Effects of infestation with Globodera rostochiensis (Wollenweber) Behrens Rol on the growth of four potato cultivars

CROP PROTECTION (1982) 1 (2), 169-179 © 1982

Effects of infestation with Globodera rostochiensis (Wollenweber) Behrens R o l on the growth of four potato cultivars K. EVANS

Nematology Department, Rothamsted Experimental Station, Harpenden, Hefts. AL5 2JQ_, England ABSTRACT. Four potato cultivars were grown in plots which, following previous treatments, contained few or many Globoderarostochiensis(Wollenweber) Behrens Rol. Soil moisture content, leaf water potential and percentage ground cover were measured each week, and whole-plant samples (including roots) were taken ~ and 9 weeks from planting. Leaf areas, fresh and dry weights and inorganic nutrient contents were measured on these samples. Stomatal resistance was measured on one occasion, 13 weeks after planting, and soil samples were taken midway between plants on the same date and roots extracted from them. Yields were measured 30 weeks after planting. In terms of yield Cara, which contains the resistance gene H1, tolerated nematode attack better than the other cultivars. Of the non-resistant cultivars, Pentland Crown was the most tolerant and Pentland Dell the least; the fact that Maris Peer yielded less than Pentland Dell in heavily infested plots is probably attributable to its early maturity date and lower yield potential. The shoot:root ratio measured 9 weeks after planting was similar for all cultivars following the same previous treatment, but was decreased from 11-9 when the previous crop was Marls Piper to 5"6 when the previous crop was Pentland Crown. Good yields in the presence of nematodes were produced by Cara (and to a lesser extent Pentland Crown) because it produced extra roots in the heavily infested plots; Pentland Dell and Marls Peer produced fewer roots in heavily infested plots. Measuring leaf water potentials and soil water contents was of little value for estimating tolerance but the least tolerant cultivar, Pentland Dell, showed the largest increase in stomatal resistance in heavily infested plots, with Cara showing the smallest. Percentage ground-cover measurements through the growing season gave the best overall picture of the reactions of the four cultivars to nematode infestation, but were time-consuming. Concentrations of potassium in haulm dry matter were decreased and those of calcium increased by nematodes, with the magnitudes of the effects least in Cara and greatest in Pentland Dell. The prospect of developing an assay for tolerance sufficiently convenient for use by plant breeders must depend upon adapting a simple test of this type to a pot scale.

170

Effects of cyst nematodes on growth of potatoes

Introduction Some potato cultivars are very sensitive to (intolerant of) attack by potato cyst nematodes and some are relatively little affected (see Oydvin, 1977; Evans and Franco, 1979; Whitehead, Tite, Fraser and French, 1980). This difference in sensitivity comes within the definition of tolerance suggested by Schafer (1971) in which tolerant plants are those which endure severe disease without severe losses in yield or quality; the term resistance is then used only in the narrow sense of describing the parasite's inability or ability to reproduce on the host. It is important that new resistant potato cultivars are not severely damaged by nematode attack as these cultivars will be the ones chosen for use in infested fields. Their performance when exposed to nematodes must therefore be assessed before they are released. The only satisfactory method currently available for assessing degree of tolerance is to grow the cultivar under test alongside a range of other cultivars in field trials which include plots with small and large nematode population densities. A number of experimental designs may be used to achieve a suitable range of population densities: 1.

Trials done on two sites, one with small the other with large nematode populations, which are as similar as possible in terms of previous cropping and soil type. Such an experiment was reported by Evans and Franco (1979). 2. Trials done on heavily infested land with various nematicide treatments to reduce nematode populations in some plots (see Whitehead et al., 1980). 3. Trials with plots spaced widely across an infested field in order to give a range of population densities, as used by Brown (1969). 4. Trials in which preparatory treatments are used to produce a range of population densities. All of the methods have disadvantages. The first type uses two sites between which there will be differences over which no control can be exerted; the second introduces the unknown side effects of treating with nematicides; the third has plots so widely spaced that fertility and other differences are bound to occur, and the trial becomes difficult to manage; the fourth requires two years to complete and differences in nutrient take-off may occur in the preparatory year. Of the types of trial available, it was decided that the fourth was likely to give the best results if large differences in nutrient take-off could be avoided. This was achieved by choosing a site with a population density of about 20 eggs/g soil (a density which has little effect on the growth of the two cultivars used in the preparatory year) and growing resistant and non-resistant cultivars, thus either bringing the population down to less than 10 eggs/g soil or increasing it to about 100 eggs/g soil. The plots from the preparatory year were then split into sub-plots on which cultivars known to differ in their tolerance of nematode attack could be grown and various aspects of their growth examined. The aim was to decide what gave the best measure of tolerance and so might be used in an assay for this feature.

