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

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

Aggressiveness of Mycosphaerella graminicola Isolates from Susceptible and Partially Resistant Wheat Cultivars

ABSTRACT The selective effect of quantitative host resistance on pathogen aggressiveness is poorly understood. Because two previous experiments with a small number of bread wheat cultivars and isolates of Mycosphaerella graminicola had indicated that more susceptible hosts selected for more aggressive isolates, we conducted a larger experiment to test that hypothesis. In each of 2 years, six cultivars differing in their levels of partial resistance were planted in field plots, and isolates were collected from each cultivar early and late in the growing season. The isolates were inoculated as populations bulked by cultivar of origin, field replicate, and collection date on seedlings of the same six cultivars in the greenhouse. The selective impact of a cultivar on aggressiveness was measured as the difference in aggressiveness between early and late isolates from that cultivar. Regression of those differences on disease severity in the field yielded significance values of 0.0531 and 0.0037 for the 2 years, with moderately resistant cultivars selecting for more aggressive isolates. In a related experiment, the protectant fungicide chlorothalonil was applied to plots of two susceptible cultivars to retard epidemic development. When tested in the greenhouse, isolates of M. graminicola from those plots were significantly more aggressive than isolates from the same cultivars unprotected by fungicide.     

Frequency of Sexual Reproduction by Mycosphaerella graminicola on Partially Resistant Wheat Cultivars

ABSTRACT The frequency of sexual reproduction has a profound effect on the population structure and the adaptive potential of a facultatively sexual parasite. Little is known about the relationship of quantitative host resistance to the frequency of sex in pathogens. We sampled over 5,000 fungal fruiting bodies from eight different wheat cultivars over a 3-year period. The cultivars possessed varying degrees of susceptibility to Mycosphaerella graminicola, a facultatively sexual pathogen that is hetero-thallic and bipolar. The fruiting bodies were classified as M. graminicola pycnidia or ascocarps (asexual and sexual fruiting bodies, respectively), other identifiable fungi, or unidentified. In all 3 years, area under the disease progress curve (AUDPC) explained a significant proportion of the variation in ascocarps as a percentage of M. graminicola fruiting bodies (P < 0.0005). The mean percentage of M. graminicola ascocarps from all cultivars was 63% in 1998, when the epidemic was intense, and 14% in 1999, a year of low disease levels. In 2000, samples were taken at 7-day intervals from 6 June to 27 June from two cultivars with substantially different AUDPCs (788 and 2,185 percentage-days). The less diseased cultivar yielded its first M. graminicola ascocarps 1 week later than the more diseased cultivar, and respective means of ascocarps as a percentage of M. graminicola fruiting bodies across sampling dates were 20.2 and 59.3%. The ratio of sexual to asexual reproduction by M. graminicola is likely to be strongly conditioned by infection density.     

A Model for Infestation Foci of Potato Cyst Nematodes Globodera rostochiensis and G. pallida

ABSTRACT Infestation foci of potato cyst nematodes (Globodera rostochiensis and G. pallida) in the provinces of Zeeland, Friesland, Groningen, and Drente of the Netherlands were sampled to validate a model describing infestation foci based on data from Flevoland. Eighty-two fields were presampled to locate infestation foci using a coarse sampling grid (8 x 3 m). Parts of 37 fields containing one or more foci were sampled intensively by extracting at least 1.5 kg of soil per square meter (1.33 x 0.75 m). Forty foci were analyzed for spatial distribution characteristics of cysts using multiple regression with generalized linear models and classical linear models. The results showed that the data from all investigated cropping areas fit well to an exponential model with two parameters, the length and width gradient parameters. Significant differences in these parameter values between cropping areas could not be demonstrated. As both parameters follow a normal distribution, the probability of any combination can be described by a bivariate normal distribution. Gradient parameters were correlated, but significant correlations between these parameters and certain variables such as the nematode species involved, the time interval between sampling and the last potato crop, soil type, cropping frequency, and cyst density in the focus center could not be demonstrated.     

