On cyst colour changes, bionomics and distribution of potato cyst-eelworm (Heterodera rostochiensis Woll.) pathotypes in the East Midlands

Pot tests and field trials in 1966 confirmed earlier observations that pathotype A potato cyst-eelworm (Heterodera rostochiensis Woll.) showed marked colour differences from pathotypes B and C during development on the roots of susceptible potatoes. In hatching tests started in July, newly formed brown cysts of pathotype A released four times as many larvae as those of pathotypes B and C, without entering an immediate diapause; the latter produced larvae after a rest period of 18–21 days. Subsequent second-generation cyst production in glass-tube culture was greater from pathotype A. Field observations suggested that all pathotypes mature at about the same time and give rise to only one flush of cysts on both early and maincrop potatoes in the growing season. Cyst colour surveys in commercial crops, and pot and field trials using resistant potato selections, confirmed that pathotypes B and C are both dominant and widespread in the East Midlands.     

The resistance of six potato cultivars to English populations of potato cyst-nematodes, Globodera pallida and G. rostochiensis

Six cultivars of potato (Santé, Morag, Paladin, Glenna and Fingal bred for resistance to both potato cyst-nematodes (Globodera rostochiensis and G. pallida) and Valiant bred for resistance to G. pallida alone) were exposed to 28 English populations of G. pallida and eight English populations of G. rostochiensis in pots. Susceptible cv. Désirée potatoes served as controls for all 36 populations. Inoculum (Pi) was 12000 eggs in cysts per 400ml pot of soil. Average increase of G. rostochiensis (Pf/Pi) on cv. Désirée was 23.5 but on cvs Sante, Glenna and Fingal it was < 1.0 and on cv. Morag it was 2.2. In contrast, cvs Paladin and Valiant were susceptible (average Pf/Pi = 17.4 and 26.5, respectively). Against G. pallida populations, average Pf/Pi for cv. Désirée was 21.7; on cvs Paladin, Santé and Glenna it was 2.9, 2.6 and 2.4, respectively; cvs Morag and Fingal were less resistant (7.4 and 5.6, respectively) and cv. Valiant was quite susceptible (11.0). Resistance to the different populations of G. pallida and G. rostochiensis varied but for the most resistant cultivars (Santé, Glenna and Paladin) the variation was usually small. The value of the six resistant cultivars studied to the integrated control of potato cyst-nematodes in England and the genetic diversity of the nematode populations to which they were exposed are discussed.     

Control of potato cyst-nematode, Heterodera rostochiensis, in peaty loam soil by D-D, aldicarb and a resistant variety of potato

In peaty loam soil in Cambridgeshire, 5.2 or 10.3 kg aldicarb/ha incorporated in the top-soil before potatoes were planted controlled potato cystnematode (Heterodera rostochiensis Woll.) better than 384, 769 or 1153 kg D-D/ha injected 15 cm deep into the top-soil in the preceding autumn. 10.3 kg aldicarb/ha applied in 1968 and 1969 permitted King Edward potatoes (susceptible to H. rostochiensis) to grow well in infested soil and prevented multiplication of pathotypes of H. rostochiensis on Maris Piper potatoes (resistant to H. rostochiensis pathotype A). Although large amounts of D-D applied in 1968 and 1969 increased the yield of King Edward potatoes in both years they did not control potato cyst-nematode in the second year.     

Testing potatoes bred from Solarium vernei for resistance to the white potato cyst-nematode, Heterodera pallida

Three clones of potatoes bred from Solanum vernei were tested for their resistance to the white potato cyst-nematode, Heterodera pallida, both by root-ball counts and by estimates of total cysts produced. Two susceptible clones were used as controls. Root-ball counts suggested a higher degree of resistance than that based on total cysts produced. There were differences in pathogenicity between the two populations. The degree of resistance found was such that in practice the resistant clones would be best used in conjunction with nematicide treatment in a system of integrated control.     

