Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    A routine test for the relative susceptibility of potato genotypes with resistance to Meloidogyne chitwoodi
    Teklu, Misghina G. ; Schomaker, Corrie H. ; Been, Thomas H. ; Molendijk, Leendert P.G. - \ 2016
    Nematology 18 (2016)9. - ISSN 1388-5545 - p. 1079 - 1094.
    The population dynamics of Meloidogyne chitwoodi on eight potato genotypes was compared to the susceptible cv. Desiree in four glasshouse experiments. The initial nematode densities consisted of log series 2x , with −4 < x < 8. Seinhorst’s logistic model was fitted to the final population densities to estimate the parameters maximum multiplication rate (a), maximum population density (M) and the ratios RSa, RSM and M/a. Average RSa and RSM of the seven resistant genotypes were smaller than 0.29%. The M/a ratios on six resistant genotypes and cv. Desiree were the same, 1.3, indicating Pi independence of RS. One genotype stood out with M/a = 8.6, whereby RSa < RSM. Both RS and M/a were unaffected by pot size or experimental conditions. Screening protocols at Pi = 24 second-stage juveniles (g dry soil)−1 in 2 or 3 kg pots were evaluated for distinctiveness between the two genotype groups. Based on the results, an optimal protocol for a routine resistance test is proposed.
    A routine test for Relative Susceptibility estimation of potato genotypes with resistance to Meloidogyne chitwoodi
    Been, T.H. ; Schomaker, C.H. ; Teklu, M.G. - \ 2016
    - p. 98 - 98.
    Several aspects of the Relative Susceptibility (RS) of eight potato genotypes, one susceptible and seven partially resistant against M. chitwoodi, were studied in four successive greenhouse experiments, in 10, 5, 3 and 2 kg pots. The cultivar Désirée was used as a susceptible control. The initial nematode densities (Pi), consisted of a 2 log series: 2x, with x ranging from - 4 till 8. The final population density (Pf) was estimated in the roots, soil and, in one experiment, also in the tubers. The proportion of nematode free tubers was estimated in all experiments. Seinhorst’s logistic model of population dynamics was fitted to the data to estimate the maximum multiplication rate (a), maximum population density (M), RSa and RSM. Apart from one genotype, resistant to G. pallida but susceptible to M. chitwoodi, the averaged estimates for RSa and RSM of all tested genotypes were smaller than 0.29% and 0.23%, respectively, with coefficients of variation of 0.21 and 0.09, respectively. The M/a ratio for six out of seven genotypes and Désirée was 1.3, indicating that for these genotypes RS is independent of Pi (RSa = RSM). RS can, therefore, be tested at one well-chosen Pi.
    Although a and M were sometimes influenced by pot size, both RS and M/a were not. A choice of 24 J2 (g dry soil)-1 as a Pi and pot sizes of 2 or 3 kg for routine testing is discussed. Tuber infestation levels after 8-10 months of storage, only estimated in Exp. 2, was 0.35 J2 (g dry soil)-1 for Désirée and less than 0.0015 J2 (g dry soil)-1 for four tested genotypes, on average 2% of the total Pf.
    Damage thresholds and population dynamics of Pratylenchus penetrans on carrot (Daucus carota L. cv. Nerac) at three different seed densities
    Teklu, Misghina G. ; Meressa, B.H. ; Radtke, Esther ; Been, T.H. ; Hallmann, Johannes - \ 2016
    European Journal of Plant Pathology 146 (2016)1. - ISSN 0929-1873 - p. 117 - 127.
    Lesion nematodes - Modelling - Population density - Quality loss - Tolerance limit and yield loss

    Yield and quality loss of carrot (Daucus carota L. cv. Nerac) caused by Pratylenchus penetrans and the population dynamics of this nematode were studied in a climate controlled glasshouse. A range of 12 nematode densities was used at three different seed densities of carrot; 2, 4 and 18 seeds pot−1. Seinhorst’s yield loss model; y = m + (1 - m) 0.95Pi/T-1 for Pi > T; y = 1 for Pi ≤ T for Tylenchina was fitted to the yield and quality loss data. Seinhorst’s model for population dynamics of migratory nematodes with multiple generations; (Formula presented.) was fitted to the data of the final population densities (Pf). P. penetrans had a significant impact on carrot taproot yield and its quality. The tolerance limits for the relative carrot taproot yield (Ty) were 1.51, 1.88, and 1.37 and those of quality yields (Tq) were 0.67, 0.18, and 0.40 P. penetrans (g dry soil)−1 at 2, 4 and 18 seeds pot−1, respectively. Both the minimum yield (0.20, 0.29, and 0.60) and the minimum quality yield (0.05, 0.07, and 0.20), expressed as a proportion, increased with seed density at 2, 4 and 18 seeds pot−1, respectively. The model for population dynamics fitted well to the Pf data obtained. The maximum multiplication rates (a) were 19.58, 9.99, and 17.54, while the maximum population densities (M) were 49.86, 43.21, and 60.37 P. penetrans (g dry soil)−1 at 2, 4, and 18 seeds pot−1, respectively. Carrot cv. Nerac can be considered a good host for P. penetrans.