Materials and methods The experiment was on Butt Furlong field at Woburn Experimental Farm and occupied a site 27 m × 14 m. In 1979, whole plots were six rows (4-27 m) by 6.1 m

K. EVANS

171

(two blocks of four plots) and grew either the resistant cultivar Marls Piper (gene Ht) or the non-resistant cultivar Pentland Crown (four plots of each). The nematode was Globodera rostochiensis (Wollenweber) Behrens Ro 1 at a population of approximately 20 eggs/g of soil. Samples taken in spring 1980 showed that populations averaged eight eggs/g soil after Maris Piper and 105 eggs/g soil after Pentland Crown. Whole plots were split into two sub-plots in 1980, each sub-plot being planted with Cara, Marls Peer, Pentland Crown or Pentland Dell, giving two replicates of each cultivar following the Marls Piper in 1979 and two following the Pentland Crown. Sub-plot size was six rows (4.27 m) by 3.05 m, with basal manuring at 1505 kg/ha of 13 : 13:20 N P K fertilizer, and the experiment was planted on 28 April 1980. Soil moisture, leaf water potential and percentage ground-cover measurements were taken each week using methods similar to those described by Trudgill, Evans and Parrott (1975) and Evans, Parkinson and Trudgill (1975). Soil moisture was measured with a Wallingford Neutron Moisture Meter (Bell, 1969) at 10cm intervals to a depth of 75 cm below ridge crests, leaf water potential of the fifth leaf of each of three plants with a pressure chamber (Dixon, 1914) and ground cover with a viewing frame. At the end of July (13 weeks after planting) the stomatal resistance of the undersurface of the fifth leaf of three plants from each plot was measured with a porometer (Parkinson and Legg, 1972) and soil samples from which roots could be extracted were taken with the apparatus described by Welbank and Williams (1968); cores of 7 cm diameter were taken to a depth of 75 cm below the ridge crests, midway between plants, and divided into 25 cm depth fractions. Whole-plant samples (two plants from each sub-plot) were taken 6½ and 9 weeks after planting, and the roots associated with each plant were extracted by sieving the soil beneath each plant through a 1 cm mesh. Leaf area and fresh weights of tops and roots were measured and inorganic nutrient contents of the tops determined by standard methods (Cosimini and Talibudeen, 1978). The yield of tubers from 16 plants in the middle two rows of each plot was measured 20 weeks after planting and post-harvest soil samples were taken from each plot to determine the post-harvest cyst nematode populations. Results

For analysis, the data were adjusted for whole-plot effects, and the levels of significance of treatment effects were assessed in F-tests. Because of wet conditions in the weeks before harvest there was some rotting of tubers and loss of yield data from some plots: no estimate of the significance of differences in yield shown in Table 1 can be made, therefore. However, the results strongly suggest that yield of Cara is little affected by the nematode population of 105 eggs/g soil, yield of Pentland Crown is affected rather more and yield of Pentland Dell is very much affected. Yield of Marls Peer was also much decreased; it was considerably less than that of the other cultivars, even in plots with few nematodes. Table 2 gives the fresh weights of shoots and roots, the shoot:root ratio and leaf-area indices of the plant samples taken 6½ and 9 weeks after planting. Although the 1979 crop (through its effect on nematode numbers) affected the amount of shoots produced (P < 0-1), it was the interaction between the 1980 cultivar and the 1979 crop that had a significant effect on root production at 6½ weeks. Cara produced more roots than any of the other cultivars and, like Pentland Crown, responded to exposure to large numbers of nematodes by producing extra roots; Pentland Dell

172

Effects of cyst nematodes on growth of potatoes TABLE 1. Yields (t/ha) of four potato cultivars grown on land infested with few or many Globodera rostochiensis Rol (i.e. after Marls Piper or Pentland Crown)

Cultivar

Yield after M. Piper Yield after P. Crown (Mean P i = 8 eggs/g) (Mean P i = 105 eggs/g)