Observations on the distribution of potato cyst-nematodes, Globodera rostochiensis and G. pallida on two fenland potato farms

The distribution of potato cyst-nematodes, Globodera rostochiensis and G. pallida was surveyed in two widely separated fenland potato farms in Eastern England. Evidence was obtained of the suppression of G. rostochiensis and increase of G. pallida populations when Maris Piper and other potatoes with the same gene (H₁) for resistance to G. rostochiensis are grown. Numbers of G. rostochiensis and/or G. pallida in the soil could not be related directly to recent potato cropping histories of the fields. Identification of the two species of Globodera by the presence and colour of females on the roots of Désirée, Maris Piper, Cromwell (A27/20) and A25/11 potatoes were confirmed by agarose gel isoelectric focussing of general proteins. Cromwell exhibited little or no resistance to G. pallida at Woodwalton. A25/11 was more resistant to this species at Woodwalton but was susceptible to G. rostochiensis. The results of these surveys emphasise the need for carefully planned and integrated control programmes based on the use of resistant cultivars of potato, crop rotations and appropriate nematicide use to combat the growing problem of G. pallida, especially in organic soils.     

Ecological studies on potato mop-top virus in Scotland

Plants with symptoms of potato mop-top virus (PMTV) occurred in many commercial seed stocks of Arran Pilot and Red Craig's Royal potato in Scotland, but their incidence rarely exceeded 5%. In nuclear stocks of seed potatoes, most varieties examined in 1967 and 1968 were infected at one or more locality, but infected plants did not occur in all clones or at all stages of propagation of any one variety. infection of nuclear stocks resulted both from propagation on virus-infested land and from unwitting selection of infected plants to start new clones. PMTV was detected in farm soils ranging from light sands to heavy loams, in five Scottish counties. Soil was infested throughout the ploughed layer but the severity of infestation varied greatly within any one field; some sites of former potato clamps were heavily infested. PMTV was detected in field soil 12 years after potatoes were grown. In glasshouse tests many British crop and wild plants were colonized by Spongospora subterranea. Within some families all species tested were moderate to good hosts. (Solanaceae, Chenopodiaceae and Cruciferae), in others, species differed greatly in susceptibility (Compositae and Umbelliferae), and in a few, species were poor hosts or were not infected (Caryophyllaceae and Gramineae). Of the British crop and weed species that were moderate to good zoosporangial hosts of S. subterranea, only Solanum nigrum, potato, spinach and sugar beet were hosts of vector-borne PMTV. Potato probably survives between potato crops mainly in the resting spores of S. subterranea. PMTV was probably first brought to Europe with potatoes from South or Central America.     

Population fluctuations of Heterodera rostochiensis Woll. when susceptible potato varieties are grown continuously

When seven potato varieties susceptible to Heterodera rostochiensis were grown every year on the same small plots for 6 years with precautions to minimize soil movements, yields were inversely proportional to the number of larvae in the soil before cropping in the first year only. With all the varieties, numbers of larvae after harvest fluctuated around mean values and the fluctuations became smaller in successive years. Although conditions from year to year were as uniform as possible, there were differences in weather, incidence of potato blight (Phytophthora infestans), seed size and quality, and in planting dates. Analysis of soil temperatures and of heat accumulation did not suggest that conditions after planting caused the fluctuations. The smaller fluctuations with repeated cropping probably reflect the influence of population density on the size of the root system and are what would be expected from the curve relating pre- to post-cropping numbers, which reaches a maximum before the reproductive rate decreases to unity. A resistant potato hybrid ex andigena, also grown every year, usually outyielded the susceptible varieties and, by the sixth year, yielded at least twice as much as the mean of the six susceptible varieties.     

Glucose phosphate isomerase polymorphism in field populations of the potato cyst nematodes, Globodera rostochiensis and G. pallida

The isozymic variation of glucose phosphate isomerase (GPI) was compared in 20 field populations of potato cyst-nematode (PCN) and related to variation in reference pathotypes of PCN. Populations pathotyped as G. rostochiensis Ro 1 by conventional differential cultivar tests were found to be different from populations of G. pallida. Variation within populations pathoptyped as G. pallida was seen and three populations were found to be similar to the reference G. pallida Pal pathotype. This seemed to be confirmed for two populations using cysts formed on a cultivar resistant to the Pa 1 pathotype when an isoenzyme pattern characteristic of G. palida Pa2 and Pa3 was found. The feasibility of the use of isozyme identification of PCN pathotypes is discussed in the light of these results.     

Influence of nematicide, resistant and susceptible potato cultivars and bare fallow on the population dynamics of Globodera rostochiensis Woll. Rol under field conditions in Spain

The susceptible potato cultivar Désirée and the resistant cv. Diamant were cultivated on untreated and nematicide-treated plots on two fields in northern Spain infested with Globodera rostochiensis Rol. Initial and final infestations of cysts and viable eggs were assessed. Multiplication rates differentiated more accurately the effects of the different treatments than final number of cysts and eggs, due to the elimination of some of the errors associated with variability of the initial infestation. Many new cysts were observed on all plots where susceptible potatoes had been grown, whether they had been treated with nematicide or not, although significantly fewer cysts were actually found on the nematicide-treated plots. With the resistant cultivar no significant effects of the nematicide were detected even though there was a noticeable reduction of viable cyst contents over and above the effect of the resistant cultivar.     