    Survival and vitality of J2 of meloidogyne chitwoodi in time in pot experiments
    Teklu, M.G. ; Been, T.H. ; Schomaker, C.H. ; Beniers, J.E. - \ 2015
    In: Proceedings of the 67th International Symposium on Crop Protection. - Ghent : - p. 21 - 21.
    Survival in time of inoculated juveniles of Meloidogyne chitwoodi (J2) in the absence of host and their vitality on a susceptible host was studied in a pot experiment under glasshouse conditions. This is, to assess if part of the Pi can survive and become part of the Pf at harvest and affect the partial resistance estimator when testing for resistance. In total 96, 2 kg pots were filled with soil, brought to 10-12% moisture content, and a nematode density (Pi) of = 55.4 J2 g-1 dry soil was added. Immediately after inoculation, at time (t = 0) and weekly, during 15 weeks, batches of 6 pots were used to collect a 500 g sub-sample, which were elutriated using the Seinhorst elutriator to estimate the number of surviving nematodes. The remainder of the soil from each batch (1.5 kg) was stored at 4°C and used to check the vitality of the surviving J2 on the susceptible tomato cv. Moneymaker. The actual average recovery at (t = 0) after inoculation was 35.05 J2 g-1 dry soil indicating a loss of about 37%. Juveniles of M. chitwoodi could be recovered during all 112 days, although population densities dropped drastically to 0.44 J2 g-1 dry soils. The data of the surviving juveniles in time fitted well to an exponential model (R2 = 0.98). According to the model the daily mortality was 10.72% and 0.73% of the nematodes at time zero survived for 112 days. The initial loss of 37% of the inoculum is the result of losses during inoculation, mortality during mixing and reduction in elutriation. The efficiency of the Seinhorst elutriator for M. chitwoodi juveniles is 92%. When eliminating this error the actual loss declines to 31.23%. The largest percentage of this loss is mortality due to mixing of the soil before sub-sampling; a small percentage is lost during inoculation. Based on the decline data, the effect of the possible survival of inoculated juveniles on the estimator for partial resistance in pot tests can be calculated. The errors were researched both by simulation and by re-analysing some pot experiments with highly resistant potatoes genotypes and fodder radish varieties and will be discussed. In practice, the problem of surviving inoculated nematodes in pot experiments seems remote as a very large, if not the whole, of the volume of soil in the pot is rooted by the test plant and no J2 of the Pi will be available in the soil. The surviving nematodes from all ages were able to infect and multiply on the susceptible tomato cv. Moneymaker. The data obtained suggests two processes in this experiment: The first pattern shows a normal multiplication rate (a) of 11.2 and a maximum population density (M) of 43.2 J2 g-1 dry according to the population dynamics model at the highest nematode densities obtained in the first 29 days of the decline. A second pattern indicates a mortality of J2 in storage at 4°C as expressed in larger multiplication rates at shorter storage time at identical nematode densities. Key words: Population decline, multiplication, resistance testing and storage temperature
    Damage thresholds and population dynamics of Meloidogyne chitwoodi on carrot (Daucus carota) at different seed densities
    Heve, W.K. ; Been, T.H. ; Schomaker, C.H. ; Teklu, M.G. - \ 2015
    Nematology 17 (2015)5. - ISSN 1388-5545 - p. 501 - 514.
    plant-parasitic nematodes - root-knot nematode - partial resistance - potato - cultivars - increase - hapla
    Yield loss of carrot (Daucus carota) cv. Nerac caused by Meloidogyne chitwoodi and population dynamics of this nematode were studied using a range of 13 nematode densities at three seed densities (2, 4, 18 seeds pot-1) in a climate-controlled glasshouse. Yield and quality data were fitted to Seinhorst’s yield models. Final population densities were fitted to the population dynamic models for sedentary and free-living nematodes. The tolerance limits for yield loss were 0.34, 0.62 and 0.50, while that of quality were 0.012, 0.142 and 0.813 second-stage juveniles (J2) (g dry soil)-1 at increasing seed densities, respectively. The minimum yield (m), increased with seed density: 0.25, 0.30 and 0.50; the minimum quality yield was 0.10, 0.08 and 0.15 J2 (g dry soil)-1 at increasing seed densities, respectively. Both maximum multiplication rates and maximum population densities increased with increasing seed density but were generally low. Carrot cv. Nerac can be considered a bad host for M. chitwoodi.
    Extraction efficiency of J2 of M. chitwoodi from roots and potato peels using the Seinhorst spray mist-chamber
    Teklu, M.G. ; Been, T.H. ; Schomaker, C.H. ; Beniers, Annelies ; Altena, L. ; Beers, T.G. van; Boomsma, D. ; Molendijk, L.P.G. - \ 2013
    Efficiency of the rebuild Seinhorst elutriator in the recovery of M. chitwoodi juveniles from the soil
    Teklu, M.G. ; Been, T.H. ; Schomaker, C.H. ; Beniers, Annelies - \ 2013
    In: Proceedings of the 65th International Symposium on Crop protection, Ghent, 21-05-2013. - Ghent, Belgium : Ghent University - p. 70 - 70.
    Methodology development for partial resistance testing of potato cultivars resistant to M. chitwoodi
    Teklu, M.G. ; Been, T.H. ; Schomaker, C.H. - \ 2013
    In: Proceedings of the 52nd Annual meeting of Society of nematologist, July 14- 17, 2013, Knoxville, Tennessee, USA. - Knoxville, Tennessee, USA : - p. 106 - 107.
    M. chitwoodi was first described in 1995 in The Netherlands (Karssen, 1995). It is now listed as a quarantine organism in the EPPO region with 4 EU member states officially infested. Since 1996, research has been initiated to identify resistant genes against M. chitwoodi from wild species of tuber bearing potatoes and integrate these genes into cultivated potatoes. Currently, several breeding companies successfully produced resistant genotypes against M. chitwoodi. Parallel to this, research was started to develop a standard methodology to screen the partial resistance of these genotypes. Population dynamical models were used to estimate their level of resistance, expressed as percentage of relative susceptibility (rs). This methodology provides farmers with quantitative information on the effect of growing resistant potatoes at any initial population density in their field. The models were first tested in a pilot project in 2010 with 3 resistant potato genotypes in (5 kg) pots at a range of 13 nematode densities. In 2011, another 8 genotypes were tested in (10 kg) pots at a range of 12 densities. The results showed that Seinhorst’s population dynamical