Effects of Potato-cyst Nematodes (Globodera pallida) and Soil pH on Root Growth, Nutrient Uptake and Crop Growth of Potato

Potato-cyst nematodes (Globodera pallida) cause severe yield losses in potato. Plants infected with potato-cyst nematodes generally have reduced concentrations of nitrogen, phosphorus and potassium in the foliage. This study investigated whether reduced growth of nematode-infected potato is caused by nutrient limitation.Experiments in the field and in containers showed that phosphorus concentration correlated best with total crop biomass at early stages of growth. The role of phosphorus in nematode damage was further investigated in the field and in the Wageningen Rhizolab. The experimental field was infested with potato-cyst nematodes and two levels of nematode density were established by fumigation with a nematicide. Prior applications of calcium carbonate resulted in pHKCl levels of 4.8 and 6.1. Two levels of phosphorus fertiliser were applied: either 0 or 225kgPha–1. In the Wageningen Rhizolab, soil of both pH levels from the field was used after treatment with 1MRad gamma irradiation to kill the nematodes. Subsequently, half of the soil was inoculated with cysts to give a nematode density of 30 viable juveniles per gram of soil.In the field, nine weeks after planting, the total crop biomass ranged from 107gm–2 for the treatment with nematodes at pHKCl 6.1 without phosphorus fertiliser to 289gm–2 for the fumigated treatment at pHKCl 4.8 with phosphorus fertiliser. The differences in total biomass for the various treatments were explained by differences in foliar phosphorus concentration. Nematodes induced or aggravated P deficiency and reduced total biomass. This was not the only damage mechanism as at high, non-limiting levels of foliar phosphorus concentration, nematodes still reduced total biomass.In the Wageningen Rhizolab, directly after planting, the number of roots visible against minirhizotrons was reduced by nematodes. However, the increase of root number in the nematode treatment continued longer than in the control, until root number was higher than that of the control. The compensary root growth of the nematode treatment was restricted to the top 30cm and nematodes reduced rooting depth.High soil pH reduced growth, mainly by reducing the availability of phosphate. Both nematodes and high soil pH reduced nutrient uptake per unit root length. Our results lead us to suggest an interaction between nematodes and soil pH, with nematode damage being higher at pHKCl 6.1 than at pHKCl 4.8.     

Decline of potato cyst nematodes, Globodera rostochiensis and G. pallida, in spring barley microplots

In microplots of nine soils freed from volunteer potatoes and sown for four consecutive years with spring barley, the eggs of five populations of Globodera pallida and four populations of G. rostochiensis declined at annual rates of 12-8-40-5%. One population of G. rostochiensis declined faster than other populations of either species, although average rates of decline did not differ significantly between the two species of Globodera. These results are compared with earlier research and their relevance to control of the nematodes by crop rotation is discussed.     

The demarcation of Globodera rostochiensis and Globodera pallida infestations in fields for seed potatoes, using a Monte Carlo approach

A Monte Carlo approach was utilized to calculate the size of the demarcation unit, necessary to cover infestations with Globodera rostochiensis and Globodera pallida (PCN), detected with the new EU-sampling method. Results from 90 sampled fields were used to obtain frequency distributions of the central population density (CPD), the length and width gradients of infestation foci and the distance between 'primary' and 'secondary' foci within one field section in the direction of cultivation. With a simulation model written in R, a free software environment for statistical computing, the dimensions of the demarcation unit were calculated, based on the dimensions of the sampling unit, the position of the 'primary' infestation within the sampling unit and the size of the infestation focus. It is concluded that a demarcation of 90% of possible foci positions and dimension, analogous to the specified detection probability of the EU sampling method, provides sufficient security to demarcate any infestation. With the preferred sampling unit of 11 m width and 300 m length (1/3 ha sampled), used in The Netherlands, only 0.61% of the cyst numbers will be situated outside the demarcated unit. These cyst numbers compare to 0.17% of total amount of cysts in the EU infestation model: 1 focus with a CPD of 100 cysts kg⁻¹ and three foci with CPDs of 50 cysts kg⁻¹. The required demarcation area has a width of 36 m and a (maximum) length of 402 m and covers an unit of 1.5 ha. The methodology described can be applied to all proposed sampling methods and sampling unit dimensions.     

