A new sampling method for the detection of low population densities of potato cyst nematodes (Globodera pallida and G. rostochiensis)

0261-2194(95)000147-6

C‘ro,) Pnmw, ,,,I Vol / 5. NC,. 4. pp. 37S-382. ,996 (‘opyqht @ IYYh Ekewer Science Ltd Pnntrd ,n Great Bntam. All rights rr,erved il?hl-2144M $ls.txl + o.cn,

ELSEVIER

A new sampling method for the detection of low population densities of potato cyst nematodes (Globodera pallida and G. rostochiensis)* T. H. Been

C. H. Schomaker

and

DLO Research

Institute

for Plant Protection

(IPO-DLO), PO Box 9060, NL-6700

GW Wageningen,

The Netherlands

To reduce the use of nematicides

in The Netherlands,

cyst nematodes

has been developed.

so eliminating

the

detected,

need

for

precautionary

soil fumigation

cost to benefit Analysis

ratio,

by farmers.

Moreover,

sampling

produced

Differences

in parameter

one detection

between

for small infestation was introduced

areas in the following

where the sampling

representing

model describing expected

values of the model

method

A pilot version of the method

fumigants

areas and a 90%

in The

Netherlands

research is directed using sampling

are

which has a poor

different

(including

originating

growing

in 1989 in the Flevopolders

Copyright

Kevwords: infestation

focus:

is estimated

spatial

growing

pattern;

and applied

in all the seed

was decreased

by 90% in system has

was realised in all seed and ware a result the total reduction areas) was 75%

methods

to generate

of soil

in 1994. Current

of the size of potato cyst nematode

new detection

@ 1996 Elscvier

As

low to

areas.

first. Since 1992, the sampling

for 1995.

the starch potato

these

areas were sufficiently

years. In 1992, soil fumigation

in the estimation

from

all growing areas in The

foci to apply to all potato growing

system was introduced

reduction

to improvements

data

systems for farmers.

a focus is

cyst numbers within infestations.

been used as a basis for an advisory system. In 1994, an 80% reduction growing

when

As many infestations

measures than soil fumigation,

data from 40 patchy infestations

a simple exponential

and ware potato growing the Flevopolders,

other control

of potato probability

can successfully be applied.

of intensive

Netherlands

potato

with a predefined

the area where a control measure has to be applied can be minimized.

still quite small at the time of detection,

enable

a new sampling method for the detection

The method detects small infestations

infestation

improved

foci

advisory

Science Ltd.

simulated

sampling;

advisory

system;

probability

of ;letection

In the 1980s 60% of the total volume of pesticides used in The Netherlands were soil disinfectants for the control of potato cyst nematodes. Research into the effectiveness of these nematicides revealed an accelerated breakdown of the active component by microorganisms in many fields (Smelt et al., 1989b; Smelt, Crum and Teunissen, 1989a). Even when this problem did not occur, the percentage mortality achieved throughout the root zone was insufficient to compensate for the density-dependent multiplication rate of potato cyst nematodes on potatoes (Been and Schomaker, 1987). Also, the scale of potato cyst nematode infestations compared with the minimum area commonly treated with a soil disinfectant was less than 10%. Moreoever, as the accuracy of the sampling methods used to detect potato cyst nematodes was unknown and statutory restrictions punitive, farmers tended to apply nematicides as a precaution. To resist the use of nematicides, a sampling method

*Presented at the 13th International Plant Protection Congress, The Hague, The Netherlands. July 1995

was needed which could detect small infestations with a predefined probability, thus providing the means to make intelligent decisions on the nature and the extent of control measures. This implies that a more effective sampling method should be based on a general model describing size and shape of an infestation. In 1987, a research programme for the development of a detection method for patchy infestations was started, firstly in the Flevopolders. This resulted in a prototype which was applied with great success (Schomaker and Been, 1993). As a result, in 1990 the research was extended to other growing areas in The Netherlands, which differ from the Flevopolders in historical development and cropping frequency. Analysis of the extended data sets has only recently been completed.