models for nematodes with multiple generations fitted well, except in one genotype tested in 2011which lacked resistance to M. chitwoodi, and a reduction of the number of densities used seems possible. In 2012, research was initiated to investigate whether the pot size can be downscaled from 10 to 5 or even to 2 kg pots – also at 12 densities -, without loss of quality of the estimated relative susceptibility. Also, growth, yield loss and quality damage as root knot index (RKI) were assessed and compared. Genotypes 2011M1, MDG2 and cv. Désirée (control) were the tested potatoes. The population dynamical model fitted well for the genotypes tested. The maximum multiplications rate “a” and the maximum population density “M” at 2, 5 and 10 kg pots were estimated and used to calculate the rsa and rsM values . Despite a decrease in “a” and “M” values with increasing pot size the rsa values were relatively stable. The rsM values were a bit higher in 5 kg pots. Seinhorst yield models used to describe the fresh tuber weight also fitted well. The RKI values obtained from the three pots sizes were also stable as a quality measure for industrial processing. Overall results indicate the possibility of downscaling the resistance test for M. chitwoodi in potato in terms of pot size and number of densities. Implications of the current research in the development of a cheap and reliable resistance test will be discussed.
    Estimation of partial resistance in potato genotypes against Meloidogyne chitwoodi
    Norshie, P.M. ; Been, T.H. ; Schomaker, C.H. - \ 2011
    Nematology 13 (2011)4. - ISSN 1388-5545 - p. 477 - 489.
    root-knot nematode - plant parasitic nematodes - globodera-pallida - population-density - increase - clones
    Three new potato genotypes, designated AR 04-4107, AR 04-4096 and AR 04-4098, with resistance towards Meloidogyne chitwoodi, and the susceptible cv. Désirée were grown at a range of population densities of M. chitwoodi in a climate-controlled glasshouse in order to establish the presence and degree of partial resistance. Tuber parts of about 12 g were planted at densities (Pi) of 0, 0.5, 1, 2, 4, 8, 16, 32, 64, 128 and 256 second-stage juveniles (J2) (g dry soil)-1. The plants were allowed to grow for a period of 105 days. Tomato cv. Moneymaker was included and inoculated at Pi = 2 J2 (g soil)-1 to verify the quality of the inoculum by measuring the multiplication rate. Plant height was measured weekly over 11 weeks. At harvest, fresh shoot, root and tuber weights, and number of tubers were measured to express yield. Final population densities (Pf) were calculated as the total number of nematodes found in soil and roots. Tubers were scored for visible symptoms and a root-knot index was calculated. The relation between pre-plant population densities (Pi) and nematode densities at harvest (Pf) was fitted using R. The multiplication rate a of M. chitwoodi on AR 04-4107, AR 04-4096, AR 04-4098 and cv. Désirée was 0.55, 0.27, 0.91 and 32, respectively. Partial resistance rsa of AR 04-4107, AR 04-4096 and AR 04-4098 was 1.7%, 0.8% and 2.8%, respectively. Partial resistance expressed as rsM was 0.2%, 0.2% and 0.1%, respectively. It can be concluded that AR 04-4107, AR 04-4096 and AR 04-4098 are strongly partially resistant to M. chitwoodi. Also, the population dynamics curves run almost parallel between both the tested genotypes and the reference cultivar, indicating that a simple and cheap partial resistance test is feasible. When tuber yields were fitted to the Seinhorst model for yield reduction, cv. Désirée showed a minimum yield (m) of 0.86, while all three resistant genotypes suffered no yields losses at all (m = 1), which indicates that the observed resistance was associated with tolerance. As a result of the remarkably high partial resistance, quality damage was low compared with cv. Désirée. The root-knot index, which takes into account internal quality damage of the potato tuber, was below 10 for all genotypes with partial resistance, the lower damage threshold used for industrial processing of consumption potatoes. Visible symptoms on the tuber skin were absent up to densities of 32 J2 (g soil)-1 for genotypes AR 04-4098 and AR 04-4096 and 2 J2 (g soil)-1 for AR 04-4107, and significantly reduced at higher densities when compared with the susceptible cv. Désirée. However, when tuber peels were investigated, egg masses were detected in tubers at almost all initial population densities.
    A scaled-up Seinhorst elutriater for extraction of cyst nematodes from soil
    Been, T.H. ; Bekkum, P.J. van; Beers, T.G. van; Beniers, J.E. - \ 2007
    Nematology 9 (2007)3. - ISSN 1388-5545 - p. 431 - 435.
    heterodera cysts
    In order to process large soil samples containing potato cyst nematodes, the Seinhorst (1964) cyst elutriator was scaled up to process both sandy and marine-clay soils in batches of up to 2.5 kg. Several modifications were implemented. To maintain the required upward current of 3.01 min¿1, an inflow of 8.01 min¿1 was necessary in the enlarged, 7.5 cm diam. sedimentation tube. Also water inflow is now regulated using a flow meter with pressuriser. Several experiments were undertaken, using artificial sandy soil and marine-clay soil, both naturally infested with potato cyst nematodes. In the final experiment, using the 8.01 min¿1 inflow for 4 min, there was a loss of 0.65% and 0.74% of cysts, and eggs and juveniles, respectively. This indicates that the yield (99%) of the scaled-up elutriator matches that of the original Seinhorst elutriator and, therefore, is suitable for statutory and scientific research
    Radopholus arabocoffeae sp. n. (Nematoda: Pratylenchidae), a nematode pathogenic to Coffea arabica in Vietnam, and additional data on R. duriophilus
    Trinh, P.Q. ; Nguyen, C.N. ; Waeyenberge, L. ; Subbotin, S.A. ; Karssen, G. ; Moens, M. - \ 2004
    Nematology 6 (2004)5. - ISSN 1388-5545 - p. 681 - 693.
    fragment-length-polymorphism - burrowing nematode - similis - reproduction - thorne - banana - dna
