Ann. appl. Biol. (1980), 95, 115-124 Printed in Great Britain

115

The effects of nematode resistant and susceptible spring oat cultivars and aldicarb on the cereal cyst nematode Heterodera

avenae and yields in contrasting soil types

BY T. D. W I L L I A M S A N D J . BEANE Nematology Department, Rothamsted Experimental Station, Harpenden, Herts, AL5 2JQ

(Accepted 18 January 1980)

SUMMARY

The effects of the cereal cyst-nematode, Heterodera auenae Woll. on resistant and susceptible oat cultivars, with and without aldicarb treatment, were compared on a clay-with-flints soil at Rothamsted and a loamy sand at Woburn. At both sites, when H. auenae was extremely scarce, yields were not further enhanced by aldicarb. At Rothamsted aldicarb increased yields by 48-72% when H. auenae averaged 10 eggs/g soil. At Woburn, aldicarb increased yields of both susceptible and resistant varieties by 80-90% with 20 eggs/g. The resistant varieties conferred yield benefits in the following oat crop equal to the residual effects of aldicarb applied before the previous crop, demonstrating that H. auenae was wholly responsible for the yield losses. Nematode resistant oats suffered as much or more damage from root invasion by H. auenae juveniles as the susceptible varieties but the resulting decrease in nematode numbers led to considerable yield improvements in the following year. At Woburn in 1977, when formalin was an added treatment, fewer females were infected by parasitic fungi and post-crop egg numbers were greater.

I N T R O D U C T I O N

The cereal cyst-nematode, Heterodera auenae Woll. occurs commonly in the loamy sand soils at Woburn Experimental Farm and outbreaks also occur from time to time on the loam over clay-with-flints at Rothamsted. There, in 1934, an oat crop failure occurred in Pastures Field where a soil fumigation experiment was sited in 1935 and 1936 (Ladell, 1936, 1937). Of four fumigants used, carbon disulphide jelly and ‘Cymag’ (sodium cyanide) almost doubled the yield of the spring oat variety ‘Marvellous’; only carbon disulphide produced a significant decrease in post-crop cyst numbers. In the second year, with no further fumigant treatment, the oat crop was a failure in all but the carbon disulphide plots (4-6 tonnedha); no treatment significantly affected cyst numbers. Less severe outbreaks have since occurred at Rothamsted from time to time with a more intense one in Pennell’s Piece Field in 1972 which afforded the opportunity for a further experiment. However, H. auenae is scarce in the soils of the Broadbalk and Hoosfield classical experiments which have grown winter wheat and spring barley respectively every year for more than a century (Corbett, Winslow & Webb, 1969). Butt Close Field, Woburn, has been the site of many trials of nematicides against H. auenae since 1964 (Williams, 1969; Williams & Salt, 1970; Williams, Beane, Berry & Webb, 1979).

The effects of resistant and susceptible barley cultivars or near-isogenic lines on H. auenae and vice-versa are well documented (Cotten, 1970; Williams, 1970; Graham & Stone, 1975; Howard & Cotten, 1978). Yield improvements in resistant cultivars from some 20 sites in England and Wales averaged 8.6% usually accompanied by substantial decreases in post-crop egg numbers.

Oats are most sensitive to H. auenae attack (Gair, 1965). With the introduction of spring oat selections based on nematode resistant Auena byzantina and A . sterilis (Cotten & Hayes, 1972; 0 1980 Association of Applied Biologists

116 T . D . WILLIAMS A N D J . B E A N E

Cook, 1974) the opportunity arose to assess the effects of H . avenae on resistant and susceptible spring oat varieties with and without nematicide treatments.

M A T E R I A L S A N D M E T H O D S

Field studies Site. 1. Pennell’s Piece, Rothamsted Experimental Station Farm

Soil type - Deep flinty silt loam (Charity Complex), a well-drained soil with a friable flinty surface. The site was previously occupied by the spring wheat series of the cereal disease reference plots where wheat had either been grown continuously for 9 yr or in rotation with beans (1 yr), oats (1 yr) then 3 yr continuous wheat. All phases of this rotation had been included. In 1972, severe but localised infestations of cereal cyst-nematode were observed and the nematode studies were initiated in 1973.