Resistance of potato varieties to field populations of the potato cyst nematode Globodera pallida in the northeastern part of the Netherlands

In the northeastern part of the Netherlands starch potatoes are grown intensively. From 392 fields in that area, soil samples that contained the potato cyst nematodeGlobodera pallida were used to analyze the resistance of 7 varieties of starch potatoes.Differences and similarities in resistance of the varieties to the field populations ofG. pallida could well be explained by their descent from distinct sources of resistance.TheG. pallida-populations that showed a relatively high virulence to the varieties Producent, Elles, and Darwina were not equally distributed over the area, but were confined to a small number of regions.     

Development and Evaluation of Sampling Methods for Fields with Infestation Foci of Potato Cyst Nematodes (Globodera rostochiensis and G. pallida)

ABSTRACT A computer program, SAMPLE, was developed to evaluate existing and create new sampling methods for the detection of infestation foci of the potato cyst nematode (Globodera rostochiensis and G. pallida). By combining a model for the medium-scale distribution of cysts that provides the expected population densities at each position within the focus and a model for the small-scale distribution within square meters, sampling procedures can be simulated. The importance of the parameters of the two distribution models-the length and width gradient for the medium-scale distribution and the aggregation factor k of the negative binomial distribution for the small-scale distribution-was investigated by sensitivity analysis. Parameter k proved to be less important when calculating the average detection probability of a focus than were the length and width gradient parameters. Several existing versions of the statutory sampling method used in the Netherlands were tested for their performance on a standard infestation focus with a central population density of 50 cysts per kg of soil, which is small enough to use resistant potato cultivars as a control measure without noticeable yield reductions in a 1:3 year cropping frequency with potato. These methods did not perform with the desired average detection probability, set at 90%; therefore, SAMPLE was used to develop several new sampling methods for focus detection. SAMPLE is a tool to develop sampling methods on demand for every possible combination of characteristics required for use by seed and ware potato growers (recommendations for optimum control measures leading to maximum returns and integrated pest management) and by governments (legislation, quarantine, and export protection).     

Quantification of Globodera rostochiensis and G. pallida in mixed populations using species-specific thermostable proteins

A method has been developed to quantify species ratios in mixed populations. The method is based on the separation of species-specific thermostable proteins by SDS-PAGE. Densitometric analyses of the 17 kD protein ofGlobodera pallida and the 18 kD protein ofG. rostochiensis revealed a high correlation (R ²=0.93) with the species ratio in the mixed samples. Within the limits of 10 to 90% of each species, one can estimate with 95% reliability the species composition with 3 to 6% deviation.Samenvatting Een methode is ontwikkeld om de samenstelling van soortenmengsels vanGlobodera rostochiensis enG. pallida te kwantificeren. Bij deze methode wordt gebruik gemaakt van de soort-specifieke thermostabiele eiwitten die met behulp van SDS-PAGE gescheiden worden. De kleurintensiteit van het 17 kD eiwit vanG. pallida en het 18 kD eiwit vanG. rostochiensis is per gel-laan bepaald m.b.v. een densitometer en heeft een lineair verband met de soortsverhouding in de mengsels (R ²=0.93). Binnen het bereik van 10 tot 90% van elke soort kan men met deze ijklijn met 95% betrouwbaarheid de soortsamenstelling bepalen op 3 tot 6% nauwkeurig.     