    Radopholus arabocoffeae sp. n., a new nematode pathogenic on Coffea arabica cv. Catimor, is described from Vietnam. Females of R. arabocoffeae sp. n. are characterised by the broad amphidial apertures with prominent margins. Males are characterised by the bursa extending to one third, rarely middle, of the tail. The new species belongs to the group of species with a long tail in the female. Radopholus arabocoffeae sp. n. is easily distinguished from R. similis by the bursa reaching to only one third of the tail vs extending to the tail terminus. Radopholus arabocoffeae sp. n. is differentiated from R. bridgei by the lateral field having three bands of equal width (vs middle one narrower than others), lateral field completely areolated over whole body vs not areolated except irregularly in neck and tail, hemizonid distinct vs indistinct, four lateral field incisures terminating far behind phasmid vs three incisures terminating at or just behind phasmid, lateral lines fusing at two thirds of tail vs fusing at one third of tail, longer spicule length (18-21 vs 15.5-18.0 mum), and male bursa usually extending to only one third of tail vs mid tail. Radopholus arabocoffeae sp. n. differs from R. colbrani by the rod-like vs round sperm, spicule length (18-21 vs 13-16 mum), tail length to stylet ratio (4.1-4.9 vs more than 5.1) and presence vs absence of a bursa. Radopholus arabocoffeae sp. n. differs from R. duriophilus by the rod-like vs kidney-shaped sperm. Males further differ from R. duriophilus by shorter stylet length (8.2-11.6 vs 11.5-15 mum), smaller distance between dorsal pharyngeal gland orifice and stylet base (1.7-3.4 vs 4-9.5 mum), shorter hyaline tail (2.6-3.4 vs 4-9.5 mum), and bursa extending to one third of tail vs mid-tail. Female R. arabocoffeae sp. n. differ from R. duriophilus by the broad amphidial aperture with prominent margin present vs absent. Males of R. arabocoffeae sp. n. differ from R. musicola by the rudimentary and amalgamated stylet base (vs with knobs), and inner lateral lines fusing at two thirds of the tail vs just posterior to the phasmid. The high level of ITS-rDNA sequence divergence of R. arabocoffeae sp. n. from other Radopholus spp. and the presence of nucleotide autapomorphies support a separate specific status of this new species. On carrot disks, the two species reproduced from 15-30°C; optimum reproduction occurring at 28°C. The reproductive capacity of R. duriophilus was higher than that of R. arabocoffeae sp. n. Radopholus duriophilus reproduced from single juveniles; R. arabocoffeae sp. n. did not. The correlation between initial densities of Pratylenchus coffeae, R. duriophilus and R. arabocoffeae sp. n. and the weight of C. arabica cv. Catimor fitted the Seinhorst model Y = ym for Pi ge T, and Y = ym · m + ym(1–m)z(Pi–T). Coffea arabica cv. Catimor was very susceptible for to all three nematode species tested, but especially so to R. arabocoffeae sp. n. The reproductive capacity of R. arabocoffeae sp. n. on C. arabica cv. Catimor was higher than P. coffeae or R. duriophilus.
    Description of the male and first-stage juvenile of Longidorus intermedius Kozlowska & Seinhorst, 1979 (Nematoda: Dorylaimida), and notes on its morphology and distribution
    Peneva, V. ; Loof, P.A.A. ; Penev, L.D. ; Brown, D.J.F. - \ 2001
    Systematic parasitology 49 (2001). - ISSN 0165-5752 - p. 127 - 137.
    Het testen van opgeschaalde Seinhorst-opspoelkannen
    Bekkum, P.J. van; Beers, T.G. van; Beniers, J.E. - \ 2000
    Gewasbescherming 31 (2000)3. - ISSN 0166-6495 - p. 65 - 67.
    aardappelen - dieren - populatiedichtheid - populatie-ecologie - mortaliteit - populatiegroei - ziekteresistentie - plaagresistentie - fysica - meting - methodologie - ? - globodera - potatoes - pratylenchus - heteroderidae - tylenchidae - animals - population density - population ecology - mortality - population growth - disease resistance - pest resistance - physics - globodera
    Om een voldoende grote statistische betrouwbaarheid voor de bepaling van populatiedichtheden van Globodera spp. in wetenschappelijke experimenten of bij het testen van aardappelcultivars ten behoeve van hun partiele resistentie-eigenschappen te verkrijgen, is het nodig om zowel de begin- als de einddichtheid van het aaltje nauwkeurig te bepalen. Twee methoden voor de bepaling van de relative vatbaarheid van aardappelrassen voor aardappelcysteaaltjes worden vergeleken
    The common relation between population density and plant weight in pot and microplot experiments with various nematode plant combinations
    Seinhorst, J.W. - \ 1998
    Fundamental and applied nematology 21 (1998). - ISSN 1164-5571 - p. 459 - 468.
    The Seinhorst Research Program
    Schomaker, C.H. ; Been, T.H. - \ 1998
    Fundamental and applied nematology 21 (1998)5. - ISSN 1164-5571 - p. 437 - 458.
    We propose the 'Seinhorst Research Program', derived from Seinhorst's empirical philosophy. All theories of the 'Seinhorst Research Program' are developed by searching for recurring regularities (patterns) in a collection of observations, named 'the empirical base'. To prevent 'ghost theories from sloppy data', all assumptions underlying the empirical base are carefully described in theories with respect to methodology and technology, including statistics. The patterns to be recognised are summarised by mathematical equations, which must be connected with biological processes to bridge the gap between 'normal' language and mathematical language for the description of biological theories. Often, the patterns result from more than one biological process. If so, the basic patterns are disentangled from one another using a method of pattern analysis. The procedure is best carried out when only a limited number of more or less congruent patterns are involved. Therefore, attention must be given to the choice of the hierarchic level and the complexity of the investigated system. Investigations proceed from simple experimental systems to complex natural systems at a hierarchic level that is neither so high that manifesting processes are very dissimilar nor so low that one runs the risk of describing processes irrelevant for the purpose of the investigation. In the 'Seinhorst Research Program', this purpose is finding methods for improvement of financial returns of host crops attacked by plant-parasitic nematodes through calculating risks of nematode population development and subsequent yield reduction. Pattern analysis yields theories about causes of phenomena observed at the investigated hierarchic level and about properties of processes at the nearest lower hierarchic level. Predictions at the next higher hierarchic level are made by synthesising several patterns in (stochastic) simulation models. Synthesis is also applied to compound patterns of processes in simple experimental systems, with the objective of explaining complicated patterns in complex systems.
    Quantitative studies on the management of potato cyst nematodes (Globodera spp) in The Netherlands
    Been, T.H. ; Schomaker, C.H. - \ 1998
    Agricultural University. Promotor(en): J.C. Zadoks. - S.l. : Been [etc.] - ISBN 9789054859604 - 319
    aardappelen - globodera - plantenparasitaire nematoden - controle - nematiciden - technieken - bemonsteren - detectie - modellen - potatoes - globodera - plant parasitic nematodes - control - nematicides - techniques - sampling - detection - models