Design. Each of the existing reference plots (12 in two randomised blocks of six) was split length-wise so that a sub-plot (7.92 x 1.14 m) could be sown with spring oat cv. Mostyn (susceptible to H . avenae), or cv. Nelson (ex. Weibull’s, Sweden) a resistant variety, or a Weibull’s selection W. 16840 (also resistant). Each sub-plot was further split to permit the application of granular aldicarb (2-methyl-2-(methylthio)propionaldehyde-O-(methylcarbamoyl) oxime) at 5 kg/ha a.i. All aldicarb plots were treated (broadcast application followed by rotavation to a depth of 15 cm) prior to sowing of all varieties on March 14, 1973. Basal manuring (per hectare) was 1260 kg 0 : 20 : 20 compound fertiliser and 250 kg ‘Nitro-Chalk 25’ by drill. The crop was harvested on 22 August 1973.

In the second (residual) year, 1974, no further treatments were applied. A uniform crop of spring oats cv. Manod was sown in all sub-plots on 5 April, fertilisers were as before. The crop was harvested on 28 August.

Nematode samples: At the beginning of the first year each whole plot was sampled (20 cores, 2.5 x 15.0 cm). The egg count from each bulked sample (Southey, 1970) constituted the pre-crop assessment for the six sub-plots. After harvest each sub-plot was individually sampled (15 cores) and post-crop egg numbers determined. Each sub-plot was again sampled after the 1974 harvest.

Site. 2 . Butt Close, Woburn Experimental Farm The site had a lengthy history of H . avenae infestation: preceding crops were wheat (1974)

and oats (1975). The loamy sand (Cottenham Series, Lower Greensand) is well drained, easily leached and liable to capping and erosion (Catt, King & Weir, 1975). H. avenae seldom reproduces freely at this site unless formalin is applied as a drench before sowing (Williams, 1969). In this experiment we studied the interactions of aldicarb and formalin and their effects on H . avenae resistant and susceptible spring oat cultivars and the incidence of the Oomycetous fungi that parasitise H. avenae females (Kerry, Crump & Mullen, 1980).

Design. In 1976 there were four blocks of four plots each split into four sub-plots (harvested area: 0.00098 ha.) Formalin was applied to half the sub-plots as a drench (38% formaldehyde at 3000 litredha) 3 wk before sowing spring oats cv. Manod (190 kg/ha) on 22 March. Basal fertiliser (20 : 14 : 14) was combine drilled at 380 kg/ha. The crop was harvested on 4 August but because of the H. avenae infestation and the severe summer drought it was a failure and no yields were taken.

In 1977 formalin was again applied (16 February) at the same rate to give sequences of formalin in both years, no formalin and formalin in first or second years only. Aldicarb was applied to half the whole plots by tractor-drawn broadcast applicator at 10 kg/ha a.i. on 15 March and rotavated to a depth of 15 cm. All plots were spring-tine cultivated on the same date.

Aldicarb and Heterodera with resistant and susceptible oats 117

Whole plots were sown with spring oat cv. Maris Tabard (susceptible to H. avenae) or cv. Nelson (resistant) at seed rate 190 kg/ha. Basal fertiliser (20 : 14 : 14) was combine drilled on the same date, 30 March.

The design permitted comparisons of all combinations of treatments including the residual effects of 1976 treatments.

Nematode and plant samples. Pre- and post-crop H. avenae egg counts were assessed as in the Pennell’s Piece experiment. Plant samples were taken in May for measurements of fresh top and root weights (4 x 12.5 cm row per sub-plot) and H. avenae invasion counts. In June, eighteen 15 x 2.5 cm cores per sub-plot were mixed and subsampled (200 g air-dried soil) to estimate the number of unhatched eggs carried over to the next year. In July five 20 x 5 cm plant row/sub-plot samples were taken to assess the number of females and the extent of fungal parasitism. The crop was harvested on 3 September.