Materials

and methods

In every growing area, about ten infested fields were selected using sampling results provided by the Dutch Plant Protection Service originating from their statutory soil sampling survey of fields (size 0.33 ha) after a

Crop Protection

1996 Volume 15 Number 4

375

Detecting

population

densities

of potato

cyst nematodes:

T.H. Been and C.H. Schomaker

Figure 1. Map 1: population densities (cysts/kg) in the central square metre of rectangular areas of 8 x 3 m (length x width) covering l/3 ha. Map 2: population densities (cysts/kg) in every square metre within the outlined area in map 1. Bottom right: three dimensional representation of population densities (cysts/kg) in map 2

crop. Figure 2 gives an example of a presampling and final sampling scheme of one of these fields. For pre-sampling, an infested field was subdivided into rectangular areas of 8 X 3 m. They measured 8 m in the direction of cultivation because the dispersion of potato cyst nematodes depends largely on the activities of the farmers (harvesting machines, soil tillage); active dispersion is almost negligible as hatched larvae migrate only a few centimetres to the root tips of their host plants. This accounts for the oval shape of foci of infestation with poorly growing potato plants at the highest nematode densities. From the central square meter (1.33 X 0.75 m) of these 8 X 3 m rectangles, 40 cores of 25 g were collected with a 25 cm long auger according to a stratified plan. The infestation focus could then be located in the* field and the area for the final, more intensive sampling established (Figure I, outlined square). At least five fields containing one of more foci were intensively mapped per growing area. The final sampling scheme depended on the probable size of the focus, estimated from the pre-sampling, but in most cases, every square metre was sampled by extracting and processing at least 1.5 kg of soil and counting the numbers of cysts. Numbers of eggs per cyst were estimated in samples originating from ten of these foci. potato

376

Crop Protection

1996 Volume 15 Number 4

Model of the large scale distribution

Examples of infestations ranging from a single focus up to a conglomerate of several foci are displayed in Figure 2. All foci appear to be more or less elliptical and population densities increase exponentially towards the centre, but more slowly in the direction of cultivation than perpendicular to it. Consequently, the relation between log cyst numbers and distance from the centre is linear. The general shape of the focus is described by: E[p(x,y)]

= p(O,O) * bbl * fix1

(1)

where: PW) E[p(x,y)]

1 b

is the population density in the r,entre of the focus; is the expected population density at a certain location in the focus with coordinates x and y; is the change in population density per metre in the direction of cultivation; is the change in population density per metre perpendicular to the direction of cultivation

Detecting

population

densities

The parameters of interest in this model are the length gradient (I) and the width gradient (b). They can be estimated by multiple regression analysis on logtransformed data. With the model, the expected population densities at any location in the focus can be calculated given a certain central population density p(O,O). The model applied to the foci in all investigated growing areas. The most important result from the analysis was that, most probably, parameter values are similar in ail growing areas. This is illustrated in Figure 3 where the average length and width gradient for five different growing areas are displayed. Therefore, one detection method suffices for all growing areas in The Netherlands. As the length and width gradient are both normally distributed, the probability of any combination of these parameters and, therefore, the probability of occurrence of a focus with a certain combination of the length and width gradient in the field, can be calculated with a bivariate normal distribution. The model and the estimations of its parameters can be used in different ways; e.g. to investigate the accuracy of a sampling method by calculating its overall performance or by adjusting it to detect infestation foci, with parameter values covering a certain volume of this distribution, with a predefined probability.

Definition

of a standard

cyst nematodes:

T.H. Been and C.H. Schomaker

ally, a focus with a central population density of 50 cysts/kg soil (standard focus) was chosen, starting from the principle that, at this population density, it would be possible to control potato cyst nematodes without severe crop losses with (partially) resistant potato cultivars grown in common rotations (1:3 and 1:4). Immediately after a susceptible potato cultivar, the number of eggs per cyst was estimated at an average of 200 at cyst densities of 100 per kg of soil and smaller. This implies that the population density at the centre of the standard focus is about 10 eggs/g soil. As the actual sampling is done immediately after harvest, 2-3 years of non-host crops follow in which the population density decreases. This annual decline has been investigated among others by Den Ouden (1960) and is averaged at 50% in the first year after a potato crop and 33% in subsequent years. (Been, Schomaker and Seinhorst , 1995). Population densities would be expected to decrease to 3.3 and 2.2 eggs/g soil in a 1:3 and 1:4 crop rotation respectively. These densities are only slightly above the tolerance limit of 2 eggs/g soil (Seinhorst, 1982b). The chances that these infestations would be detected by current statutory soil sampling methods are very low. Moreover, resistant potato cultivars, which differ from susceptible ones only in their quality as a host, will suffer hardly any growth reduction at these densities. As potato cyst nematodes can occur at depths of up to 80 cm, as do potato roots (Vos & Groenwold, 1986), the effect of resistant cultivars greatly surpasses the effect of soil fumigation, as the latter will only reduce population densities by an average of 60% and only in the upper 30 cm of the tilled zone (Been and Schomaker, 1987). For accuracy

focus

Before a sampling method for detection can be developed, a standard focus must be chosen which should be detected with a certain probability. Eventu-

A

of potato

1

1

Figure 2. Several infestation foci ranging from a single infestation focus up to a conglomerate 386 cysts/kg; B: 295 cysts/kg; C: 283 cysts/kg; and D: 560 cysts/kg

Crop Protection

of foci.