    Potatoes are among the most profitable agricultural crops in arable farming in the Netherlands and consequently are grown as frequently as conditions allow. As a result Dutch farmers experienced huge problems with potato cyst nematodes during the last 50 years. In Chapter 1 an outline is presented of the situation of potato cyst nematode control in the 1980's, the problems, possible solutions and the research initiated, of which a part is presented in this thesis.

    In 1984 emphasis was put into research concerning the efficiency of soil fumigation on heavy marine clay soils where the majority of Dutch seed and consumption potatoes are grown. Both laboratory and field experiments were carried out to investigate the efficiency of 1,3 dichloropropene and metam-natrium. This required the processing of thousands of larvae suspensions. One of the most labourious and tedious occupations in nematological research is the counting of individuals of the pathogen.

    In Chapter 2 A GOP-302 image analysis system - Context Vision, Sweden - was used to automate the counting of large numbers of larvae-suspensions of Globodera rostochiensis and G. pallida . These suspensions originated from hatching tests, which were conducted to estimate percentage mortality in field and lab experiments of nematodes exposed to nematicides. The result is called ANECS ( A utomatic NE matode C ounting S ystem), a software program that can count up to 64 compartments with larvae suspensions successively without the aid of an operator. A special object carrier was developed. Images of up to eight object carriers (512 larvae suspensions) can be stored and image analysis can be suspended to off-office hours. The time needed to count one compartment was reduced by 80% to one minute compared to 'manual' labour while the time for probe preparation remained the same. The percentile error is highest at very low larvae densities (<20 per suspension) and is caused by pollution with small fibres carried by air during the handling of the larvae suspensions. This problem can be minimised by setting up clean-laboratory procedures. At least 95% of the larvae originating from hatching tests were recognized and counted. The program can and has been be adapted to count other nematode species or to suit more complicated problems like counting both larvae and eggs in one suspension.

    In Chapter 3 errors due to subsampling and laboratory procedures affecting the expected value and variance of cyst counts were investigated. Several fields, infested with potato cyst nematodes ( Globodera rostochiensis and G. pallida ) were sampled by collecting bulk samples of approximately 70 cores amounting to 1.8 or 2.5 kg soil from a number of square metre plots located in a regular grid pattern over a 0.33 ha area. Bulk samples from five fields, I-V, were thoroughly mixed and from one field, VI, lightly mixed, and subsequently divided into three subsamples of approximately equal weights. Two, sometimes three, subsamples were elutriated separately. Cysts were elutriated by two commercial laboratories, 1 and 2, and separated from the debris and counted at two research laboratories, 0 and 3.

