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.     

Management of the potato cyst nematode (Globodera pallida) with bio-fumigants/stimulants

Field trials evaluated the effect of four plant-based bio-fumigants/stimulants on population levels of G. pallida and the resulting potato yields and quality. Three formulations contained seaweed biostimulants (Algifol, Nutridip and Metastim) and one bio-fumigant containing mustard and chilli pepper extracts (Dazitol). These were compared with the fumigant nematicide Nemathorin and untreated control plots. The effect of G. pallida on growing potato crops was assessed by recording haulm characteristics which indicated that the nematicide treatment gave most protection. Levels of PCN juveniles and migratory nematodes were assessed during the trial. Plots treated with Nemathorin and Dazitol had fewest PCN, whilst the highest number of migratory nematodes occurred in fallow plots. Sixteen weeks after planting the nematicide treatment produced highest yield and tuber numbers. Dazitol treatment produced a lower yield but the largest tubers.     

The effect of granular nematicide incorporation depth and potato planting depth on potatoes grown in land infested with the potato cyst nematodes Globodera rostochiensis and G. pallida

Field experiments at Harper Adams, Shropshire and Wisbech, Cambridgeshire investigated the effect of nematicide incorporation and seed tuber planting depth on the yield of the potato (Solamum tuberosum L.) cultivars Estima and Maris Piper and the population control of the potato cyst nematodes Globodera rostochiensis Woll. (Skarbilovich) and G. pallida (Stone). The nematicide fosthiazate was applied at 3 kg^?1 ha and either not incorporated, or incorporated to 20 cm or 35 cm. Potatoes were mechanically planted to three depths; approximately 10 cm, 15 cm and 25 cm. Incorporation to 20 cm with tubers planted at a depth of 10 cm or 15 cm, reduced root invasion compared with the other treatments. Incorporating nematicide to 20 cm also gave consistently higher ware yields and better nematode control than the other incorporation methods, which were not significantly different to the control. However, ware yield and nematode multiplication rate were not significantly affected by planting depth.     

The effect of partially resistant potato clones and aldicarb on the multiplication of the potato cyst nematode Globodera pallida in field trials

Potato cultivars with different degrees of resistance to Globodera pallida (PCN) were trialled at three infested sites in 1983, 1984 and 1985. These trials were primarily intended to assess the effects of PCN damage on the yield of tubers and consequently the sites chosen had high initial population densities of G. pallida. The population density of G. pallida was determined from samples taken both pre-planting and post-harvest and results showed that there were significant differences between the clones in the nematode multiplication rates. Although actual rates of multiplication varied between sites the relative differences between clones were maintained across sites. Under the susceptible control cultivars the populations generally increased even in the presence of nematicide. The partially resistant clones gave best control, as measured by nematode multiplication, at the sites with the highest initial population densities but gave good control at all sites when treated with nematicide irrespective of initial population density. The role of partial resistance in integrated control systems and the effectiveness of nematicides against G. pallida is discussed.     

Effects of a mixed-isolate mycorrhizal inoculum on the potato – potato cyst nematode interaction

Inoculation of microplants of potato cv. Golden Wonder with Vaminoc, a mycorrhizal inoculum of three arbuscular mycorrhizal fungi (Glomus spp.), resulted in an increase in in‐sand hatch of Globodera pallida, but not G. rostochiensis, within 2 weeks. By this time, mycorrhized plants also supported a larger number of feeding nematodes of both PCN species (50% higher for G. rostochiensis) than did non‐mycorrhized plants, with a higher proportion of the G. pallida population being fertilised females than for G. rostochiensis. After 12 weeks, the multiplication rate of G. rostochiensis on mycorrhized plants was significantly greater than on non‐mycorrhized plants, whereas no such difference was observed for G. pallida. The principal component of PCN multiplication affected by mycorrhization was increased cyst number per plant from 6 to 12 weeks. Over this period, there was no increase in cyst number per plant for either PCN species on non‐mycorrhized plants, whereas the value increased on mycorrhized plants for both G. rostochiensis (by almost 200%) and G. pallida (57%). Mycorrhization resulted in significant increases in the root and shoot dry weights of plants grown in the absence of PCN. Although mycorrhized plants carried a larger PCN burden than non‐mycorrhized plants when grown on PCN‐infested medium, as a result of the increased PCN multiplication rate, they produced larger root systems than did nonmycorrhized plants, suggesting increased tolerance to PCN of the mycorrhized plants, particularly to G. rostochiensis. Of morphological characters investigated in the absence of PCN, only stem height (increased) was significantly affected by mycorrhization. Colonisation by mycorrhizal fungi resulted in increased tuber yield both in the absence (significant increase) and presence (non significant) of PCN, as a result of increased tuber number per plant. These results are discussed in the light of the possible use of AMF as part of an integrated PCN management plan.     