Multiplication of English isolates of potato pale cyst- nematode, Globodera pallida, on potatoes of differing nematode resistance and the identity of potato cyst- nematode populations

In pots, 25 populations of potato pale cyst-nematode, Globodera pallida Stone, differed significantly in their ability to multiply on potato clones P55/7 and ZC83/ 6, both fully resistant to G. pallida pathotype Pal. Neither clone was fully resistant to any of the populations. For 21 populations common to this and an earlier experiment, increase on the more resistant potatoes (P55/7, ZC83/6, cvs Sante, Paladin and Glenna) was correlated with their increase on less resistant potatoes (cvs Morag, Fingal and Valiant). Variation in virulence on these partially resistant potatoes was not matched by differences in the electrophoretic patterns of the nematodes' proteins. The identification of populations of G. rostochiensis (Woll.) Skarbilovich used in these experiments was confirmed by electrophoresis. All populations of G. pallida Stone, appeared to contain very small numbers of G. rostochiensis after subculture on susceptible potatoes (cv. Désirée).     

Resistance and tolerance of potato varieties to potato rot nematode (Ditylenchus destructor) and stem nematode (Ditylenchus dipsaci)

Ditylenchus destructor and Ditylenchus dipsaci are economically important plant‐parasitic nematodes, affecting potato production mostly in temperate climates. Management through crop rotation is not feasible because of their wide host range. These nematodes are listed as quarantine pests in many countries. Limited information exists on the resistance and tolerance of currently cultivated potatoes to D. destructor and D. dipsaci. Two greenhouse experiments were conducted to screen 25 potato varieties for resistance to and tolerance for D. destructor and D. dipsaci infections. Reproduction factor (RF) and relative susceptibility (RS) were used to evaluate resistance, while potato tuber damage and tuber weight reduction was used to evaluate tolerance. Based on the RF, 16 varieties were evaluated as susceptible (S) while 5 varieties were evaluated as resistant (R) to D. destructor; varieties ‘Innovator’, ‘Aveka’ and ‘Spunta’ were resistant to D. dipsaci based on RF. ‘Désirée’ was observed to be highly susceptible to D. destructor and D. dipsaci in both experiments and was used as the standard susceptible control variety for the calculation of RS. A scale of 1–9 was used to classify RS of the potato varieties to D. destructor and D. dipsaci, where 9 indicated the highest level of resistance. All classes of resistance to D. destructor and D. dipsaci were observed in the potato varieties tested in the experiments. Six varieties had significantly lower RS to D. dipsaci than the standard susceptible control variety. Tolerant to highly sensitive potato varieties to both nematodes were also observed. RS and external potato tuber damage were identified as suitable methods for resistance and tolerance determination, respectively. This study provides essential information on the status of resistance and tolerance in potato varieties against D. destructor and D. dipsaci but needs to be confirmed under field conditions.     

Observations on the control of potato cyst-nematodes, Globodera rostochiensis and G. pallida in 35 English potato soils in pots by oxamyl and resistant potatoes

The increase of 35 English field populations of potato cyst-nematodes (Globodera rostochiensis and/or G. pallida) was measured on Désirée, Maris Piper, Caxton (A25/11), Cromwell (A27/20) and clone 11233 ab 22 in pots of sandy, silty or peaty loam soil. Désirée was susceptible to all populations tested and, as in field soils, the final population (Pf) was inversely related to the initial population of potato cyst-nematode eggs (Pi) in the soil. Maris Piper and Cromwell were resistant to all G. rostochiensis populations, with one possible exception. Maris Piper was susceptible to all G. pallida populations. Caxton was susceptible to some and fairly resistant to other populations of G. rostochiensis, indicating the existence either of two biotypes within the one pathotype (Rol) as yet encountered in Britain, or the existence of an additional pathotype. Caxton and Cromwell were fairly resistant to G. pallida. Clone 11233 ab 22 was only moderately resistant to both species. Resistance to potato cyst-nematode increase varied considerably, especially in Caxton (to G. rostochiensis) and in 11233 ab 22 (to both species). Oxamyl greatly reduced the increase of G. rostochiensis populations on Désirée potatoes, with the notable exception of one population but it generally had much less effect on G. pallida populations, regardless of soil type. The difference in effect on the two species may be due to a longer period of hatching in G. pallida than in G. rostochiensis and also perhaps to a second generation in G. pallida.     