In 1978, the last year, the residual effects of all previous treatments were tested on a uniform crop of Manod oats (seed rate 202 kg/ha) sown on 31 March. All cultivations and sampling procedures were as for 1977. The crop was harvested on 4 September and post-crop H. avenae samples taken a week later.

All grain yields are given as 85% dry matter.

R E S U L T S

Site 1 . Pennell’s Piece, Rothamsted 1973. Table 1 gives the yields of the resistant and susceptible oat varieties. Without aldicarb all varieties yielded similarly; with aldicarb, selection W 16840 significantly

outyielded both Mostyn and Nelson. Yields of both Nelson and W16840 were significantly improved by aldicarb but not the susceptible variety Mostyn.

The effects of the cereal disease cropping sequences were assessed in the yield analysis, the only significant effect was that in the absence of aldicarb yields were smallest after the 9 yr continuous wheat sequence and were significantly increased by aldicarb. H. avenae was most abundant in these continuous wheat plots at the beginning of the experiment.

Table 2 shows the effects of aldicarb in plots with 10 eggs/g H. avenae and those with less than 1 egg/g. It emphasises the magnitude of yield responses, less clearly seen in Table 1 because of the uneven distribution of H. avenae.

The percentage yield increases in aldicarb-treated sub-plots for the experiment as a whole were: Mostyn ( S ) + 12% from 5.59 t/ha, Nelson (R) + 20% from 5.31 t/ha, W16840 (R) + 22% from 5-73 t/ha.

The post-crop H. avenae egg counts are given in Table 3. Without aldicarb, egg numbers were significantly greater after Mostyn (S) than after Nelson

and W16840. With aldicarb all varieties produced equally few eggs but numbers were significantly decreased in Mostyn plots. Population changes in individual sub-plots were much more marked. Mostyn increased egg numbers in one non-aldicarb sub-plot from 9 to 23/g soil and Nelson decreased them from 18 to 2/g in another. Because H. avenae was unevenly

Table 1. Grain yields of spring oat cultivars (tlha) Pennell’s Piece, 1973

Variety Mostyn (S)* Nelson (R)* W16840 (R) 5.59 6.26 Aldicarb kg/ha a.i. 5-31 5.73

6.40 6.98

LAD. Vertical comparisons (P = 0.05) = 0.79, (P = 0.01) = 1.1 1,

* (S) susceptible; (R) resistant. horizontal comparisons (P = 0.05) = 0.50, (P = 0.01) = 0.69.

118 T , D . W I L L I A M S A N D J . B E A N E

Table 2. Grain yields of spring oat cultivars (tlha) in plots with and without Heterodera avenae and aldicarb, Pennell’s Piece I973

With H. avenue (10 eggs/g) A

\

Aldicarb O h Yield change (5 kg/ha ai.) No aldicarb with aldicarb

Mostyn (S) 6.33 4.28 +48 Nelson (R) 6.23 3.63 +72 W16840 (R) 6.97 4.52 +54

Few H. avenue (< 1 egg/g) r 7

h

Mostyn (S) 6.27 6.78 -8 Nelson (R) 7.08 6.76 +5 W16840 (R) 7.41 7.62 -3

Table 3. Post-crop Heterodera avenae eggslg soil, log transformation (x + I ) , (untransformed means in parentheses), after oat cultivars and aldicarb I973

Variety r

Aldicarb kg/ha a.i. Mostyn (S) Nelson (R) W16840 (R)’ 0 0.492 (5.0) 0.151 (0.8) 0.133 (0.5) 5 0.114 (0.4) 0.069 (0.3) 0.034 (0.1)

Pre-crop means: without aldicarb 4.3 eggslg with aldicarb 3.6 eggslg

L.S.D. Horizontal: (P =0.05) 0.171, (P =0.01) 0.240, (P =0.001) 0.339; vertical: (P = 0.05) 0.149, (P =0.01) 0.209, (P =0.001) 0.295.