Maris Peer Pentland Dell Pentland Crown Cara

24"8 46.3 55"9 59"4

11.0 13.4 31.3 54-6

Pi: initial population TABLE 2. Fresh weights (g) of shoots and roots and leaf-area indices for four potato cultivars, measured 6½ and 9 weeks from planting. Figures are means of four plants. 6½ weeks

Previous crop Maris Piper

Cultivar Maris Peer Pentland Dell Pentland Crown Cara

Mean Pentland Crown Maris Peer Pentland Dell Pentland Crown Cara Mean

Levels of significance in F tests Previous crop Cultivar Previous crop × cultivar

FW FW shoot root (A) (B) A/B

9 weeks

LAI

FW shoot (A)

FW root (B)

A/B

LAI

1-06 1.11 0-90 1.27

471 698 657 1009

46.7 56-0 48"0 89.1

11.3 12.1 13"0 11.1

2-05 3.17 2"84 4"49

230 297 239 293

26-3 36"0 28-2 24-0

8.8 7-7 8"3 11.8

265

28-6

9-1

1 - 0 8 709

60.0

11-9

3.14

48 17.4 105 26.6 141 37"3 208 45"7

3.1 4-3 3-9 4-5

0.24 0"39 0-49 0"86

25.4 14-6 62.9 119"0

6"3 5-9 4"8 5-3

0.72 0-48 1"27 2-92

126 31-7

3-9

0 " 5 0 299

55.5

5.6

1.35

0-1 --

--

145 118 283 652

0"05 0.1 0'1 0-1 -0-05 O.1 . . . .

-0"05 - 0"05 - 0-05 . .

FW: fresh weight (g) LAI: leaf-area index and Maris Peer produced fewer roots when exposed to large n u m b e r s of nematodes. It was the increased root production 9 weeks after planting ( P < 0 . 0 5 ) that led to significant differences in shoot weights between cultivars, as shoot: root ratios were not significantly different between cultivars; in fact they were remarkably constant within previous crop treatments but significantly decreased (from 11-9 to 5.6 at P < 0.05) where the 1979 crop was Pentland Crown. W h e n soil samples were taken midway between plants 13 weeks after planting, those taken from plots of Cara contained m o r e roots than those from other plots, and

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173

samples from Cara after Pentland Crown contained more than twice as much root as those from Cara after Marls Piper (Table 3). Amounts of root from all other cultivars were less after Pentland Crown than Maris Piper: Marls Peer had the least and Pentland Crown the most, with Pentland Dell midway between the two. (The effect of the 1980 cultivar was significant at P < 0-05 and the interaction between the 1980 cultivar and the previous crop was significant at P < 0 - 1 . ) As these samples were taken from soil cores 7 cm in diameter approximately 19 cm from the plants, the results further demonstrate the increased root production by Cara compared with that of the other cultivars, and that Pentland Crown produces more roots than the TABLE 3. Fresh weights (g) of root per litre of soil at three depths in samples taken midway between plants of four potato cultivars 13 weeks after planting. Figures are means of four samples, plants at 38 cm spacings Fresh weight of root (g) Depth below soil surface (cm) Previous crop Maris Piper

Cultivar

0-25

25-50

50-75

Total

Maris Peer Pentland Dell Pentland Crown Cara

0-52 0"48 1-08 1.19

0"14 0.18 0.12 0.16

0-00 0"00 0.01 0.01

0.66 0-66 1"20 1-35

0"81

0.14

0-01

0"96

0"16 0-23 0"36 2.71

0"02 0-07 0"06 0.17

0"00 0"00 0"00 0.01

0"18 0"30 0.42 2.88

0.86

0-08

0.003

0"94

Mean Pentland Crown Maris Peer Pentland Dell Pentland Crown Cara Mean

other non-resistant cultivars. Few roots below 25 cm and virtually none below 50 cm were found in any plot. Effects on shoot production through the growing season are seen very clearly in Figure 1, in which percentage ground cover is plotted against time. Plants growing in plots with many nematodes reached a smaller maximum ground cover than those in plots with few nematodes and all plots of Marls Peer, Pentland Dell and Pentland Crown senesced completely at the same time (18 weeks after planting)• However, in plots with many nematodes, Marls Peer reached only 62 O~o ground cover, Pentland Dell 72~o and Pentland Crown 93~o. Cara was exceptional in that it still had some green leaves on all plots at harvest time, 20 weeks after planting, and that it reached 100~o ground cover on all plots. Figure 2 shows leaf water potentials measured through the growing season: up to 11 weeks after planting there were no differences between cultivars and no significant differences at any time between plots with few or many nematodes. In the twelfth week after planting, leaf water potentials became much more negative in all plots, and measurements were subsequently discontinued on Marls Peer because of a shortage of suitable leaves. Measurements on Pentland Dell and Pentland Crown were continued up to weeks 15 and 16 respectively, and it was possible to record leaf