Maximum

densities; A:

1996 Volume 15 Number 4

377

Detecting population densities of potato cyst nematodes: T.H. Been and C.H. Schomaker

“._

Flevq~~ldera

Friesland

Zealand

Gmningen

wentho

Figure 3. Average length (Cl) and width (A) gradients of foci in five growing areas in The Netherlands with their standard deviation. The soil type of the first four provinces consists mainly of (heavy) marine clay, that of the latter of sand and peat

of detection of the standard focus, 90% probability was chosen. Figures 4 and 5 give two and three-dimensional images of the standard focus, which, apart from its central population density, is defined by a length and width gradient according to the 10% lower percentile of the bivariate distribution. This means that 90% of all foci will have these dimensions or larger ones and, therefore, are detected with probabilities equal to or larger than the predefined one. Simulating

sampling

procedures

A special computer programme, SAMPLE, was developed to automate testing and development of

E Figure 4. Three-dimensional image of the standard focus (50 cysts/kg soil in the centre) according to equation (1) with length- and width gradients according to the 10% lower percentile of the bivariate normal distribution

sampling methods for detection. The programme uses the following procedure to calculate detection probabilities In every gridpoint where a subsample is taken, the expected population density is calculated using equation (1). Within a square metre, the expected population density is assumed to be constant and numbers of cysts distributed according to the negative binomial distribution (Seinhorst, 1982a, 1988). The probability of detection is defined as the probability of

Figure 5. Two-dimensional map of the same focus as in Figure 4 with population densities (cysts/kg) per square metre (1.33 x 0.75 m) with a 5 x 5 m sampling grid superimposed. Arrows indicate shifting of grid in both directions to calculate the average detection probability and the variance. The distance by which the grid will be shifted can be set to any value from 1 cm upwards (default 10 cm).

378

Crop

Protection

1996 Volume

15 Number

4

Detecting

population

densities

extracting one or more cysts from the focus, which is the same as I minus the probability of finding no cysts at all. For this purpose the negative binomial distribution function can be simplified to the following equation: P’[PCx,y) =

01= {kl{k+

qP(~,Yl>>k

(2)

where: is the input of equation (1); is the coefficient of aggregation of the negative binomial distribution; P’[P(X.Y)= 0] is the probability of finding zero cysts at location (x,y). HP(GY )I

k

Seinhorst (1988) and Schomaker and Been (1993) assumed k to be independent of the mean density and proposed a ‘common’ k of 70 for soil samples of 1.5 kg originating from one square metre, based on data from the literature and their own observations. Sensitivity analysis of the effect of variation of k on the average detection probability revealed that the latter hardly changed when k increased from 40 to 90 (Schomaker and Been, 1993). The probability of finding zero cysts in a soil sample from a focus is defined as the product of the outcomes of equation (2) for all subsamples taken within the focus. Then, the complement of this last calculation is the detection probability of a focus. If II subsamples are taken: Pr @ L 1) =

1 - II+,..,,

Pr (p~x,Y)i= 0)

= Zi=i,.N) Pri@aO)IN

cyst nematodes:

T.H. Been and C.H. Schomaker

dimensions starting with a 2.5 by 2.5 m grid up to a 11 X 12 m grid. From Figure 6 it can be concluded that the wider the grid, the larger the soil sample needed to detect the standard focus with 90% probability, although the relation is by no means linear. For instance, the amount of soil required to detect the standard focus with a average detection probability of 90% is 6.7 kg when using a 2.5 X 2.5 m grid and 15.6 kg using a 11 X 12 m grid. This difference is caused by the unfavourable grid dimensions compared to the size and shape of the focus. Further, an increase in grid size in the direction of cultivation proved to be less sensitive than the same increase perpendicular to that direction. In Figure 7, another consequence of differences in grid dimensions is demonstrated by comparing the frequency distributions of detection probabilities of the same two sampling grids. Although sample sizes were adjusted so that with both grids the standard focus is detected with an average probability of 90%, the variance of the detection probability is much smaller in the 2.5 X 2.5 m grid than that of the 11 X 12 m grid.