distributed, overall effects on egg numbers, as for yields, were less marked but nevertheless some significant treatment effects were recorded. Most H. avenae (8 eggslg) followed Mostyn ( S ) in untreated sub-plots previously in the continuous winter wheat sequences 1963- 1972. H . avenae was also numerous on the site of a previous continuous wheat plot, where grain yield was least (Nelson, 2.16 tlha). Variety yields were almost doubled by aldicarb in this whole plot. Apart from this highly localised association between a previous crop sequence and H. avenae, none of the earlier crop sequences significantly influenced H . avenae numbers.

Table 4. Post-crop Heterodera avenae eggslg soil in 1974, transformed to log (eggs/ 1000 g + 1 ) (untransformed means in parentheses), residual efJ’ects of oat cultivars and

aldicarb, 1973

Variety (73) I

Aldicarb (73) kglha Mostyn (S) Nelson (Rf W 16840 ( S ) a.i. 0 0.993 (4.9) 0 .553 (1.3) 0.354 (1.3) 5 0.528 (1.8) 0.301 (0.9) 0.475 (1 .5 )

(overall mean 2.0 eggs/g soil)

L.S.D. Horizontal comparison: (P = 0.05) = 0.0349, (P= 0.01) = 0.489; vertical comparison: (P = 0.05) = 0.390.

Aldicarb and Heterodera with resistant and susceptible oats 119

1974. A uniform crop of the susceptible variety Manod was sown on all plots; aldicarb was not applied. Yields were poor, averaging only 2.26 t/ha compared with 6.05 t/ha in 1973. Neither varieties nor aldicarb ( I 973) had significant residual effects. Without aldicarb egg numbers after Mostyn (73) significantly exceeded those after Nelson and W 16840. With aldicarb, all egg counts were small and did not differ. The residual effects of aldicarb were significantly detectable in the ex-Mostyn plots only (Table 4).

Site 2. Butt Close, Woburn Experimental Farm 1976. The yields of Manod oats, when only formalin was applied to half the plots, were not

taken. The crop was a failure because of the exceptional drought and considerable H. auenae invasion. H. auenae was fairly evenly distributed before the 1976 crop, averaging 16, 17, 20 and 16 eggs/g respectively in Blocks I-IV. Samples of soil, roots and plant tops were taken during the season from 16 sub-plots (two formalin treated, two untreated per block) to measure nematode invasion, population ‘carry-over’ and crop growth. Pre- and post-crop egg numbers were recorded for all 64 sub-plots.

The effects of formalin were negligible because of the very dry spring and summer, when the January-August rainfall was 254 mm less than average, with a deficit in every month. Some 400 juvenile H. auenaelg were found in both formalin treated and untreated roots in May. During June, three counts of females were made, numbers reaching their peak in midJune (2400/five x 20 cm x 5 cm row/sub-plot). Of these, 80% were fertilised. Disease incidence was low (7%) and unaffected by formalin in this very dry year. Post-crop egg numbers were identical in treated and untreated plots.

1977. Aldicarb (10 kg/ha a.i.) and further formalin were applied. Spring oat cultivars, Maris Tabard (susceptible to H. auenae) and Nelson (resistant) were sown. The yields are shown in Table 5.

Formalin 1977 slightly but significantly (P = 0.05) increased yield over-all, but had no effect on varieties at the same level of aldicarb treatment. The main effect was that of aldicarb (P = 0.001), increasing the yield of both varieties with or without formalin 1976 or 1977. Maris Tabard significantly (P = 0.001) outyielded Nelson when treated with aldicarb and formalin 1977, otherwise yields were similar or barely achieved significant improvement. Averaged for all treatments, Tabard outyielded Nelson by 0.4 t/ha (P = 0.0 1).

Tables 6, 7 and 8 show changes of H. auenae populations, effects of treatments on plant growth, invasion, and female counts respectively. Only 1977 treatment effects are shown since there were no residual effects of the 1976 formalin application.

