^-1 soil. Oxamyl incorporated in the seedbed at 5 kg ha^-1 before planting prevented significant loss of yield and damage to the tubers and minimised nematode increase. Cara and Maris Piper potatoes, which were resistant and tolerant to G. rostochiensis usually responded less to oxamyl than did susceptible cultivars. In a range of cultivars, yield responses to oxamyl treatment of soil infested with G. rostochiensis often differed from those in soil infested with G. pallida. In field experiments, oxamyl controlled G. pallida less than G. rostochiensis. In pots, such differential control of the two species by oxamyl was not observed.     

The effect of stem nematode on establishment and early yields of white clover

The effects of stem nematode (Ditylenchus dipsaci) infestation on the establishment of white clover sown in mixed swards with perennial ryegrass, were investigated in two field trials. Clover cultivars known to have different degrees of resistance were sown on land in which stem nematode was controlled either by aldicarb (Experiment 1) or crop rotation (Experiment 2). The establishment of white clover was greatly improved and subsequent clover yields were inversely related to stem nematode infestation. At the first harvest after sowing, clover yields were 3.6 and 3.3 times greater from aldicarb and rotation treatment plots than from check plots; over the first nine months, total clover yields were increased by 3.5 and 8.9 times by aldicarb and rotation treatments. In both trials, plots of resistant cultivars had less stem nematode infestation and yielded better than the very susceptible cultivar, S184. Stem nematode infestations eventually developed on all plots, after the establishment phase. This is the first report showing that resistant cultivars improve establishment of clover in mixed swards on stem nematode infested soils. In both experiments, plots became dominated by clover and even cv. S184 eventually produced good clover swards. Aldicarb and rotation treatments also controlled clover cyst and root lesion nematodes, and numbers of these were inversely related to first harvest yields. Other soil borne pests and diseases, although not prominent, have not been ruled out as possible confounding factors. There was no correlation between grass yield and aldicarb treatment.     

Control of potato cyst nematode (Globodera rostochiensis) by nematicides and a resistant potato cultivar at four sites in Scotland

The control of potato cyst nematode (Globodera rostochiensis) by the oxime-carbamates aldicarb and oxamyl was tested in four fields in Scotland. Dazomet was tested in three of these fields and carbofuran in one. In untreated plots in the three most heavily infested fields Maris Piper (resistant) yielded better than Pentland Crown (non-resistant). All nematicides increased the yields of both potato cultivars but had a greater effect on the yield of Pentland Crown. Dazomet increased yields of tubers most. Heavy nematode infestation reduced yield of tubers more in a sandy soil than in two sandy loams. In a field with few potato cyst nematodes nematicides did not significantly affect tuber yields. Although the nematicides greatly increased yields, they were not completely effective in controlling potato cyst nematodes. In treated plots in the lightly infested field, there were more nematode eggs following a crop of Pentland Crown than before. In contrast, Maris Piper markedly decreased post-cropping populations and except at one site, where dazomet further decreased nematode numbers, combining nematicides with the resistant cultivar failed to decrease nematode numbers further. Nematicides decreased the numbers of larvae invading potato roots by up to 95%, oxamyl at 5–6 kg/ha being consistently the best treatment.     

Survival and duration of development of Oscinella spp. (Diptera: Chloropidae) on different Gramineae in the laboratory

The duration of development and survival of Oscinella frit, O. nitidissima and O. nigerrima at different growth stages of several plant species was investigated in the laboratory. The effect of the infestation on the growth of plants was also quantified. Survival of O. frit larvae was significantly lower (P < 0.01) on barley than on either oats or wheat, between which there was no significant difference. Survival was usually highest when plants were infested at the two-leaf stage and lowest when plants were infested at the five-leaf stage. The duration of development of both male and female O. frit was longer on barley than on either oats or wheat and generally increased when older plants were infested. The proportion of male flies was greatest when older plants were infested. Duration of development and survival of O. frit was similar on oats and Lolium multiflorum, but larvae did not survive on Dactylis glomerata. Of the other Oscinella species, O. nitidissima larvae did not survive on oats, wheat and barley and O. nigerrima larvae did not survive on either oats or barley. The main shoots of cereal plants infested at the two-, three- or four-leaf stages were rapidly killed by O. frit larvae whereas many of those infested at the five-leaf stage continued to grow slowly. In response to attack proportionately more tillers were produced by oat plants than by either wheat or barley plants. When cereals were infested with O. nigerrima larvae the central shoots were damaged, but most recovered and continued to grow. Damage symptoms were most obvious when cereals were infested at the two-leaf stage, oats being more severely affected than either wheat or barley.     

