Ann. appl. Biol. (1964), 53, 77-93

Printed in Great Britain 77

The effect of motley dwarf virus on yield of carrots and its transmission in the field by Cavariella aegopodiae Scop.

BY MARION WATSON AND E. P. SERJEANT Rothamsted Experimental Station, Harpenden, Herts

(Received 24 July 1963)

S U M M A R Y

In 1959 and 1961, when the willow-carrot aphid, Cavariella aegopodiae, infested carrots at Rothamsted and Woburn early and severely, crops were infected with motley dwarf virus in late May or early June and yielded about 6 tons of roots per acre. In 1962, when invasion was late and sparse, the crops remained almost uninfected and yielded 24-25 tons/acre. In 1960, when aphids invaded early but multiplied slowly, about 85 yo of carrots on unsprayed plots became infected in July and August, and the yield was 9.2 tons/acre. Spraying three times with Metasystox starting at an early stage of growth affected yield little in 1961, or in 1962, but increased yield by about 3 tonslacre in 1960.

Field-plots experimentally infected in 1962 by aphids fed on infected plants in the glasshouse, lost I I tons/acre from infection in early June, 8 tons/acre from infection in late June and 6 tons/acre from infection in July. Plots cultured with virus-free aphids in early June yielded as much as control plots. Experimental infection did not affect yield in 1959 and 1961, when the crop became naturally infected before treatment.

The yields in different years were linearly related to the log. mean weekly numbers of C. aegopodiae caught on sticky traps near the sites, and the regression accounted for much of the variance in yield. The residuals of the log. mean weekly trap-counts were negatively related to residual weekly rainfall in inches ; I in. of rain above average approximately halved the increase in aphids. This may explain the failure of early invading aphids to become numerous at Woburn in 1960, when an inch of rain fell in three consecutive weeks in June.

Treating seed or seedlings with systemic insecticide did not prevent young plants from becoming infected when infective aphids were cultured on them 10-14 days after treatment.

Aphids taken from willow in the spring did not transmit motley dwarf to healthy carrots, but did so after they had fed on infected carrots. Aphids from wild umbellifers often transmitted motley dwarf to healthy carrots.

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

Field experiments were done on Woburn and Rothamsted farms in 1959 and 1962, inclusive, to find the effect of motley dwarf virus (CMDV) (Stubbs, 1948, 1952; Watson, 1960) on yield of carrots. Two kinds of treatment were used: one, experi- mental infection, in which plots were deliberately infected by feeding infective aphids

78 MARION WATSON AND E. P. SERJEANT on the young plants, and the other, insecticide treatment to control spread of infection by naturally invading aphids.

M E T H O D S

Experimental infection. Willow-carrot aphids (Cavariella aegopodiae, vector of CMDV) were fed for a week on infected carrots. They were collected in groups of 12-20 in small specimen tubes which were opened and placed at 3 in. intervals along two 4 ft. rows of carrots covered by cages of muslin on a wooden support with windows of cellulose nitrate. Other cages received virus-free aphids or no aphids and plots of the same size were left uncovered to measure the effect of shading by the cages. After some days the cages were removed and the aphids killed by insecticide.

Insecticide treatments. Metasystox at the rate of 12 02. in 40 gallons of water per acre was applied with a knapsack sprayer at Woburn in 1959, 1960, and Rothamsted 1962. In 1961 a hand-propelled power-operated spray-boom (Burt, Broadbent & Heathcote, 1960) was used and at Woburn in 1962 a tractor-drawn agricultural sprayer. In 1962 the effects of Menazon seed dressing at 2 7 , (80% powder) by weight of seed, was tested in the field and also in a glasshouse experiment. In the glasshouse rows of carrots were grown in yard-square wooden boxes of soil, which were divided into plots.

Basal dressing of 8 cwt./acre compound fertilizer (10: 10: IS) was applied im- mediately before sowing to all plots. From 1960 onwards the roots were examined after harvesting when the tops were separated from them, and samples were kept for top : root ratio and dry-matter content estimation. I n 1962 a few roots showed violet root rot (Helicobasidium purpureum) lesions, but otherwise there was little recognizable damage from other causes than virus.