    Random bulk samples from five fields, I-V, were divided into three portion after thoroughly mixing and taken to Laboratory 0, to compare elutriation precision and accuracy of commercial and scientific laboratories and to check the quality of mixing. To this purpose, pairs or triples were divided into classes. The expected value of the variance within pairs was estimated per class and could be described by a distribution function analogue to a negative binomial distribution, but with three in stead of two parameters. Cysts appeared to be randomly distributed in the well mixed samples, resulting in a binomial or trinomial distribution between pairs or triples. The expected values of the coefficient of variation associated with elutriation were 3.6, 9.6 and 5.5% in the Laboratories 0, 1 and 2, respectively. The upper 95% confidence limit,δ 0.95 , of coefficients of variation associated with elutriation in Laboratories 1 and 2, were estimated by the differences in 95% upper limits of coefficients of variation between the Laboratories 1 and 2 on the one hand and Laboratory 0 conversely. This difference,δ 0.95 , ranged from 73% to 42% for Laboratory 1 and from 43% to 19% for Laboratory 2 if 10 to 100 cysts were counted in samples. The consequences of these laboratory errors for the accuracy of sampling methods for both research and extension purposes are discussed.

    Nematicide trials require reliable results concerning the effect caused by the fumigant. The percentage mortality of potato cyst nematodes can be estimated by comparing the hatchability of untreated larvae with that of nematicide treated larvae. In Chapter 4 , research is described to improve the quality of hatching tests. Hatching tests using potato root diffusate are labourious and yield quite variable results. Sources of variability were identified and analysed, and solutions were presented. A method was developed to conduct hatching tests using inert materials so that the total variation at the end of the test is minimized.

    A number of hatching tests was carried out to increase reliability, optimize the method and limit the amount of work. Thus, it was possible to obtain a coefficient of variation ( cv ) of the hatching process which is in accordance with the combined errors expected when a certain number of cysts is treated and eggs are used in a hatching test. An Appendix is provided listing the different errors and ways to calculate and cope with them.

    The results indicate that the hatching process is no longer an important source of variation for the end result. All variation higher than expected could be explained by variation between replications of batches with the same treatment, indicating that small differences in nematicide application cause major differences in the end result. The treatment effect was more important in field experiments than in laboratory experiments.

    The hatching curve could be described adequately by a log-logistic curve with 3 parameters (λfinal number of hatched larvae,αtime,βslope parameter). Addition of a fourth parameter (γ, incubation time) improved the fit of the hatching curve significantly. Using the log-logistic model, final hatch can be predicted with a certain error before the actual hatching test ended, but in general final hatch is underestimated. When an error of 5% is accepted, the length of time required to perform a hatching test of a laboratory experiment can be reduced by 80% for untreated batches and by 40 to 80% for batches treated with nematicides. Acceptable reduction is negatively correlated with the concentration of the fumigant used.

    Hatching tests with cysts originating from field experiments are unsuitable for prediction using a time limited data set. In cyst batches from the field compound hatching curves could be distinguished in 4 out of 6 fields, indicating that the soil samples contained at least two fractions of cysts with different hatching responses. Prediction would cause a significant underestimation of final hatch and consequently an overestimation of mortality.

    Because of its high vapour pressure, 1,3-dichloropropene is primarily used on marine clay soils. In Chapter 5 a laboratory experiment is described investigating the two stereo-isomeres of 1,3-dichloropropene for their efficiency in killing nematodes. Batches of increasing numbers of Globodera rostochiensis cysts were exposed to a range of concentrations of the (E)- and (Z)-isomers of 1,3-dichloropropene. The cysts were of identical origin. Temperature during treatment was 10 oC, humidity 100% and time of exposure 8 days. The integrals of concentration time products ( CT ) created were 0, 3, 7, 14, 31, 60, 125, 242, and 437μg/ml·day for the (E)-isomer and 0, 3, 16, 59, 240, and 419μg/ml·day for the (Z)-isomer. Survival was estimated with hatching tests 1.5, 3, and 7 months after treatment.

    The relationship between dosage of (E)-isomer and numbers of hatchable nematodes followed a log-logistic equation at all hatching dates. Hatchability, and therefore lethal dosages, increased as hatching tests were more delayed. Seven months after treatment, practically all treated nematodes had recovered and hatchability of treated and untreated nematodes was the same. A log-logistic relationship was also found for dosage (Z)-isomer and numbers of hatchable nematodes 1.5 month after treatment. When hatching tests of nematodes treated with the (Z)-isomer were delayed till 3 and 7 months after treatment, the results were better explained by a compound model, assuming two independent log-logistic effects, one stimulating hatch at low dosages and one reducing hatch at all dosages. Only the (Z)-isomer of 1,3-dichloropropene was effective as a nematicide.

    Chapter 6 presents research concerning the efficiency of standard doses of 1,3 dichloropropene in fields with a high silt content. Three fields of marine clay soil were fumigated with 150 l/ha 1,3-dichloropropene (DD) (Teleone II TM, Shell 95 TM). On three dates after application, concentrations of Z- and E- 1,3-dichloropropene were measured per 5 cm layer of soil to a depth of 40 cm and integrals of concentration time products were calculated. When the fumigant was no longer detectable, a top soil treatment with either 150 l/ha metam-sodium or 180 kg/ha dazomet (active compound methyl isothiocyanate) was applied, followed immediately by autumn ploughing. Soil samples were taken before and after fumigation and after the top soil treatment to extract potato cyst nematodes (PCN). Survival was determined by means of hatching tests. Mortalities after the DD treatment, defined as 100 - % survival, were estimated per 5 cm layer of soil to a depth of 30 cm to construct dosage response curves. Fumigation with DD killed 48, 48 and 72% of the PCN per field, respectively. Accelerated breakdown of DD by microorganisms accounted for the two lower mortality rates.