Variation in the development of stem nematodes, Ditylenchus dipsaci, in susceptible and resistant crop plants

Dicotyledonous plants were reliably inoculated with stem nematodes by placing drops of 1.3% carboxymethylcellulose containing the surface sterilised worms between the cotyledons or in the leaf axils of recently emerged seedlings raised in pots of cool, moist soil. Inoculating onion leaves, bean stems and potato tubers or potato leaves gave variable results and inoculating onion, tulip and narcissus bulbs and lucerne and red clover stems was usually unsuccessful. An attempt to characterise 67 stem nematode populations by the reactions of a number of different plants failed for lack of useful differential hosts. Lucerne was, however, resistant to all but lucerne populations. Multiplication of stem nematode populations varied greatly between cultivars of lucerne or red clover. Some cultivars were resistant to some populations of lucerne or red clover stem nematodes and susceptible to others. These differences could not be ascribed to differences in viability of the nematodes or to differences in success of the inoculations. They indicate the presence of different pathotypes or biotypes in different populations of a so-called ‘host race’ and indicate the need for new resistant cultivars to be tested against a range of populations before they are released for general use. Amongst lucerne cultivars tested, Vertus was resistant to some lucerne stem nematode populations and susceptible to others. The supposedly resistant lucerne cultivars Euver and Lifeuil were as susceptible as was Europe. Amongst red clover cultivars tested, Redhead, Kühn, Changins and Mt Calme were susceptible, Britta, Lucrum and Temara were the least resistant, Renova, Rittinova and Quin were intermediate and susceptible to one or more of the populations tested but Norseman and especially Sabtoron were very resistant.     

Structure of Syncytia Induced by Heterodera avenae Woll. in Roots of Susceptible and Resistant Wheat (Triticum aestivum L.)

The structure of syncytia induced by Heterodera avenae in roots of susceptible Triticum aestivum cv. ‘Capa’ and resistant wheat AUS 10894 was investigated. In susceptible plants the first syncytia were observed 4 days after placing the seedlings into a substrate containing nematode cysts. They were located on different root levels. In resistant plants syncytia were first found after 14 days, exclusively in the root maturity zone. In both wheats syncytia consisted mostly of stelar parenchymatic cells. The syncytia induced in roots of Triticum aestivum AUS 10894 were characterized by less immediate contact with the host tracheary elements than the syncytia induced in roots of susceptible Triticum aestivum cv. ‘Capa’. In both wheats a stimulation of pericycle cell divisions, giving rise to lateral roots, was found in parts where the syncytia developed.     

Control of pea cyst-nematode, Heterodera goettingiana, by small amounts of aldicarb, Du Pont 1410, Ciba-Geigy 10576 or Dowco 275 applied to the soil at planting time

Aldicarb, or Du Pont 1410 (S-methyl-I-(dimethylcarbamoyl)-N-[(methyl-carbamoyl)oxy]thioformimidate), at 2.8–22.4 kg a.i./ha incorporated in the seed-bed before sowing greatly increased the yield of peas in a clay loam and two sandy clay soils infested with pea cyst-nematode, Heterodera goettingiana, and lessened or prevented increase in the number of nematodes. CibaGeigy 10576 (an organophosphorus compound) at 5.6–22.4 kg a.i./ha was similarly effective in a sandy clay soil. Dowco 275 (O, O-diethylO-(6-fluoro-2 pyridyl) phosphorothioate) at 5.6 or 11.2 kg a.i./ha also controlled the nematode well in the clay loam and in a sandy clay soil but although it greatly increased the yield of peas in the clay loam, it did not increase yield in the sandy clay.     

Possible roles of esterase, glutathione S-transferase, and superoxide dismutase activities in understanding aphid–cereal interactions