Except at Woburn in 1959 sticky traps (Broadbent, Doncaster, Hull & Watson, 1948) were placed near all experiments to estimate weekly fluctuations in numbers of Cavariella aegopodiae. The estimates used for Woburn, 1959, were from Rothamsted, figures for Rothamsted and Woburn having been similar in other years. Counts of aphids and observations were made on the plots and on plants removed to the glasshouse.

We are indebted to Dr G. D. Heathcote for help in identifying aphids caught on traps, and for supplying data from his records.

The experiments will be described in chronological order and distinguished by year and situation.

E X P E R I M E N T S

Woburn 1959 Table I (main treatment experimental infection; var. James Scarlet Intermediate). Carrots were sown on 26 April for an experiment consisting of five randomized

(I and 2) early and late experimental infection; (3 and 4) caged controls receiving virus-free aphids on the same dates. The early experimental infections and controls were done on 8 June, but at this

time the crop was already heavily infested with aphids and the plants appeared

blocks testing the following treatments :

Effect of motley dwarf virus on yield of carrots 79 diseased. Samples tested in the glasshouse contained CMDV. The second experi- mental infection was therefore omitted but uncaged control areas were harvested from the unused plots.

The whole experiment was sprayed with Metasystox on 14 June, and harvested 7 September. Experimental infection had no effect on yield (Table I) presumably because the plants were already infected when cultured with infective aphids. The uncaged plots yielded about half a ton more of roots than the caged ones, which may have been an effect of shading or of slightly increasing aphid population and spread of virus under the cages. The mean yield was less than 43 tons/acre, and this was not exceptional for the area. Another experiment at Woburn also had many infected plants when sampled on 8 June and yielded 6 tons/acre of roots.

Table I. Woburn, 1959. Effect of experimental infection on carrots naturally infested early (before the$rst infection date) with Cavariella aegopodiae. Fresh weight, tonslacre

Caged Caged infected healthy aphids aphids Uncaged S.E. Mean S.E.

Roots 3'9 3'5 4'7 0.47 4'0 0.28 Tops 1.9 1.8 2' I 0.17 2'0 0'12

Table 2. Woburn, 1960. Counts of Cavariella aegopodiae on plants taken from differently sprayed plots. Expressed as aphids1 100 plants

Spraying treatments (see text).

Date of sampling

Early + None Early Late late S.E. Mean S.E.

0 6 - - 9 June Winged 15 5 3

28 June Winged 5 25 20 22

Nymphs 410 I 0 0 411 140 70.6 265 49.6

Nymphs 432 403 502 448 70.6 359 49.6

- 18 -

Woburn 1960 Table 2 (main treatment insecticide spraying; var. James Scarlet Intermediate). A Latin square arrangement of plots each of about 1/600 acre, sown on 29 April,

received four insecticide spraying treatments, namely: ( I ) sprayed 26 May (early); (2) sprayed 13 June and also on 4 July (late); (3) sprayed early and late; (4) no spray. The late spray was repeated on 4 July because heavy rain on both 13 and 14 June probably rendered it ineffective.

Aphids were not seen on 26 May when the plants seemed free of virus infection. A few alatae were seen in situ on 9 June. Samples of ten plants were taken from each plot and examined under a binocular miroscope and groups of small nymphs were found mainly in the heart leaves and concave faces of the petioles (Table 2). Early spraying depressed their numbers.

Similar samples were taken on 28 June. The numbers of winged aphids and nymphs had increased on early-sprayed plots where they had previously been depressed, but spraying on 13-14 June had little effect. The nymphs appeared to be the progeny of recently arrived alatae for there were very few apterous adults. Probably earlier

80 MARION WATSON AND E. P. SERJEANT generations had been killed by the heavy rainfall in early June. Four of the I 60 plants sampled on 28 June were infected with CMDV. On 6 August 44% of plants on unsprayed and an average of 23 yo on sprayed plots were infected. The ratio was similar at harvest, when unsprayed plots had an average of 87 % infection, and was reflected in the yields (Table 3) which were increased by early and late spraying though the increase was only significant when both were given.