Effects of cyst nematodes on growth of potatoes

174

Maris Peer

Pentland Dell

100 80 60 40

/ 2O o

~

o

i

> 0 u

Pentiand C r o w n

g

Cara

100

80

60

40

I

•

20

05

1(3

i

15

i

5 10 - -20 Time from planting (weeks)

1'5

2'0

FIGURE 1. Percentage ground cover, measured at weekly intervals, of four potato cultivars growing in plots with few or many G. rostochiensis Rol. © Few nematodes; • many nematodes.

water potentials on Cara up to harvest. Cara had a greater leaf water potential than other cultivars throughout the experiment, but this difference was not significant at any time during which the other cultivars were growing. Measurements of stomatal resistance (Fig. 3) were made on one occasion only, during the thirteenth week from planting, and the effects of previous cropping and cultivar were both significant at P < 0-1. Stomatal resistance was lowest in Cara and greatest in Pentland Dell and Marls Peer; it was slightly increased by many nematodes in Cara but was more than doubled in Pentland Dell. T h e ability of a plant to maintain low stomatal resistances and high leaf water potentials depends both on the availability of water and the efficiency with which its root system is able to extract water. Figure 4 shows that rainfall was plentiful during 1980: differences between the water relations of the four cultivars were, therefore,

K. EVANS 10

175 Pentland Dell

Marls Peer Q

8 b

•

6

>(

g_ 4 ~2 2

o

I

I

I

I

I

c~

E

Pentlond Crown

2 g

8

a_

6

Cara

•

-0

4 2 0

I

10

15

210

5

10

1~5

210

Time from planting (weeks)

FIGURE 2. Pressure chamber readings of leaf water potential, made on leaves of four potato cultivars growing in plots with few or many G. rostochiensis Rol. O Few nematodes; • many nematodes. Many [ ] nematodes [ ] Few nematodes

~4.0 J®3 . 2 u

o 2.4 P 1.6

E u~ o

0.8

0

Maris Pentland Pentland Cara Peer Dell Crown

FIGURE 3. Stomatal resistance, measured 13 weeks after planting of four potato cultivars growing in plots with few or many G. rostochiensis Rol. probably attributable to differences in their ability to extract water from the soil. T h e volumetric water contents of the soil 20 cm below the surface (estimated from neutron probe readings and a calibration curve) o f plots of Cara and Maris Peer with few or m a n y nematodes are also s h o w n in Figure 4. It is clear that neither cultivar extracted so m u c h water w h e n many nematodes were present and that Cara consistently extracted more water than Maris Peer, particularly when only few

Effects of cyst nematodes on growth of potatoes

176

Ivla ny n e m a t o d e s

26

®

22

~E 18 o

14

o q; _Q

E u O

10

OJ

I

I

I

I

I

I

I

I

I

I

I

I

I

I

Few n e m a t o d e s

26 c

O u

22 .u 18 L

E O >

14

\,/

•

10 I

I

I

i

f

I

~

I

~

50

I

I

I

I

85.8

1

40

v

I

E 30

z

20

10

9

16 23 June

30

7

14 21 July

28

4

11 18 Aug.

25

1 8 Sept.

FIGURE 4. Volumetric water content of soil, 20 cm below the surface, in plots of Marls Peer and Cara with few or many G. rostochiensis Ro 1, and daily rainfall through the growing season. O Marls Peer; • Cara.