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.     

The use of foliage assessment to improve the identification of tolerance to damage by nematodes (Globodera pallida) in potatoes

Six potato trials, two in each of three years, were conducted in collaboration with the Agricultural Development and Advisory Service (ADAS) at sites infested with potato cyst nematodes (G. pallida Pa 2/3). The trials were part of a selective screen to identify PCN tolerant and intolerant clones with each trial consisting of four blocks divided into nematicide treated and untreated sub-blocks. A range of partially resistant and susceptible material was assessed for yield losses due to PCN damage to the roots and for the effect on the foliage by comparison between the nematicide treated and untreated areas. The relationships between the foliage symptoms, untreated yields, treated yields, proportional yield loss, initial PCN population and the post-harvest PCN population levels are examined. Significant correlation coefficients were obtained between foliage symptoms and yield of clones in PCN infected soil and also between foliage symptoms with percentage yield loss due to PCN infestation. The conclusions were that the assessment of PCN damage to foliage vigour/development can contribute positively to a more accurate identification of tolerant or intolerant potato genotypes.     

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.     

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.     

Control of potato cyst nematodes and economic benefits of application of 1,3-dichloropropene and granular nematicides

The two species of the potato cyst nematodes (PCN) Globodera pallida and G rostochiensis are the most problematic pests of the potato crop in the UK. There are no commercially available cultivars with full resistance to G. pallida and both crop rotation and granular nematicides are less effective at controlling this species than G. rostochiensis. In situations of very high PCN levels it may be possible to reduce populations and yield losses by using an autumn application of the soil fumigant 1,3-dichloropropene (1,3-D) followed by a spring application of a granular nematicide. Two field experiments were done to look at the integration of methods for the control of PCN. The Common Field experiment (G. rostochiensis infested) compared the use of 1,3-D with the granular nematicides aldicarb, oxamyl and fosthiazate when growing the susceptible cv. Estima. The Four Gates experiment (infested with both PCN species but mainly G rostochiensis) compared the performance of cv. Santé (partially resistant to G. pallida, fully resistant to G. rostochiensis) with that of the susceptible cv. Estima when treated with 1,3-D and oxamyl at full and half-rates. The results of the experiments show that an integrated approach to nematode control on heavily infested sites, including granular and fumigant nematicides and cultivar resistance, can lead to significant decreases in nematode population densities and reduce yield losses. An economic evaluation of the experiments modelled the gross margins from the different nematicide treatments. In Common Field, the highest gross margins were achieved with the combined use of fumigant and granular nematicides. In Four Gates, there was a clear economic benefit for both cultivars from the use of 1,3-D. In this experiment, oxamyl was of economic value to Estima but not to Sante and full-rate oxamyl was of more benefit than half-rate to Estima.     

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.     

The relationship of potato yield with and without nematicide to density of potato cyst nematodes, lobodera rostochiensis and G. pallida

Trials relating response to nematicide to potato cyst nematode density were conducted initially in the West Midlands and later in other Regions. Thirteen trials, only three of which were within the intensive potato growing areas, conformed to a general pattern with yield losses being largely recouped by nematicide treatment. Five trials, four within the intensive areas, gave no correlation between potato yield and nematode density and an unpredictable response to nematicide. The control of the nematode appeared to be poorer in the latter trials but other factors affecting yield and nematode multiplication may have been involved. It is speculated the main factor might be interaction with fungal organisms e.g. Rhizoctonia or Verticillium. Evidence is presented to show that on several trials the nematicide has an effect other than by controlling potato cyst nematodes. Yield losses caused by potato cyst nematodes are more variable than previous work indicated, probably due to varietal, seasonal or environmental influences.     