Population dynamics of the potato cyst nematode,Globodera pallida,in relation to temperature,potato cultivar and nematicide application

The impact of increasing temperatures on the population dynamics of the soil-dwelling nematode Globodera pallida, a persistent and economically important pest of potatoes, was investigated. The reproductive factor (final population⁄initial population) and length of life cycle were found to be temperature sensitive. Pot experiments performed over 4 months allowed comparison of the effect on development of G. pallida of two temperature regimes: an average temperature comparable to current field conditions (14.3 °C) and an average temperature above current field conditions (17.3 °C). A larger second generation of juveniles was observed at 17.3 °C compared to 14.3 °C. Multiplication of G. pallida at field sites in Shropshire and East Lothian (average soil temperatures of 15.5 and 14.1 °C, respectively, during potato cropping) was also examined. A quantitative PCR assay and visual examination of roots were used to monitor the dynamics of the G. pallida populations in both field sites at 4-weekly intervals. Four cultivars, Desirée, Cara, Maris Piper and Estima, were grown with and without nematicide treatments. Nematicide treatments suppressed population increases at both sites. Females were observed on the roots of cvs Cara and Desirée at the end of the growing season in Shropshire, but not at East Lothian, and are likely to represent a second generation.     

Resistance to Peach-potato Aphid,Myzus persicae Sulzer (Hemiptera: Aphididae) in Potato Cultivars

Previous research suggests that potato cultivars offer little aphid resistance. However, few studies have measured the effects of host cultivar on aphid age dependent life table statistics or related these measures to field performance. In this study, laboratory and field studies assessed four commercial potato cultivars (Anya, Desiree, Pink Fir Apple, Santé) for resistance to Myzuspersicae. Cultivars were found to show considerable differences in resistance. In the laboratory, the intrinsic rate of increase (r_m) and effective fecundity of M. persicae was lowest on cv. Anya and highest on cv. Desiree. Aphids also took longer to develop to adults and were smaller in size on Anya compared with Desiree. Field studies over two seasons found that M. Persicae infestations started earlier on Desiree than on Anya and that the greatest weekly counts were recorded on the former.     

Effect of Juglone on Active Oxygen Species and Antioxidant Enzymes in Susceptible and Partially Resistant Banana Cultivars to Black Leaf Streak Disease

The black leaf streak disease (BLSD), caused by Mycosphaerella fijiensis, is the most destructive disease of bananas and plantains around the world. Breeding for resistance is the most promising strategy to fight this disease especially in small farmer plantations. Mycosphaerella fijiensis produces many phytotoxins such as juglone, which can be used, jointly with field and inoculations under controlled conditions, for screening banana cultivars for BLSD-resistance. This non-host specific phytotoxin has been shown to act on chloroplasts and disturbs the proton electrochemical gradient across the plasmalemma membrane. Moreover, an involvement of the oxidative burst during the interaction has been suggested. The present study was carried out using two cultivars that differed for either their juglone-responses or their resistance to BLSD (cv. Grande Naine susceptible to BLSD and juglone and cv. Fougamou partially resistant to BLSD and highly tolerant to juglone). The production of active oxygen species (AOS) and the enhancement of the enzymatic and/or non-enzymatic AOS-scavenging systems were investigated after treatment of the two cultivars with juglone. The time-course of AOS-production and AOS-scavenging was shown to be the key difference between these two tested cultivars after treatment with juglone. Thus, an early release of AOS (O₂⁻ radical and H₂O₂) and a quick stimulation of a preferment anti-oxidant system (superoxide dismutases, catalases, and peroxidases) was observed for cv. Fougamou as compared to cv. Grande Naine for which a late and weak generation of AOS accompanied by a late stimulation of the anti-oxidant systems were detected.     

Specific Behaviour of French Aphanomyces Euteiches Drechs. Populations For Virulence and Aggressiveness on Pea, Related to Isolates from Europe, America and New Zealand

The pathogenic variability of Aphanomyces euteiches on pea was investigated using a collection of 88 pea-infecting isolates from France and 21 isolates from Denmark, Sweden, Norway, USA, Canada and New Zealand. Aggressiveness and virulence were assessed by scoring the root symptoms on a differential set of six pea genotypes. Eleven virulence types were characterised. The virulence type I, previously described as virulent on the whole set, was predominant and included the most aggressive isolates of all geographical origins. The other types were much less prevalent, existing as one to five isolates. Three virulence types (III, IV and V) contained no French isolates. The type III, avirulent on MN313, was composed of American isolates only, and resembled the major group recently described in the USA. A wide range of aggressiveness was found within the virulence type I, and the French isolates appeared globally more aggressive than the foreign isolates. These findings indicate that isolates from the virulence type I should be used as references in breeding programs, and that pea lines PI180693 and 552 may be the most interesting resistance sources to date, despite their only partial resistance.     