(3)

As the exact location of the focus within the field is unknown, the sampling grid can touch a focus in many different ways and a separate detection probability can be related to all these possibilities. Therefore, the computer programme simulates a systematic shifting of the grid in both directions and calculates the detection probability for each new location of the grid. This is illustrated in Figure 5 where a 5 X 5 m sampling grid is superimposed over the standard focus with arrows indicating the directions in which the grid is shifted. Finally, a frequency distribution of detection probabilities is obtained per sampling grid. If the grid was shifted N times, this average is calculated as: Praver @al)

of potato

Figure 6. Sensitivity analyses for grid dimensions. The sample size per 0.33 ha required to detect the standard focus with an average detection probability of 90% was calculated for sampling grids varying between 2.5 X 2.5 m and 11 X 12 m

(4)

In 1988, several then current Dutch sampling methods, defined by sampling grid and sample size, were evaluated for their efficiency (Schomaker and Been, 1989) but were rejected as detection methods because of their poor performances. As a consequence new sampling methods had to be developed.

f 8

3000

2 8

2000

1000

Developing

a new detection

method

The dimensions of the sampling grid affect the sample size necessary to obtain a certain average detection probability. Using SAMPLE, an optimal sampling grid was established by calculating the sample size required for a 90% detection probability for a variety of grid

0 50

60

70

60

probability(%) Figure 7. A comparison of the frequency distribution of detection probabilities when using 2.5 X 2.5 m (m) and 11 X 12 m (0) sampling grids (length x width). Both sampling grids have the same average detection probability (90%) for the standard focus but differ in variance

Crop Protection

1996 Volume 15 Number 4

379

Detecting population densities of potato cyst nematodes: T.H. Been and C.H. Schomaker

agency. The new method detects foci with a central density of 150 cysts/kg soil and higher with high (approximately 100%) probability. Therefore, if an infestation focus is present in a field, but not detected with the new sampling method, it can only be very small. The probability of detection by statutory soil sampling is low (e.g. conditional probability 0.018% for the standard focus) while the probability for a yield reduction larger than 1% in the next potato crop is nil. Problems with the execution of the new sampling method in farmer’s fields are the laborious collecting and processing of the large soil samples. These problems have been successfully addressed by local firms in The Netherlands using automated sampling devices mounted on a jeep and elutriators for the extraction of cysts from soil samples of up to 3 kg.

Figure 8. Graphical display of the foci detected with a 90% probability by the statutory and the new sampling method (5000 and 50 cysts/kg in the centre, respectively)

R

0

290

statutorymsthod

SO0 pcquliotioil

750

a nc

1000

1250

1500

density of centfe (cysts/kg)

Figure 9. Detection probabilities with the statutory and the new sampling method of foci with central population densities ranging from 50 to 1500 cysts/kg

For the individual farmer, who draws only once from this frequency distribution, a sampling method with a closely spaced grid is more reliable. Based on these analyses, grid dimensions somewhere near 5 X 5 m were recommended as the best compromise between two conflicting aims: minimizing the sample size and the variance of the detection probability on the one hand and minimizing the time needed to collect and process the samples on the other. As the statutory soil sampling method with a 7.5 X 7.5 m grid and a core size of 3.3 g (sample size 200 g/O.33 ha) is still used in The Netherlands, and even worse variations of it in other countries, it is interesting to compare this sampling method with the detection method proposed, i.e. a 5 X5 m sampling grid and a core size of 52 g (sample size 6.9 kg/O.33 ha) on two aspects: (1) the size of the focus detected with a 90% probability (Figure 8) and (2) their detection probabilities for foci of different sizes (Figure 9). Given a average detection probability of 90%, the new sampling method detects foci with central densities a hundred times smaller than does the statutory sampling method, but costs only 3-4 times as much to perform, depending on the sampling