The white potato cyst nematode (Globodera pallida) – a critical analysis of the threat in Britain

Over the last 30 years, there has been an epidemic of the white potato cyst nematode (wPCN, Globodera pallida). It has progressively replaced the yellow species (yPCN, G. rostochiensis) throughout most of England and Wales and is now a widespread problem. As damaging populations of wPCN are enormous (>10⁹ eggs ha^?1), several crops of potato cultivars resistant only to yPCN were required to produce this change. The threat it poses is reflected in an increase in the numbers of soil samples being tested and in nematicide use, which has increased to > 25 000 ha of potatoes being treated annually. Computer modelling shows that current management of wPCN is mostly ineffective and populations will continue to increase. The multiplication rate of wPCN is inversely related to its population density at planting and, because of this, modelling shows that sufficient eggs are likely to survive to enable large populations of wPCN to “rebound” following nematicide treatment. This is supported by recent trial results showing that wPCN population increase was almost as great in nematicides‐treated plots as in the untreated. Modelling also showed that current rotations (typically potatoes once every 5 or 6 years) are too short to prevent wPCN populations from progressively increasing, even when used in conjunction with a nematicide. Similarly, except with avirulent populations, the partially resistant cultivars currently available will not prevent wPCN from increasing. However, as the effectiveness of partially resistant cultivars is independent of population density, they can be very effective when integrated with a nematicide. Unfortunately, only c. 8% of the potato area is planted with partially resistant cultivars, and much of that is in land not known to be infested with wPCN. Consequently, the current epidemic of wPCN is likely to become progressively more serious. However, many farmers are failing to recognise and respond to this threat until it is too late because of the slow rate of increase of wPCN, the difficulties of detecting small populations and the costs of nematicides. To respond to the current epidemic of wPCN, the greatest priority is to have available an increased number of commercially‐attractive partially resistant cultivars.     

Single nucleotide polymorphism (SNP) genotyping as basis for developing a PCR-based marker highly diagnostic for potato varieties with high resistance to Globodera pallida pathotype Pa2/3

Globodera pallida is a parasitic root cyst nematode of potato, which causes reduction of crop yield and quality in infested fields. Field populations of G. pallida containing mixtures of pathotypes Pa2 and Pa3 (Pa2/3) are currently most relevant for potato cultivation in middle Europe. Genes for resistance to G. pallida have been introgressed into the cultivated potato gene pool from the wild, tuber bearing Solanum species S. spegazzinii and S. vernei. Selection of resistant genotypes in breeding programs is hampered by the fact that the phenotypic evaluation of resistance to G. pallida is time consuming, costly and often ambiguous. DNA-based markers diagnostic for resistance to G. pallida would facilitate the development of resistant varieties. A tetraploid F₁ hybrid family SR-Gpa segregating for quantitative resistance to G.␣pallida was developed and evaluated for resistance to G. pallida population ‘Chavornay’. Two subpopulations of 30 highly resistant and 30 susceptible individuals were selected and genotyped for 96 single nucleotide polymorphism (SNP) markers tagging 12 genomic regions on 10 potato chromosomes. Seven SNPs were found significantly linked to the nematode resistance, which were all located within a resistance ‘hotspot’ on potato chromosome V. A haplotype model for these seven SNPs was deduced from the SNP patterns observed in the SR-Gpa family. A PCR assay ‘HC’ was developed, which specifically detected the SNP haplotype c that was linked with high levels of nematode resistance. The HC marker was only found in accessions of S.␣vernei. Screening with the HC marker 34 potato varieties resistant to G. pallida pathotypes Pa2 and/or Pa3 and 22 susceptible varieties demonstrated that the HC marker was highly diagnostic for presence of high levels of resistance to G. pallida pathotype Pa2/Pa3.Amirali Sattarzadeh and Ute Achenbach contributed equally to the work     

A method for estimating the contribution of seed potatoes, machinery and soil tare in field infestations with potato cyst nematodes on a national scale

In order to make a cost benefit analysis for the management of the potato cyst nematodes Globodera rostochiensis and G. pallida, we developed a method to estimate the relative importance of three basic distribution channels of potato cyst nematodes: seed potatoes, machinery and soil tare. The baseline is determined by the area planted with potatoes, the area infested with potato cysts, the proportion of resistant potato cultivars and the distribution of cysts trough different channels. This quantification forms a basis for the evaluation of the effects of different control measures for potato cyst nematode on a national scale. The method can be useful as an example for application in other countries.     