Selection for reproductive ability in Globodera pallida populations in relation to quantitative resistance from Solanum vernei and S. tuberosum ssp. andigena CPC2802

Four field populations of the nematode Globodera pallida were subjected to selection pressure for increased reproductive ability by rearing sub-populations continuously on four partially resistant potato genotypes for 12 generations. The resistance was derived from either Solanum vernei or from S. tuberosum spp. andigena CPC2802. After the 12th generation the original and sub-populations of nematodes were assessed for their reproductive ability on a susceptible genotype and on each of the partially resistant genotypes. Selection pressure was shown to have increased reproductive ability but the increases were specific to the source of resistance used. The average increase on the ex S. vernei clones was from 11% reproduction by the unselected populations to 35·5% reproduction after selection. On the clones derived from CPC2802, which had higher levels of resistance, the increases were larger with an average of 6·6% reproduction for the unselected but 47·4% reproduction after selection. The response to selection differed amongst the initial field populations with some rates of reproduction increased to as much as 79%. A RAPD based analysis of the original and sub-populations after selection indicated small but consistent changes in the genetic structure, which could have been a result of the selection pressure per se and/or the bottlenecks that the populations had gone though.     

Variation in Pathogenicity and Multiplication of Beet Necrotic Yellow Vein Virus (BNYVV) in Relation to the Resistance of Sugar-beet Cultivars

Some partially resistant cultivars varied in their response to beet necrotic yellow vein virus (BNYW), which could be due to the occurrence of different pathotypes. In the past three different types of BNYVV could be identified. Since in the field no consistent cultivar×virus source interaction could be detected, greenhouse trials were carried out under more standardised conditions, starting with a similar initial density of BNYVV. Cultivars with different degrees of resistance varied in their response to various types of beet necrotic yellow vein virus (BNYVV). The B type appeared to be less damaging than the A and P types. The virus content in the tap roots and the ratio of the virus content in tap roots to that in lateral roots were both higher in P type than in A or B type infections indicating that the P type moves more rapidly in the plants than the two other BNYVV types. The percentage of plants in which the virus reaches only a low concentration (less than 56ng/ml of sap) is much lower in P type than in A or B type infections. Frequency distribution diagrams of individual plants showing different BNYVV levels reveal considerable differences between various cultivars.     

Assessing reproduction of potato cyst nematodes (Globodera rostochiensis and G. pallida) on potato cultivars for National Listing1

To investigate testing procedures for assessing the reproduction of potato cyst nematodes (PCN) on potato cultivars, and to minimize variation in results, trials were done jointly for 3 years at two UK National Listing test centres (DANI in Northern Ireland, DAFS in Scotland). Test clones comprised susceptible, resistant and ‘partially resistant’ cultivars. The factors under investigation included the effect of season, site, location, temperature, PCN population, inoculation method, pot size, hatch rates and methods of assessment of reproduction. Standard procedures are recommended for testing potato cultivars for resistance to PCN.     

The potential value of British wild Solanum spp. as trap crops for potato cyst–nematodes, Globodera rostochiensis and G. pallida

Two forms of Solanum nigrum subsp. nigrum, two forms of S. nigrum subsp. schultesii, S. americanum, S. chenopodioides, S. sarrachoides, S. scabrum, S. villosum subsp. villosum and S. villosum subsp. puniceum were grown in pots for over 4 months to measure their effect on numbers of potato cyst–nematodes, Globodera rostochiensis and G. pallida, in the soil. No Solanum tested significantly reduced numbers of G. rostochiensis or G. pallida. Solanum americanum, S. sarrachoides and S. villosum puniceum increased numbers of G. pallida slightly.