380

Crop Protection 1996 Volume 15 Number 4

Practical results

The effect of the prototype sampling method on nematicide use was evaluated in 1989 in a joint effort of the Groene Vlieg Ltd and the Ministry of Housing, Physical Planning and Environment (VROM) on approximately 226 ha in the Flevopolders (Figure 20). A decrease of 84% of nematicide application on the tested fields was realised. A higher decrease would have been possible, had farmers growing potatoes in a 1:3 rotation not been obliged to use soil fumigation. In 1990, a slightly adapted method (90% detection probability of a focus with 100 cysts/kg soil in the centre) was adopted by the Dutch government as an instrument for remission of obligatory soil fumigation when a 1:3 crop rotation was practised. The sampling method was combined with an ELISA test (Schots, 1988) for species identification (G. rosfochiensis or G. pallidu) to enable farmers to grow potato cultivars with appropriate resistance. As the new methods require the collection of large soil samples, the area from which a sample is taken was reduced from l/3 ha to l/9 ha. As agricultural

seed potatoes

after

remission

Figure 10. Results of a joint research project of De Groene Vlieg and the Ministry of Housing. Physical Planning and Environment (VROM) to evaluate the effect of the new sampling method on the use of soil fumigation in the Flevopolders. ‘Before’: 226 ha (100%) were intended for treatment with a nematicide. ‘After’: Number of ha (16%) actually treated after soil sampling results became available. ‘Remission’: Number of ha (6%) treated if remission of obligatory soil fumigation for farmers’ who grow susceptible potatoes in a 1:3 rotation had been possible. Results are divided between areas with seed and ware potatoes

Detecting

population

densities

machinery distributes cysts mainly in the direction of cultivation, farmer fields were divided into strips 5 m wide and about 300 m long from which one bulk sample was collected. A typical sampling result consisted of one to several successful strips with cyst counts. As a result, farmers get very precise information about the location and size of an infestation enabling them to limit the area where control measures must be applied. Prices range from 140 to 200 Dutch guilders per ha, depending on the method used (100 or 50 cysts/kg in the centre), whereas soil fumigation costs approximately 1300 gilders. If the incidence of infestation is low, and it proved to be 3% in the last years, farmers improve their economical returns using this sampling method. In 1992, soil fumigation was reduced by 90% in the Flevopolders, where the sampling method was first introduced. In 1992, Been, Schomaker and Molendijk (1992) also introduced a prototype advisory system based on the new sampling method, estimating the size of the infestation with the number of infested strips. Combined with the relation between pre-plant densities of potato cyst nematodes (small-medium densities) and yield reduction of tubers at harvest (Seinhorst, 1986) the necessary information was provided for farmers to make informed decisions about the choice of cultivars and control measures in order to maximise their economic return. Advice was supplied by the sampling agencies using the sampling method described. Since 1989, the area over which this method is applied has increased to 13,000 ha per annum and in 1994, soil fumigation was reduced by more than 80% in those areas where seed and ware potatoes are grown. For 199.5, a 90% reduction is predicted. As a result, the total reduction of nematicides in The Netherlands (including the starch potato growing areas) was 75% in 1994, considerably more than the 40% target required by the Multi Year Crop Protection Plan (Anonymous, 1991) of the Government for 1995.

Current

research

Besides the number of infested strips, the number of cysts per strip and cysts per focus are useful quantities

0,254.

--

-

-

_ -._

__

_. -

.

_

-_ .-.

-

-.

_-

.-

_.

-;

1 I

.^

__^__

~_ ._ --

-

-

-

.-

-1 /

of potato

cyst nematodes:

number

T.H.

Been and C.H. Schomaker

of cysts

Figure 12. Frequency distribution of cyst numbers to be collected when sampling of the standard focus, with the new sampling method in simulated 25,000 times for each strip within the standard focus

for the estimation of the size of the infestation focus. By using the 10 and 90% percentiles of the bivariate distribution of the length and width gradients, a fair amount of possible foci sizes can be dismissed as highly improbable. For the remaining possible foci dimensions, these indices have to be related to focus size. Recently, using a Monte Carlo approach (the rejection method), SAMPLE has been used to simulate cyst numbers per subsample and provide reliable estimates of the number of cysts per focus or per sampled strip, given a certain focus size and sampling method (grid dimensions and subsample size). In Figure II, the frequency distribution of the number of cysts detected by applying the new sampling method on the standard focus is depicted. A 10% probability of a zero cyst count was calculated for the sampling method developed to detect the standard focus with 90% probability, which demonstrates the reliability of the method. Figure 12 displays the same approach, this time cyst numbers per sampled and infested strip (5 X 300 m) covering the standard focus. As expected, the majority of detections occur in the central infested strip of the focus. All three indices can provide the farmer with more detailed information about the probable size of an infestation, and perhaps even on the number of foci present in one strip. More precise information about the level and extent of the infestation will reduce the uncertainty of farmers about the financial risk they take and the control measures to apply. This will decrease the pressure to apply nematicides as a precautionary measure.