K. EVANS

177

nematodes were present. T h e effects of heavy periods of rain in restoring the water content of the soil and narrowing the differences between the cultivars are also clearly seen. Inorganic nutrient concentrations in haulm dry matter of plant samples taken 6½ or 9 weeks after planting are shown in Table 4. T h e only significant effects were on potassium (K) and calcium (Ca) concentrations, with K concentration decreased and Ca concentration increased in plots with many nematodes. Cara usually had the greatest K concentration and the smallest Ca concentration (suggesting that it was the least affected by nematodes) and Pentland Dell the smallest K and greatest Ca concentrations. T h e concentration of phosphorus in Cara measured 9 weeks after planting was also significantly greater than in all other cultivars (P<0.05). TABLE4. Nutrient content (~/o)of dry matter ofhaulms of four potato cultivars, 6½ and 9 weeks from planting. Figures are means of four plants. Nutrient content of dry matter (°/o) 6½ weeks 9 weeks Previous crop Maris Piper

Cultivar

P

K

Ca

Mg

P

K

Ca

Mg

Maris Peer Pentland Dell Pentland Crown Cara

0.38 0.41 0-49 0-49

6-13 5-52 6.73 6.33

2.07 2-19 1.81 1.89

0.58 0.53 0.48 0.55

0.36 0.35 0-44 0-57

6-02 4.88 5.91 6.15

1.70 2-15 1.93 1-72

0.52 0.55 0-53 0.54

Mean Pentland Crown

0.44 6.18 1.99 0.54 0-43 5.74 1.88 0.54 Marls Peer Penfland Dell Penfland Crown Cara

Mean Levels of significance in F tests Previous crop Cultivar Previous crop × cultivar

0-35 0.35 0.44 0.47

4.60 4.14 5.66 5.66

2.73 2.79 2.17 2.67

0.62 0.58 0.55 0.61

0-36 0-32 0.39 0.63

4.67 3.38 4.36 5.82

2.66 3-19 2.58 2.04

0.59 0.62 0.61 0.55

0.40 5.02 2-59 0.59 0-43 4.56 2.62 0.59 m

0.05 0.05 0-05 0.05 - -

0.05 0.05 0.1

Discussion T h e usefulness of any method of assaying tolerance to nematode attack depends on the simplicity of the method and on the ease of its adaptation to a simple pot test. T h e results obtained must also be correlated consistently with yield data taken from field experiments; it is therefore important that the method should be tested thoroughly and seen to give reliable results. Of the various methods used to assess plant growth in the presence of small or large nematode populations, measurements of leaf water potential were among the most tedious and were of least value in this experiment. Estimating volumetric water content of the soil was only slightly more meaningful and could be used only in a field-scale experiment. Measurements of stomatal resistance were non-destructive, could be adapted to use in pot tests and, ifa large stomatal resistance in the presence

178

Effects of cyst nematodes on growth of potatoes

of nematodes indicates a lack of tolerance, correctly rated Pentland Dell as the least tolerant cultivar. Although Maris Peer had the smallest yield in the presence of nematodes (Table 1) the greatest loss of yield, both proportionately and in absolute terms, occurred in Pentland Dell. Table 5 shows that this loss occurred even though the initial nematode population in Pentland Dell plots after Pentland Crown was lower than that for the other three cultivars. Nematode multiplication was greatest on Pentland Dell, followed by Maris Peer and Pentland Crown. Perhaps the greater tolerance of Pentland Crown is because it is in some way a poorer host than Pentland Dell. The relatively poor performance of Marls Peer may be partly due to it being an TABLE 5. Initial and final populations and multiplication rates of Globodera rostochiensis in plots of different potato cultivars Previous Crop Maris Piper

Cultivar Maris Peer Pentland Dell Pentland Crown Cara

Pi

Pf Multiplication

5 110 9 298 12 192 7 3

22.00 33.10 16.00 0"43

Pentland Crown Marls Peer 95 158 Pentland Dell 79 185 PentlandCrown 131 275 Cara 115 16

1.66 2"34 2"10 0-14

Pi: initial population Pf: final population early-maturing cultivar which therefore has a smaller yield potential than the other cultivars used. Measuring ground cover through the season, as in Figure 1, indicates that Cara performs better than Pentland Crown in the presence of nematodes and that both of these are better than Pentland Dell and Marls Peer. However, the method requires a field-scale experiment and is therefore only a supplement to the yield data that such an experiment would provide. Integrating the data into arbitrary ground-cover/time units and expressing them as a ratio for plots with many or few nematodes, gave values of 0.57 and 0.59 for Maris Peer and Pentland Dell respectively, and 0.81 and 0-86 for Pentland Crown and Cara. Measurements of inorganic nutrient content of haulm dry matter showed significant effects only on K and Ca concentrations. Concentrations of K were consistently decreased by nematodes and those of Ca consistently increased; Cara was the cultivar least affected and Pentland Dell the most affected. As mentioned above, this is probably the correct ranking for tolerance of these cultivars and a clearer separation might be obtained by using the ratio of K to Ca concentrations. However, these results were obtained from an experiment done in a wet year and Evans (1982) points out that cultivars can be ranked in different orders on the basis of Ca concentrations when tested in different years, i.e. when rainfall and availability of