Effects of potnetial trap crops and planting date on soil infestation with potato cyst nematodes and root-knot nematodes

In 1997 and 1998 the stimulation of hatch of potato cyst nematodes (PCN) by a trap crop was studied at various times during the growing season in a container and a field experiment. Solanum nigrum‘90‐4750‐188’was used as the trap crop in both experiments and was sown on 1 May, 16 June or 1 August in two successive years on different plots. Neither experiment revealed much seasonal variation in hatchability of PCN juveniles under a trap crop. In the container experiment, the hatch of the Globodera pallida Pa3 population was equally and strongly stimulated (89%) at all sowing dates in both years, except for the 1 August sowing in 1998 (when the hatch was 77% under extremely wet soil conditions). In the control treatment with non‐hosts (flax followed by barley) the total spontaneous hatch was 50% over 2 yr. In the field experiment, the hatch of PCN, averaged over the four populations, was also equally stimulated (71%) at all sowing dates in both years. In the control treatment with non‐hosts (flax‐barley) the total spontaneous hatch was 36% over 2 yr. Total hatch under the trap crop over 2 yr varied between the four PCN populations from 63% to 80%. In 1998 and 1999, control of potato cyst nematodes (PCN) by the potential trap crops Solanum sisymbriifolium and S. nigrum‘90‐4750‐188’was studied in the field. Potato was also included as a trap crop. In the 1998 experiment, potato, S. sisymbriifolium and S. nigrum strongly stimulated the hatch of PCN compared with the non‐host white mustard (Sinapis alba). Roots of potato and white mustard were mainly found in the top 10 cm of soil, whereas roots of S. sisymbriifolium and S. nigrum were also abundant at depths of 10–20 cm and 20–30 cm. In the 1999 experiment, soil infestation with PCN decreased markedly with potato and S. sisymbriifolium as trap crops. In plots moderately to severely infested with 2‐yr old cysts (2–29 juveniles ml^?1 air dried soil), potato reduced soil infestation by 87% and S. sisymbriifolium by 77%. In plots moderately to severely infested with 1‐yr old cysts the reductions were 74% and 60%, respectively. The reduction was least on plots very severely infested with PCN (110–242 juveniles ml^?1 soil): 69% and 52% for potato and S. sisymbriifolium, respectively. Soil infestations of plots that were initially slightly to severely infested with the root‐knot nematode Meloidogyne hapla were greatly reduced under fallow and S. sisymbriifolium but increased under potato. From these and previous experiments it was concluded that, for several reasons, S. sisymbriifolium is a promising trap crop.     

Use of resistant and susceptible potato cultivars in the trap cropping of potato cyst nematodes, Globodera pallida and G. rostochiensis

The effects of planting date and growing period of potato cultivars on their efficiency as trap crops for potato cyst nematodes (PCN) were studied. Plots were planted with susceptible or resistant cultivars in April, June and August and these were grown for 5, 6 or 7 wk before removal of the plants by hand lifting. Crops planted in June provided the best overall reductions in PCN population density of up to 95%, with cv. Santé significantly more effective than the other cultivars. Population reductions from the August planting were only slightly less than from planting in June but the tuber yields obtained were much greater: Maris Piper and Maris Bard produced 16.4 and 21.4 t ha^-1 respectively, with 37% and 43% respectively, of a size useful for canning (i.e. between 20 and 40 mm diameter).     

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.     

Effect of the fungus Pochonia chlamydosporia and fosthiazate on the multiplication rate of potato cyst nematodes (Globodera pallida and G. rostochiensis) in potato crops grown under UK field conditions