Acknowledgements

o

1

2

3

4

5

6

7

8

S

10 11 12 13 14 15 16 17 1% 19

number of cysts Figure 11. Frequency distribution of cyst numbers to be collected when sampling of the standard focus with the new sampling method is simulated 25,000 times using the Monte Carlo approach

The field work was a combined effort of the Institute for Plant Protection (IPO-DLO), the Research Station for Arable Farming and Field Production (PAGV), the Plant Protection Service (PD) and the H.L. Hilbrands Laboratory for soilborne pests (HLB). The Regional Inspection Services of the Netherlands Inspection Service (NAK) processed and counted thousands of soil samples.

Crop

Protection

1-996 Volume

15 Number

4

381

Detecting population densities of potato cyst nematodes: T.H. Been and C.H. Schomaker Gommers and Paul W.Th. Maas), Decker and Huisman, Wildervank, The Netherlands, pp. 182-194

References Tweede Kamer. Plantijnstraat,

Schots, A. (1988) A serological approach to the identification of potato cyst nematodes. Ph.D. thesis, Wageningen Agricultural University, I18 pp

Been, T.H. and Schomaker, C.H. (1987) Fumigation of marine-clay soils infested with potato cyst nematodes. 10th Triennial Conference of the European Association for Potato Research (EAPR), Aalborg, Denmark, pp. 410-411

Seinhorst, J.W. (1982a) The distribution of cysts of Globodera rostochiensis in small plots and the resulting errors. Nematologica 28, 285-297

Anon. (1991) Meerjarenplan 1990-1991,

Vergaderjaar

‘s-Gravenhage,

Gewasbescherming. Uitgeverij Sdv

21777, 3-4

Been, T.H., Schomaker, C.H. and Molendijk, L. (1992) Adviezen naar aanleiding van uitslagen van de intensieve AM-bemonstering voor de poot- en consumptieaardappelteelt in gebieden waar besmettingen pleksgewijs voorkomen. IPO-DLO Rapport No. 92-17, 3’ PP T.H., Schomaker, C.H. and Seinhorst, J.W. (1995) An advisory system for the management of potato cyst nematodes (Globodera spp). In: Potato Ecology and Modelling of Crops under Conditions Limiting Growth (Ed. by A.J. Haverkort and D.K.L. MacKerron), Kluwer Academic Publishers, London, pp. 305-322

Been,

Den Ouden, H. (1960) Periodicity in spontaneous hatching of Heterodera rostochiensis in the soil. Report of the Fifth International Symposium in Plant Nematology. Nematologica, Supplement 11: C.H.

geleide bestrijding 2/1989, l-3-16

T.H. (1989) Bemonsteringssystemen aardappelcysteaaltje. Gewasbeschermingsdossier

and Been,

Schomaker, C.H. and Been, T.H. (1993) Sampling strategies for the detection of potato cyst nematodes; developing and evaluating a model. In: Nematology from Molecule to Ecosystem (Ed. by Fred J.

382

J.W. (1986) Agronomic aspects of potato cyst nematode infestation. In: Cyst Nematodes (Ed. by F. Lamberti and E.C. Taylor), Plenum Press, New York, USA/London. UK, pp. 21 l-228 Seinhorst,

Seinhorst, J.W. (1988) The estimations of densities of nematode populations in soil and plants. Vdxtskyddrapporter. Jordbruk 51 Swedish University of Agricultural Sciences. Research information centre, Uppsala. Zweden, I08 pp Smelt, J.H., Teunissen,

Accelerated

W., Crum, S.J.H. and Leistra, M. (1989b) transformation of I ,3-dichloropropene in loamy soils.

Neth. J. Agric. Sci. 37. 173-183 Smelt, J.H.,

101-105 Schomaker,

Seinhorst, J.W. (1982b) The relationship in field experiments between population density of Globodera rostochiensis before planting potatoes and the yield of potato tubers. Nematologica 28.277-284

Crop Protection 1996 Volume 15 Number 4

Crum, S.J.H. and Teunissen, W. (1989a) Accelerated transformation of the fumigant Methylisothiocyanate in soil after repeated application of Metham-sodium. J. Environ. Sci. Health, part B. Pesticides, Food Contaminants and Agricultural Wastes 2415, 437-455

Vos, J. and Groenwold, J. (1986) Root growth of potato crops on marine-clay soils. Plant and Soil 94, 17-33