Nelson shoot weights significantly (P = 0.05) exceeded Maris Tabard (all-treatment average), and were also more improved (P = 0-001) by aldicarb. In May both varieties grew significantly better after 1977 formalin in the absence of aldicarb. They responded better to aldicarb in July.

Table 5 . Grain yields of Maris Tabard and Nelson spring oats (t /ha) after formalin 1977 and aldicarb 1977

Aldicarb (77) 0 10 kg/ha a.i. &&

Formalin (77) 0 30001/ha 0 3000 I/ha M. Tabard (S) 2.12 2.30 3.93 49 19 Nelson (R) 1.76 2.02 3.57 3.54 Variety ’77(

L.S.D. Horizontal, within each aldicarb rate and for same variety, (P = 0.05) = 0.302, ( P = 0.01) = 0.408, ( P = 0.001) = 0.544; vertical, and between each aldicarb rate (P = 0.05) = 0.356, (P = 0.01) = 0.481, (P = 0.001)

0.64 1 .

120 T . D. W I L L I A M S A N D J . B E A N E

Table 6 . Pre-crop, carry-over and post-crop Heterodera avenae populations, eggslg. Resistant and susceptible spring oats, Woburn 1977

Aldicarb (77)

Formalin (77) Maris Tabard (S)

Pre-crop Carry-over, 8 June Post-crop

Nelson (R) Pre-crop Carry-over, 8 June Post-crop

7 0

19.4 2.8

11.0

21.4 4.3 3.2

0 + 3000 I/ha

18.6 9.3

17.6

19.0 7.1 3.5

10 kg/ha a.i. &

0 3000 I/ha

19.6 20.9 4.2 6.8 1.1 3.5

21.6 22.5 5.5 7.1 1.5 2.2

Table I . Fresh weights of shoots, seminal roots and heights, resistant and susceptible oats. Woburn 1977

Aldicarb (77)

Formalin (77)

Seminal root, g, 25 May Top weight, g, 25 May Top weight, g, 4 July Shoot height, cm, 4 July

Seminal root, g, 25 May Top weight, g, 25 May Top weight, g, 4 July Shoot height, cm, 4 July

Maris Tabard (S)

Nelson (R)

0 r -I

0 3000l/ha

17 11 107 124 148 163 61 59

14 10 99 125

165 192 74 72

10 kg/ha a.i. &

0 3000l/ha

4 5 126 113 214 218

66 66

5 4 135 145 236 215

77 78

25 May samples, weights are totals from 4 x 12.5 cm row, 4 July samples, weights are totals from 5 x 5 cm row.

Seminal root weights of both varieties increased considerably ( P = 0.001) with greater invasion by H. avenae in the absence of aldicarb (Williams & Salt, 1970). Formalin 1977 had an equally significant effect with no aldicarb but not with. Plant heights (to top of flag leaf, 4 July) were not much affected by aldicarb, but Nelson was the taller variety overall ( P = 0.001). Formalin, with or without aldicarb had no effect on height. Tiller number was little affected by any treatment and was similar (40how length sampled) in both varieties.

Table 8. Heterodera avenae juvenileslg seminal root (25 May) and females (4 July) from 5 x 5 cm x 20 ern soil cores per sub-plot, 1977

Aldicarb (77) 0 10 kg/ha a.i.

Formalin (77) 0 30001/ha 0 3000I/ha Maris Tabard (S)

Juveniledg seminal roots 272 176 22 11 Females/sample 156 (30%)' 147 (l5%)* 0 1

Juveniledg seminal roots 66 25 8 5 Females/sample 3 0 0 0

Nelson (R)

Percentage females attacked by fungi.