    The additional top soil treatment with metam-sodium increased mortality to 90% or more. Dazomet, however, was less effective (53 and 80%) considering that twice as much of the active compound was applied as in the metam-sodium treatment. Multiplication of hatched larvae originating from the injection layer after the DD treatment was 25% less than that from untreated plots. This was caused by a lower fraction of larvae developing into cysts. PCN could be retrieved from soil layers as deep as 80 cm below the surface. Fumigation reached only a fraction of the infested soil, down to 25-30 cm. The infestation foci were so small compared to the standard minimum area fumigated (1 ha) that 90% of the active compound would be wasted on non-infested soil. Soil fumigation, whether or not combined with an additional top soil treatment, will seldom be profitable. Monitoring for infestation foci is recommended.

    As soil fumigation was not a viable option to keep potato cyst nematodes in check on heavy marine clay soils, another way of control had to be found. Research was focussed at the development of sampling methods for the detection of small infestation foci with high reliability (≥90% probability of detection). Precautionary soil fumigation can be avoided, the area where a control measure has to be applied can be minimized to the actual infestation, and detection occurs so early that (partially) resistant potato cultivars can be grown without significant yield reduction as population densities are still low.

    Research in the Flevopolders yielded promising results. Therefore, in 1990, a research program was initiated to develop new sampling methods for the detection of patchy infestations of potato cyst nematodes ( Globodera rostochiensis and G. pallida ) with known accuracy in all potato cropping areas of The Netherlands. Patchy infestations in cropping areas of the provinces of Zeeland, Friesland, Groningen and Drente were sampled to validate a model based on data from cropping areas in Flevoland and to determine whether one detection method could meet the requirements of all cropping areas in The Netherlands. The results are presented in Chapter 7 . Eighty two fields were presampled to locate patchy infestations using a coarse sampling grid (8 · 3 m). Parts of thirty seven fields, containing one or more foci, were sampled intensively by extracting at least 1.5 kg of soil per square metre (1.33 · 0.75 m). Forty foci were analysed for spatial distribution characteristics of cysts using Generalized Linear Models (GLM's) and classical Multiple Linear Regression Analysis, differing in assumptions about the distribution of the input variable (number of cysts per kg of soil).

    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 of these parameters 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 ( G. pallida or G. rostochiensis ), the time interval between sampling and the last potato crop, soil type, cropping frequency and cyst density in the focus centre could not be demonstrated. It can be concluded that one detection method for small infestation foci suffices for all investigated cropping areas. Its expected accuracy is independent of soil type, potato cyst nematode species, cropping frequency or time interval between sampling and last potato crop.

    In Chapter 8 the model for infestation foci developed in the previous chapter was applied for practical usage. A computer program called SAMPLE was developed to evaluate existing and create new sampling methods for the detection of patchy infestations or 'foci' of the potato cyst nematode ( Globodera spp.). By combining a model for the medium scale distribution of cysts, which provides the expected population densities at each position within the focus, and a model for the small scale distribution within square metres (negative binomial distribution) SAMPLE allows to simulate sampling procedures.

    The importance of the parameters of the two distribution models - the length and width gradient parameters for the medium scale distribution and the aggregation factor k of the negative binomial distribution for the small scale distribution - was investigated by sensitivity analyses. The aggregation factor k proved to be less important when calculating the average detection probability of a focus than 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/kg soil.

    The standard focus is small enough to use resistant potato varieties as a control measure without noticeable yield reductions in a 1:3 potato crop rotation. As the statutory soil sampling methods did not perform with the desired average detection probability, set at 90%, the program was used to develop several new sampling methods for focus detection and to investigate their performance. 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, Integrated Pest Management) and by governments (legislation, quarantine and export protection).

    For advisory purposes a model is required describing the relation between the number of potato cyst nematodes and tuber yield. A stochastic model with biologically relevant parameters was available. In Chapter 9 the direct relation between the number of potato cyst nematodes and plant growth is described and used to deduce the relation between nematode density and yield reduction of total plant weight and tuber yield. The relation between small and medium initial population densities and the relative total plant weight was derived as cross sections at right angles to the time axis of a growth model with three dimensions: time after planting t , relative total plant weight Y and relative growth rate r p /r 0 . The relative growth rate is the (constant) ratio between the growth rate r p of plants of a certain weight at a nematode density P and the growth rate r 0 of (younger) plants of the same weight without nematodes. Therefore, the ratio between the time after planting that plants need to reach a certain weight in the absence of nematodes and at nematode density P, t 0 /t p equals the ratio r p /r o (2).

    The relative growth rate r p /r o = k + (1 -k )0.95 P/T -1for P > T and = 1 for P ≤T (3). Formally, k is the minimum relative growth rate as P →∞. As a result the arbitrary equation y = m + (1- m )0.95 P/T- 1for P > T and = 1 for PT (6) also applies to the relation between small and medium initial population densities and relative total plant weight. T is the tolerance limit, below which growth and yield are not reduced by nematodes; m is the relative minimum yield.