Modifications in dispersal and oviposition of Bt-resistant and Bt-susceptible Colorado potato beetles as a result of exposure to Bacillus thuringiensis subsp. tenebrionis Cry3A toxin

Laboratory strains of Colorado potato beetle, Leptinotarsa decemlineata (Say), physiologically resistant and susceptible to Bacillus thuringiensis (Berliner) subsp. tenebrionis Cry3A toxin were reared to adults on caged potato plants. Influence of three different diets (transgenic potatoes, regular potatoes, and regular potatoes followed by the transgenic potatoes) on beetle mortality, fecundity, and flight behavior were tested under laboratory conditions. A computer-linked flight mill system was used to quantify beetle flight, and dissections were performed to determine the level of flight muscle development. Susceptible beetles continuously fed on transgenic foliage suffered heavy mortality, did not develop flight muscles, and did not produce any eggs. Resistant beetles continuously fed on transgenic foliage were capable of flight and reproduction; however, it took them longer to initiate flight behavior, and their fecundity was lower than fecundity of other treatments. In both strains, detrimental effects became significantly less severe when the beetles were allowed to feed on regular foliage prior to toxin ingestion. In the resistant strain, ingestion of Cry3A toxin significantly increased flight activity, indicating that physiological resistance was probably reinforced by the behavioral escape from toxic environments. No such response was observed for susceptible beetles. When fed on regular foliage, resistant Colorado potato beetles engaged in significantly fewer flights than susceptible beetles. Behavioral differences between resistant and susceptible beetles observed in the present study are likely to affect gene flow between transgenic crops and adjacent refugia, and should be taken in consideration when designing resistance management plans for transgenic potato crops.     

Electrophoretic studies on populations of Myzjus persicae in Scotland from March to July, 1976

Specimens of Myzus persicae have been collected from field populations in Scotland and scored electrophoretically for their phenotypes at three polymorphic loci. These phenotypes were almost invariably present in individuals in one of a small number of favoured combinations, suggesting that these populations may be oligogeneous, that is, composed of large numbers of copies of a few favoured combinations of alleles. Temporal and spatial sampling at sites in East and West Scotland in 1976 has shown that different allele combination types were present in the two areas. Populations in E. Scotland contained very few specimens belonging to the organophosphorus (OP) resistant clone types (using esterase activity to infer resistance); instead there were large numbers of an OP susceptible type which has not been detected in England, but which came to predominate on cabbage and potato host plants at every site sampled during the period of rapid population increase. In contrast, populations in W. Scotland were characterised by substantial numbers of specimens belonging to OP resistant clone types and by an almost complete absence of this susceptible type. Two other distinguishable OP susceptible types were found to be common to both areas. The W. Scotland populations closely resemble N. England populations while those in E. Scotland appear to be unique to that area. Specimens of OP resistant types were only detected in E. Scotland after the beginning of June and in such relative proportions as to strongly suggest that they were migrants from W. Scotland or N. England.     

The resistance of potato varieties to Heterodera schachtii Schmidt, the potato-root eelworm

The potato varieties Epicure and Doon Star permitted the production of fewer and smaller cysts of Heterodera schachtii on their roots than the varieties Golden Wonder and Majestic, when all were grown under standard conditions. Under the stimulus of potato-root excretion the number of larvae which emerged from Epicure and Doon Star cysts was less than that from Majestic or Golden Wonder cysts of the same volume. Accepting Lapage's (1938) criteria of resistance to nematode infestation in animals and applying them to plants, it is concluded that this deleterious effect of the variety on the eelworm is probably of the nature of a degree of resistance to eelworm attack specific to each variety and is physiological and not anatomical in action. Of the varieties tested, Epicure is judged the most resistant, closely followed by Doon Star. Majestic is fairly susceptible but not so much so as Golden Wonder on which the eelworm forms large cysts that are extremely productive of larvae.     