K. EVANS

179

water differs. An adaptation of this method to a pot test in standard conditions might overcome these problems and also provide a suitable assay. Because the prime effect of nematodes is on root development, those methods which assess root growth in the presence and absence o f nematodes probably give the best estimation of degree o f tolerance. This certainly appears to be so in Tables 2 and 3. It was noteworthy that, 9 weeks after planting, the shoot:root ratio of all the cultivars was very similar when the previous crop was Maris Piper (mean value 11.9) or Pentland Crown (mean value 5.6). Differences in growth and therefore yield were attributable to production of extra roots in Pentland Crown and Cara, thus allowing more shoots to develop. A pot-scale test would perhaps give equivalent results. Particularly tolerant lines might even be selected simply on the basis of general vigour, even in the absence of nematodes: in observation boxes Cara, growing in a nematode-free medium, produced a deeper root system than Pentland Crown, with an overall length of roots approximately twice that of Pentland Crown. Economy of effort is the main concern in developing assays for plant-breeding work, and most of the methods of assessing growth and development of plants in field trials are therefore unsuitable for such assays. However, shoot: root ratios appear to be consistent between cultivars for a given nematode population, so simple tests of shoot growth in the presence or absence o f nematodes would probably give consistent estimates of degree of tolerance. Less laborious would be estimations of the K : C a ratio in shoot dry matter of plants grown under defined conditions, but until this or other novel methods of assessing tolerance, such as monitoring ABA (abscisic acid) levels (see Evans, 1982) become available, the only reliable method of assessing tolerance will remain the replicated field trial. References BELL,J.P. (1969). A new design principle for neutron soil moisture gauges: The 'Wallingford' neutron probe. Soil Science 108, 160-164. BROWN,E.B. (1969). Assessment of the damage caused to potatoes by potato cyst eelworm, Heterodera rostochiensis Woll. Annals of Applied Biology 63, 493-502. COSXMINI,V. ANDTALmUDEEN,O. (1978). Analysis of Crops, Soils and Fertilisers, 2nd edn. Rothamsted Experimental Station, Harpenden, UK. 201 pp. DIXON, H.H. (1914). Transpiration and the ascent of sap in plants. London: Macmillan. 216 pp. EVANS,K. (1982). Water use, calcium uptake and tolerance of cyst-nematode attack in potatoes. Potato Research 25, 71-88. EVANS,K. ANDFRANCO,J. (1979). Tolerance to cyst-nematode attack in commercial potato cultivars and some possible mechanisms for its operation. Nematologica 25, 153-162. EVANS,K., PARKINSON,K.J. ANDTRUDGILL,D.L. (1975). Effects of potato cyst-nematodes on potato plants. III. Effects on the water relations and growth of a resistant and a susceptible variety. Nematologica 21, 273-280. OYDVIN,J. (1977). [Resistance and tolerance to potato cyst nematode Heterodera rostochiensis Woll. pathotype A among thirteen potato cultivars.] Forskning ogforsok i landbruket 28, 255-266. PARKINSON,K.J. ANOL ~ , B.J. (1972). A continuous flow porometer. Journal of Applied Ecology 9, 669-675. SCHAFER,J. (1971). Tolerance to plant disease. Annual Review of Phytopathology 9, 235-252. TRUDG~LL,D.L., EVANS,K. AND PARROTT,DIANAM. (1975). Effects of potato cyst-nematodes on potato plants. I. Effects in a trial with irrigation and fumigation on the growth and nitrogen and potassium contents of a resistant and a susceptible variety. Nematologica 21, 169-182. WELBANK,P.J. ANDWILLIAMS,E.D. (1968). Root growth of a barley crop estimated by sampling with portable powered soil-coring equipment. Journal of Applied Ecology 5, 477-481. WHITEHEAD,A.G., TITE,D.J., FRASER,JANETE. ANDFRENCH,EILEENM. (1980). Effects ofaldicarb and oxamyl in peaty loam soil on potato cyst-nematode, Globodera rostochiensis, and on resistant and suceptible potatoes. Journal of Agricultural Science 95, 213-217. Received 19 October 1981