Aldicarb and Heterodera with resistant and susceptible oats 121

Table 9. Grain yields of Manod (tlha) 1978, after aldicarb, formalin and resistant and susceptible oat cultivars in 1977

10 kg/ha a.i. Aldicarb (77) 0 &-

Formalin (77) 0 3000I/ha 0 30001/ha 1977 Maris Tabard (S) 1.02 1.14 2.42 2.47 1977 Nelson (R) 2.58 2.84 2.77 2.54

L.S.D. ( P = 0.05) = 0.61, ( P = 0.01) = 0.81, ( P F 0.001) = 1 .O7

Maris Tabard was the most heavily invaded variety. Aldicarb and formalin considerably decreased invasion in both varieties (P = 0.001). H . auenae females were only numerous on the roots of Maris Tabard not treated with aldicarb (Table 8). Although total numbers of females were not affected by previous formalin treatment the incidence of fungal disease was halved by 1977 formalin. This might explain the increased post-crop H. avenae egg numbers in Maris Tabard plots treated with formalin but not with aldicarb.

Post-crop egg counts were significantly (P = 0.001) larger after Maris Tabard not treated with aldicarb. 1978. Grain yields are given in Table 9. Heavy spring rains washed away plants in some

sub-plots and only in one block (4 tlha) did yields approach those of 1977. The residual benefit of a H. avenae resistant oat variety is clearly equal to that of aldicarb (10 kglha a.i.). Aldicarb (77) gave no extra benefit in ex-Nelson plots but after Maris Tabard it improved yield by 1.37 tlha. Formalin (77) had no residual effects.

The residual effects of 1977 treatments on H. auenae populations and incidence of fungal attack are given in Table 10.

Pre-crop 1978 H. auenae numbers in ex-Nelson plots were only 1-8 eggs/g after aldicarb, 3.4/g without. Post-crop populations were counted in all plots, and were still significantly

Table 10. Residual effects of aldicarb, formalin and spring oat cultivars (1977) on Heterodera avenae, Woburn 1978. (Ex Maris Tabard plots only, except where stated)

Aldicarb (77) 0 10 kg/ha a.i.

Formalin (77) 0 3000I/ha 0 3000I/ha &*

Pre-crop eggs/g 11.0 17.6 1.0 3.5 Eggs carried-over/g 1.8 6- 2 0.4 0.9 Juvenileslg seminal roots 26 1 294 42 52

L.S.D. (P = 0.05) = 48, (P = 0.01) = 66, (P = 0.001) = 90

Females/S x 5 cm x 20 cm 258 (5%). 245 (6%) 39 (8%) 39 (1%) row

L.S.D. (P = 0.05) (P = 0.01) 137, (P = 0.001) 187

Post-crop eggdg after: 1977 Maris Tabard (S) 14.9 21.3 3.0 5.9 1977 Nelson (R) 5 . 8 7.6 4.3 4.8

L.S.D. for comparisons with same variety only (P = 0.05) = 3.3, (P = 0.01)

* Percentage females attacked by fungi. = 4.4, (P = 0.001) = 5.8.

122 T. D. W I L L I A M S A N D J . B E A N E

greater (P = 0.001) after Maris Tabard (77). Egg numbers were much fewer (P = 0.001) in the ex-Maris Tabard plots treated with aldicarb but were not decreased by aldicarb in the ex-Nelson plots. Aldicarb (77) greatly decreased invasion ( P = 0.001) by H. avenue juveniles after Maris Tabard, but formalin (77) had no effect.

Numbers of females were also much decreased (P = 0.001) after aldicarb. Formalin (77) did not affect their numbers but it appeared to prevent fungal infection in the ex-aldicarb plots.

D I S C U S S I O N

The most important finding from these experiments is that the benefits in yield conferred by genes for H. avenue resistance in oats are the same in the following crop as from aldicarb (Table 9, Woburn 1978). This demonstrates that H . avenue is responsible for the recorded yield losses, since although other nematodes and invertebrate pests are controlled by aldicarb they are not affected by H. avenue resistance. The yields at Rothamsted in 1973 (Table 2) confirm that losses are due to H . avenue alone, since they were equally good whether the nematode was initially plentiful and controlled by aldicarb or very scarce with no further gain from aldicarb treatment.