This is the first report of the successful use of Pochonia chlamydosporia as a biological control agent against potato cyst nematodes (PCN) (Globodera pallida and G. rostochiensis) in potato crops grown under commercial field conditions and represents an important step in the development of biological control for PCN. Two field experiments were established in consecutive years (2006 and 2007) at different field sites in Shropshire, England. Treatments comprised of (1) untreated control, (2) P. chlamydosporia, (3) P. chlamydosporia with the nematicide fosthiazate and (4) fosthiazate alone. In both experiments, significant reductions in the nematode multiplication rate (Pf/Pi) for P. chlamydosporia treated plots were observed (48% and 51% control, respectively). The P. chlamydosporia treatment did not differ significantly from both fosthiazate treatments in terms of Pf/Pi in spite of the trend towards increased control particularly in Experiment 1. P. chlamydosporia therefore provided similar levels of nematode population control as fosthiazate. The combined treatment did not provide any additional reduction in Pf/Pi but demonstrated that P. chlamydosporia was compatible with fosthiazate. Over the different developmental stages of the juvenile nematodes, there was evidence of parasitism of adult females on the plant root by P. chlamydosporia. Root colonization by P. chlamydosporia was higher in the P. chlamydosporia treatment due to increased levels of nematodes in plant roots. Results from both experiments demonstrated the efficacy of P. chlamydosporia as a biological control agent of PCN and indicate its potential for use as part of an integrated pest management strategy.     

Co-limitation of potato growth by Potato Cyst Nematode (Globodera rostochiensis) and Rhizoctonia solani

Globodera rostochiensis and Rhizoctonia solani are the most important growth limiting factors influencing potato production in Iran. The effects of inoculation with Potato Cyst Nematodes (PCN) (0, 50, 75 and 100 cysts/3.5?kg soil) and R. solani (with or without inoculation) on potato growth and development were investigated in cultivars Santé and Marfona. Inoculation with R. solani induced severe damage, especially when inoculation was accompanied with high density of PCN. The damage caused by R. solani tended to increase with an increase in PCN density, especially in Marfona. In Santé, number of stems or branches per plant significantly increased by inoculation with R. solani, while in Marfona it was significantly affected either by R. solani inoculation or PCN density. In Santé, number of stolons per plant was significantly increased by PCN, but not by R. solani. In Marfona, however, the number of stolons per plant was significantly affected either by R. solani inoculation or by presence of PCN, but not affected by PCN density. The general effect of R. solani or PCN inoculation treatments on shoot, below-ground and total dry weight of potato was significant, but strongly affected by cultivar. In general, our study supports the synergistic interaction between R. solani and PCN and its moderation by the use of a resistant cultivar such as Santé.     

Effect of the trap crop Solanum sisymbriifolium and two biocontrol fungi on reproduction of the potato cyst nematode,Globodera pallida

The potato cyst nematode, Globodera pallida, is one of the most important pests of potato worldwide. Owing to regulatory considerations and potential environmental impact, control options for this nematode are becoming increasingly limited. Solanum sisymbriifolium and biological control agents offer viable alternative options for controlling G. pallida. Therefore, experiments were conducted to determine the effect of the nematode trap crop S. sisymbriifolium, alone or in combination with the biocontrol agents Trichoderma harzianum or Plectosphaerella cucumerina, on population decline of G. pallida. Experiments were conducted for three different ‘cropping systems’: potato (Solanum tuberosum), S. sisymbriifolium, or soil only (fallow), each followed by a potato crop. Soil was amended with P. cucumerina, T. harzianum or left unamended, and then infested with nematodes at a rate of five eggs g^?1 of soil. After 16 weeks in the greenhouse, plants were removed and the soil containing cysts was refrigerated at 4°C for 8 weeks, and then planted to potato. Cysts of G. pallida were counted after an additional 16‐week period. The P_f/P_i of G. pallida was significantly reduced by 99% in potato following S. sisymbriifolium compared to both the potato‐following‐fallow and the potato‐following‐potato treatments. Amendment of soil with T. harzianum significantly reduced P_f/P_i of G. pallida by 42–47% in the potato‐following‐potato but not in either the potato‐after‐fallow nor in the potato‐after‐S. sisymbriifolium cycles which supports evidence that the plant species may play a role in the biocontrol activity of this fungus. Addition of the fungus P. cucumerina resulted in a 64% decrease in P_f/P_i in the potato‐following‐fallow in one experiment, and an 88% decrease in P_f/P_i in potato‐following‐potato but the decrease in P_f/P_i was not consistent over all experiments. However, both biocontrol fungi resulted in lower numbers of progeny cysts after an initial 16‐week incubation with potato. To look at the effect of varied population density of the nematode on efficacy of S. sisymbriifolium to reduce G. pallida populations, potato, S. sisymbriifolium, or barley were planted into soil infested with G. pallida at rates of 5, 20 or 40 eggs g^?1 soil applied as cysts (20, 80 or 160 cysts pot^?1). After 16 weeks, numbers of cysts produced in each treatment were determined for each infestation rate. No new cysts were recovered from either S. sisymbriifolium or barley treatments, confirming that neither plant is a host for G. pallida. High numbers of cysts were recovered with potato. Soil from each treatment (containing original cysts and newly‐formed cysts when present) were then planted with potato. After an additional 16 weeks, few cysts were found in the potato‐after‐ S. sisymbriifolium treatments regardless of initial infestation rate. When potato followed barley, numbers of cysts were similar to those found after a single cycle of potato, indicating that the barley crop had no effect on the survival of initial inoculum. Overall, these results suggest that S. sisymbriifolium has potential to significantly reduce G. pallida populations, and also that the cropping system (i.e. the sequence of non‐host and host plants) may play a significant role in the efficacy of fungal biological control agents.     