Multiple alleles for resistance and susceptibility modulate the defense response in the interaction of tetraploid potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis>) with <Emphasis Type="Italic">Synchytrium endobioticum</Emphasis> pathotypes 1, 2, 6 and 18

The obligate biotrophic, soil-borne fungus Synchytrium endobioticum causes wart disease of potato (Solanum tuberosum), which is a serious problem for crop production in countries with moderate climates. S. endobioticum induces hypertrophic cell divisions in plant host tissues leading to the formation of tumor-like structures. Potato wart is a quarantine disease and chemical control is not possible. From 38 S. endobioticum pathotypes occurring in Europe, pathotypes 1, 2, 6 and 18 are the most relevant. Genetic resistance to wart is available but only few current potato varieties are resistant to all four pathotypes. The phenotypic evaluation of wart resistance is laborious, time-consuming and sometimes ambiguous, which makes breeding for resistance difficult. Molecular markers diagnostic for genes for resistance to S. endobioticum pathotypes 1, 2, 6 and 18 would greatly facilitate the selection of new, resistant cultivars. Two tetraploid half-sib families (266 individuals) segregating for resistance to S. endobioticum pathotypes 1, 2, 6 and 18 were produced by crossing a resistant genotype with two different susceptible ones. The families were scored for five different wart resistance phenotypes. The distribution of mean resistance scores was quantitative in both families. Resistance to pathotypes 2, 6 and 18 was correlated and independent from resistance to pathotype 1. DNA pools were constructed from the most resistant and most susceptible individuals and screened with genome wide simple sequence repeat (SSR), inverted simple sequence region (ISSR) and randomly amplified polymorphic DNA (RAPD) markers. Bulked segregant analysis identified three SSR markers that were linked to wart resistance loci (Sen). Sen1-XI on chromosome XI conferred partial resistance to pathotype 1, Sen18-IX on chromosome IX to pathotype 18 and Sen2/6/18-I on chromosome I to pathotypes 2,6 and 18. Additional genotyping with 191 single nucleotide polymorphism (SNP) markers confirmed the localization of the Sen loci. Thirty-three SNP markers linked to the Sen loci permitted the dissection of Sen alleles that increased or decreased resistance to wart. The alleles were inherited from both the resistant and susceptible parents.     

The population genetics of the potato cyst-nematode, Heterodera rostochiensis: mathematical models to simulate the effects of growing eelworm -resistant potatoes bred from Solatium tuberosum ssp. andigena

To follow population changes when potato varieties with resistance to Heterodera rostochiensis derived from Solanum tuberosum ssp. andigena were grown on infested land, a computer programme was written including three mathematical relationships: (1) a law relating multiplication to pre-cropping density; (2) two mathematical models of inheritance of ability to overcome resistance; (3) a law relating the proportion of larvae able to become female to pre-cropping population density. The programme also included four parameters: (1) the maximum possible reproductive rate; (2) the fraction of the population (eggs) not participating in reproduction when potatoes are grown and carried over to the following year unchanged; (3) the fraction carried over annually when other crops are grown; (4) the frequency of larvae able to become female in the population initially. Population density was measured relative to the equilibrium density and was therefore independent of the units in which density is usually measured. After supplying a range of parameters for all the above to include those likely to be encountered in practice, the changes expected (a) in the frequency of larvae able to become female in the roots of resistant varieties and (b) in population density were computed for resistant varieties grown continuously or alternately with susceptible varieties in crop rotations of different lengths. Because well established field populations are relatively dense, observed reproductive rates are small and rarely approach the maximum possible. Reproductive rate is therefore a relatively unimportant determinant of genetic change. The fraction of the population carried over to the following year is more important because it affects the length of a crop rotation necessary to make loss of potato yield acceptable, determines what the multiplication rate will be and influences the speed of genetic change by providing a reservoir of initial type males which backcross with any genetically different females that may develop on the roots of resistant plants. No experiment seems to have been done specifically to determine the parameters needed to calculate population changes. Some values can be obtained from the literature but mostly they must be guessed. When the law relating the proportion of larvae able to become female to pre-cropping population density was included in the computations, it had little effect initially but later, after several generations, it delayed genetic change. Two field experiments, one by Huijsman (1961) another by Williams (1958) and Cole & Howard (1962 a), provide some of the variables needed to compute trends in population density. Best fitting variables were computed for the data in these experiments by the method of maximum likelihood. The computed parameters for one experiment were not very realistic but those for the other were in line with what would be expected in practice and tended to favour the hypothesis that larvae able to become female in the roots of resistant plants are double recessives (aa). The computations lead us to suggest that the best policy for potato growers who have fields suitable for resistant varieties is to alternate resistant with susceptible varieties in a crop rotation containing potatoes every 3 or 4 years.