Oat yield gains when H. avenue was very scarce in Pennell’s Piece (1973/74) or effectively controlled by aldicarb at both sites were considerable. At Rothamsted in 1973 they were 72%, from 3.6 to 6 .2 t/ha, and 86% at Woburn in 1977, from 2.2 to 4.1 t/ha. The Woburn site yields are usually less even without pests or diseases.

The residual effects of the 1973 treatments on the 1974 oat crop at Rothamsted were not significant because the already patchy distribution of H . avenue was exacerbated by the resistant oat and aldicarb treatment of some of the more heavily infested plots. However, the smallest Manod yield (1.67 t/ha was in the ex-Mostyn ( S ) no-aldicarb plot where there was most H. avenue in 1973, and the best yield (2.91 t/ha) after Mostyn with no detectable H. avenue. Aldicarb applied at Woburn in 1977 in plots where H. avenue was already scarce did not further increase Manod yield in 1978, additional evidence of the nematode’s responsibility for the yield losses.

Aldicarb and resistant varieties decreased by equal amounts the numbers of females on roots and eggs in soil after harvest. Aldicarb decreased egg numbers after a susceptible crop but caused no further decrease with a resistant one. Egg numbers were still fewer in ex-aldicarb plots after a second susceptible crop at Rothamsted and Woburn. Female H. avenue were abundant only on untreated Maris Tabard at Woburn; they were almost entirely absent on treated M. Tabard and on both treated and untreated Nelson (R) roots.

In the absence of aldicarb, formalin usually increased H. avenue egg numbers after susceptible oat crops, as in earlier experiments with spring wheat and barley (Williams, 1969; Williams 8z Salt, 1970). Kerry et ul. (1980) have shown that formalin kills fungi parasitic on H. avenue and that the infective stage of an Oomycetous species is a motile zoospore dependent on soil water. This explains the absence of formalin effects in seasons of small or excessive rainfall. Our estimates of parasitism of H. avenue females (Table 8 ) and egg counts (Table 6) at Woburn agree with the results of Kerry et ul. The persistence of the effect of 1977 formalin through 1978 at Woburn may have resulted from the larger numbers of H . avenue eggs carried over in formalin treated plots. Williams, Beane & Salt (1973) found that 10 commonly occurring genera of soil fungi were suppressed for at least 5 months after formalin treatment. While we did not detect nematode parasitic fungi on the media used, they might be similarly affected.

Another important observation is the severe damage caused to the roots of resistant oats by invading juvenile H. avenue. The nature of the damage to ‘resistant’ oats merits further attention since it cannot be due to the diversion of plant resources to developing female nematodes and can sometimes be worse in the potentially best yielding resistant varieties (Chew, 1979).

.

Aldicarb and Heterodera with resistant and susceptible oats 123

Resistant barley varieties are much more tolerant to H. avenue, as indeed are susceptible barley varieties (Gair, 1965). Damage assessment trials using near-isogenic selections developed by the Welsh Plant Breeding Station and commercial breeders have shown an average 9% yield advantage from resistant barleys and as much as 26% at some sites (Cotten, 1970; Williams, 1970; Graham & Stone, 1975; Howard & Cotten, 1978). In oats, where resistance carries no yield advantage in the current crop, the converse occurs and careful use of the term ‘resistance’ is necessary to distinguish between the failure of females to develop and the ability to withstand the effects of invasion.

It is recommended that where farmers wish to control cereal cyst-nematode by resistant varieties only, H . avenue resistant barleys be used first. When nematode numbers have declined sufficiently, resistant oats can be sown with less risk of yield loss.

We thank D. B. Slope for reporting the H. avenue outbreak at Rothamsted, the Rothamsted and Woburn Farm Staffs, D. J. Tite for applying the aldicarb at Woburn and J. H. Dunwoody for the statistical analyses. We also thank Rothwell Plant Breeders for supplying seed of Nelson and W 16840 oats.

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(Received 23 April 1979)