Estimation of genetic variability and heritability for biofuel feedstock yield in several populations of switchgrass

Information on heritability and predicted gains from selection for increased biomass yield for ethanol production in switchgrass is limited and may vary among breeding populations. The purpose of this study was to estimate heritability and predicted gains from selection for higher biomass yield within a lowland ecotype switchgrass population, Southern Lowland 93 (SL‐93), and two upland ecotype switchgrass populations, Southern Upland Northern Upland Early Maturing (SNU‐EM) and Southern Upland Northern Upland Late Maturing (SNU‐LM). Narrow‐sense heritabilities (h_n²) for biomass yield in each of the three populations were estimated via progeny–parent regression analysis. Half‐sib (HS) progeny families from 130 randomly selected plants from the SL‐93 population were evaluated for biomass yield in replicated trials in 2002 and 2003. Clonal parent plants were evaluated for biomass yield in separate environments to provide unbiased h_n² estimates from progeny–parent regression. Yield differences were highly significant among SL‐93 HS progenies within and over years. For the SL‐93 population, h_n² estimates were 0.13 and 0.12 based on individual plant and phenotypic family mean (PFM) selection, respectively. Predicted genetic gains (ΔG) per selection cycle were 0.15 kg dry matter (dm) plant^?1 and 0.10 kg dm plant^?1 for PFM and individual plant selection methods, respectively. For the SNU‐EM and SNU‐LM populations, year and year × HS family effects were highly significant (P < 0.01) and the HS family effect over years was nonsignificant (P < 0.05). However, HS family effects were highly significant within respective years (P < 0.01). Estimates of h_n² for the SNU‐EM and SNU‐LM populations based on PFM and individual plant selection were similar, ranging from 0.44 to 0.47; ΔG per selection cycle ranged from 0.22 to 0.33 kg dm plant^?1. The magnitudes of the estimates of additive genetic variation suggest that selection for higher biomass yield should be possible. The substantial effect of environment on biomass yields in the upland populations and the failure of families to respond similarly over years stress the importance of adequately testing biomass yield over years to assess yield.     

Effects of Zygorrhynchus moelleri on the growth of potato and reproduction of potato cyst nematodes

Laboratory, pot and field experiments investigated the effects of the fungus Zygorrhynchus moelleri on the growth of potato and on the reproduction of the potato cyst nematodes (PCN), Globodera pallida and G rostochiensis. Preliminary laboratory tests showed that Z. moelleri growth was favoured by temperatures and pH ranges commonly present in field soils. The fungus colonised potato roots in vitro and in compost or field soil. It also stimulated in vitro root growth of three potato cultivars. In pot experiments Z. moelleri stimulated potato growth, particularly in the presence of PCN attack. In field plots infested with a mixture of G pallida and G. rostochiensis, tuber yields were not increased after application of the fungus but, in G pallida‐infested plots, yields were significantly increased after drills were inoculated with Z. moelleri. The application of Z. moelleri had no apparent effects on nematode reproduction. Factors influencing the interactions between Z. moelleri, potato and potato cyst nematodes are discussed and the potential role of the fungus as a plant growth promoter in organic potato production considered.