Activities of the detoxification enzymes esterase, glutathione S‐transferase, and of superoxide dismutase in aphids and aphid‐infested cereal leaves were assayed using polyacrylamide gel electrophoresis and a spectrophotometer to elucidate the enzymatic mechanisms of aphid resistance in cereal plants. A chlorosis‐eliciting Russian wheat aphid, Diuraphis noxia (Mordvilko), and non‐chlorosis‐eliciting bird cherry‐oat aphid, Rhopalosiphum padi (L.), and four cereals were used in this study. The four cereal genotypes were ‘Arapahoe’ (susceptible) and ‘Halt’ (resistant) wheat (Triticum aestivum L.), ‘Morex’ (susceptible) barley (Hordeum vulgare L.), and ‘Border’ (resistant) oat (Avena sativa L.). Esterase isozymes differed between the two aphid species, although glutathione S‐transferase and superoxide dismutase did not. Esterase, glutathione S‐transferase, and superoxide dismutase activities in either aphid species were not affected by the level of resistance of a cereal to D. noxia. The assays of cereal leaf samples showed that D. noxia feeding elicited an increase in esterase activity in all four cereal genotypes, although R. padi feeding did not. The increase of esterase activity in cereals, however, was not correlated to aphid resistance in the cereals. The time‐series assays of aphid‐infested cereal leaves showed that D. noxia‐infested Morex barley had a significant increase in esterase activity on all sampling dates (3, 6, and 9 days) in comparison with either uninfested or R. padi‐infested barley. No difference in glutathione S‐transferase activity was detected among either aphid infestations or sampling dates. The electrophoretic assays, however, revealed that aphid feeding elicited a significant increase in superoxide dismutase activity, which served as the control of glutathione S‐transferase activity assays. The increase in esterase and superoxide dismutase activities suggested that D. noxia feeding imposes not only toxic, but also oxidative stresses on the cereals. The ramification of using these enzyme activity data to understand the etiology of D. noxia‐elicited chlorosis is discussed.     

A comparison of cereal root eelworm resistant and susceptible spring barley genotypes at two sites

A comparison between otherwise identical cereal root eelworm resistant and susceptible spring barley lines was made on two sites, one lightly infested the other heavily infested with the nematode. On the lightly infested site, the grain yields of the resistant and susceptible lines did not differ, but on the heavily infested site, the resistant lines out-yielded the susceptible lines by 20%. A similar comparison was also made between five reputedly eelworm-resistant spring-barley genotypes and three cultivars known to be susceptible. On the lightly infested site, the three susceptible cultivars out-yielded all other genotypes, while on the heavily infested site they were out-yielded by all other genotypes except one. Development of the nematode was compared in resistant and susceptible genotypes. Although the nematode was as numerous in resistant genotypes as in susceptible genotypes, either invasion or development of the nematode was retarded in resistant genotypes. Although not conclusive, there was evidence that the male was less retarded in development than the female, which was rarely found in the adult form on resistant plants.     

Use of Grafting and Calcium Cyanamide as Alternatives to Methyl Bromide Soil Fumigation and their Effects on Growth, Yield, Quality and Fusarium Wilt Control in Melon

Grafting melon (Cucumis melo L.) seedlings on to the Fusarium oxysporum f. sp. melonis (Fom) commercial resistant squash rootstocks ‘Mamouth’ and ‘Nun 9075 RT’ and soil sterilization with calcium cyanamide (CaCN₂, Perlka) were tested in 2001 and 2002 as alternatives to methyl bromide (MB) soil fumigation. Ungrafted seedlings of the F₁ melon hybrid ‘Galia’ were cultivated: (i) in soil sterilized by MB and then artificially infested with Fom (this served as a control), (ii) in soil artificially infested with Fom and then sterilized by MB, (iii) in soil artificially infested with Fom and then sterilized with CaCN₂ (Perlka), grafted seedlings on the commercial rootstocks, (iv) ‘Mamouth’ and (v) ‘Nun 9075 RT’ were cultivated in soil sterilized with MB and then artificially infested with Fom. The grafted plants on ‘Mamouth’ and ‘Nun 9075 RT’ and plants in the Perlka treatment (2001) developed mild symptoms, as indicated by the significantly lower leaf symptom index (LSI; average values 1.06, 1.08 and 1.07) and disease index (DI; average values 1.60, 1.25 and 2.33), respectively, when compared with the controls (average values of LSI = 2.65 and DI = 5.06). Plants grafted on ‘Mamouth’ and ‘Nun 9075 RT’ and plants in the Perlka treatment (2001) were more vigorous than the controls as assessed on plant height, stem diameter and root biomass. When compared with the controls, this resulted in an increased (over years) early production (326.3, 265.8 and 489.1%) and late production (371.0, 357.0 and 404.2%). Fruit size was also larger in early production (29.2, 50.9 and 32.3) and late production (4.3, 15.2 and 26.0). The total soluble solids (^oBrix) increased (over years) in early production (27.4, 39.6 and 47.9) and late production (7.59, 10.07 and 5.6) when compared with the controls. Thus, grafting on resistant squash rootstocks ‘Mamouth’ and ‘Nun 9075 RT’ and soil sterilization with Perlka had positive effects on growth, production and fusarium wilt control in melon.     

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.