    The relations between small and medium initial population densities of potato cyst nematodes and relative tuber weight of potatoes can be derived from the growth model in an analogous way. However, there is one complication: tuber initiation does not start at the same haulm weight in plants with and without nematodes, but at the smaller haulm weight the larger the nematode density. As a consequence, tuber weights of plants with a certain total weight at nematode density P are not equal to those of plants with the same total weight without nematodes, but r p Δ t units of weight larger,Δ t being the difference between the actual time of tuber initiation and the time total plant weight becomes the same as that of plants without nematodes at the initiation of tuber formation.

    Relative total and tuber weights of plants with 'early senescence' and at large nematode densities are smaller than estimated by the model and equation (2). This indicates that at large initial population densities growth reducing mechanism(s) become active that were not operating at smaller densities.

    In Chapter 10 an advisory system is presented for the management of potato cyst nematodes ( Globodera pallida) . It emphasizes the use of partially resistant potato cultivars, which provide the possibility of keeping population densities of potato cyst nematodes at a low level in short fixed rotations. Using stochastic models based on the population dynamics of potato cyst nematodes and the relation between pre-plant nematode densities and relative yield it is possible to calculate the probabilities of population development and the reductions in yield caused by these population densities. A simulation model is developed which integrates both models, using the frequency distributions of some of the most variable parameters relevant to a particular combination of potato cultivar and nematode population. Also, the natural decline in population density when non-hosts are grown is incorporated in the model.

    The model makes it possible to calculate the probability of a certain yield reduction, given a certain potato cultivar, nematode population and rotation. Therefore, it becomes feasible for a farmer to evaluate risks and the costs of different control measures in fixed rotations. The application of this model in the starch potato growing areas could lead to significant improvements in financial returns and a major reduction of the use of nematicides.

    In Chapter 11 we describe the 'Seinhorst research program' initiated by Dr J.W. Seinhorst, former head of the Nematology Department of the IPO-DLO. It consists of an empiric philosophy, the scientific methods applied, and the models developed at the IPO-DLO during the last 45 years of nematological research, including the 13 years in which the research described in this thesis was carried out. All theories of the Seinhorst research program are developed by searching for recurring regularities (patterns) in a collection of observations, named 'the empirical base'. To prevent " ghost theories from sloppy data " all assumptions underlying the empirical base are carefully described in theories with respect to methodology and technology, including statistics.

    The patterns to be recognized are summarized by mathematical equations, which must be connected with biological processes to bridge the gap between 'normal' language and mathematical language for the description of biological theories. Often, the patterns result from more than one biological process. If so, the basic patterns are disentangled from one another using a method of pattern analysis. The procedure is best carried out when only a limited number of more or less congruent patterns are involved. Therefore, attention must be given to the choice of the hierarchic level and the complexity of the investigated system. Investigations proceed from simple experimental systems to complex natural systems at a hierarchic level that is neither so high that manifesting processes are very dissimilar nor so low that one runs the risk of describing processes irrelevant for the purpose of the investigation.

    In the 'Seinhorst Research Program' this purpose is finding methods for improvement of financial returns of host crops attacked by plant-parasitic nematodes through calculating risks of nematode population development and subsequent yield reduction. Pattern analysis yields theories about causes of phenomena observed at the investigated hierarchic level and about properties of processes at the nearest lower hierarchic level. Predictions at the next higher hierarchic level are made by synthesizing several patterns in (stochastic) simulation models. Synthesis is also applied to compound patterns of processes in simple experimental systems, with the objective to explain complicated patterns in complex systems.

    In the Conclusions ( Chapter 12 ) an overview is presented of the practical results and aspects of the research effort described in this thesis. Some comments are made on the present state of affairs concerning potato cyst nematode control in the Dutch seed- and ware potato growing areas.

    Het Seinhorst onderzoeksprogramma
    Schomaker, C.H. ; Been, T.H. - \ 1997
    Gewasbescherming 28 (1997)6. - ISSN 0166-6495 - p. 145 - 146.
    An advisory system for the management of potato cyst nematodes (Globodera spp)
    Been, T.H. ; Schomaker, C.H. ; Seinhorst, J.W. - \ 1995
    In: Potato ecology and modelling of crops under conditions limiting growth : proceedings of the second international potato modeling conference, held in Wageningen 17 - 19 May, 1994 / Haverkort, A.J., MacKerron, D.K.L., - p. 305 - 355.
    automatisering - computersimulatie - heteroderidae - nematoda - planning - aardappelen - pratylenchus - simulatie - simulatiemodellen - bodem - solanum tuberosum - tylenchidae - automation - computer simulation - potatoes - simulation - simulation models - soil
    Relative susceptibilities of eleven potato cultivars and breeders' clones to Globodera pallida Pa3, with a discussion of the interpretation of data from pot experiments
    Seinhorst, J.W. ; Oostrom, A. ; Been, T.H. ; Schomaker, C.H. - \ 1995
    European Journal of Plant Pathology 101 (1995). - ISSN 0929-1873 - p. 457 - 465.
    A growth model for plants attacked by nematodes
    Schomaker, C.H. ; Been, T.H. ; Seinhorst, J.W. - \ 1995
    In: Potato ecology and modelling of crops under conditions limiting growth : proceedings of the second international potato modeling conference, held in Wageningen 17 - 19 May, 1994 / Haverkort, A.J., MacKerron, D.K.L., - p. 197 - 214.
    computersimulatie - heteroderidae - nematoda - aardappelen - pratylenchus - simulatie - simulatiemodellen - bodem - solanum tuberosum - tylenchidae - oogsttoename - oogstverliezen - opbrengsten - computer simulation - potatoes - simulation - simulation models - soil - yield increases - yield losses - yields
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