The experiment was harvested between 23 and 25 August. Plants were weighed on the field and samples from each row were bulked so that tops and roots could be weighed separately. The roots were separated by 'topping' as with sugar beet instead of 'twisting' as with commercial carrot crops, so the yield of roots may be under- estimated. This is one reason why the yield of roots for 1960 seems low in relation to aphid numbers compared with other experiments (Fig. 3).

Table 3. Woburn, 1960. ESfect of spraying early and late with Metasystox. Fresh weight, tonslacre

Roots Tops - - Spraying treatments - L Mean - L Mean

9'2 10'2 9 Y 4'2 4.6 4.4O E 9'9 12'0 11.0" 4'5 5 '5 j.Ob

Mean 9.5 11'1 10.3 4 3 5'1 4'7

-

- 0.561. S.E. of difference 0 . 8 6 ~ -

* Also appropriate for u. 1- also appropriate for b . E, sprayed 26 May. L, sprayed 13-14 June and 4 July.

- treatments 0 Nil Q E S L e EL

I I I I I I I I I \ 10 20 30 40 50 60 70 80 90 100

Percentage infection with motley dwarf virus

Fig. I . Relation of yield of roots to percentage infection with CMD virus at harvest. Woburn, 1960. Correlation coefficient b = -0.056 k 0.0266.

Infected plants were counted when they were weighed. All plants showing mottling or reddening of the leaves were included although many were infected too late to affect yield, so the mean loss attributable to infection was less than in other experiments.

A multiple regression of total yield on percentage infection (b,) and plant number (b,) showed that yield was decreased with increasing proportion of plants infected (b, = 0~114+-0~040), but was not affected by plant number (6, = 0.001 +0-093) although plant number varied greatly from plot to plot. The yield of roots is plotted against percentage infection in Fig. I and a regression line ( b = 0.056 & 0.027) fitted. It suggests a loss of about 9 ton/acre for every additional 10% of infection.

EfJect of motley dwarf virus on yield of carrots 81

Woburn 1961 Main treatments experimental infection and spraying; var. James Scarlet Inter-

mediate. This experiment, designed to combine sowing date and insecticide treatment with

early and late experimental infection, was a ' Plaid-square ' arrangement (Yates, 1937) of thirty-two plots each split for experimentally infected and uncaged control plots. Four strips of four main plots were sown on 13 April and four on 16 May (early and late drilling). Each early-drilled strip had two half-plots infected on 23 May and two on 15 June, and each late-drilled strip had two half-plots infected on 15 June and two on 3 July (early and late infection).

Early-drilled plots were sprayed 15, 29 May and 19 June; late-drilled plots were sprayed 5,19 June and 7 July. The first was soon after the plants had germinated, when many were still cotyledons. The spray treatments were (SI) plots sprayed on all three dates, (S2) the second two dates, (S3) the last only, (S4) none.

The caged subplots were harvested on 31 August and I September, the remainder as whole plots on 28 September.

Experimentally infected subplots (Table 4). As in 1959 all plants became naturally infected, but experimental infection decreased the yields of early sprayed plots for both sowing dates. Probably the earliest spraying delayed natural spread of infection long enough for the experimental infection to take effect, but by the time of the later sprayings all plants had become infected (Table 4).

Dzjterently sprayed main plots (Table 5 ) . Spraying three times increased the yield of roots of early-sown carrots by I tonlacre and of late-sown carrots by 3 tons/acre,

Table 4. Woburn, 1961. EfJect of experimental infection on crop infested early with Cavariella aegopodiae. Split-plot comparisons and interaction with spraying treatments. Fresh weight, tonslame

Times sprayed S.E. Of S.E. O f

SI s2

Drilled" 13 April Control 7.04 6.51 Experimentally 4.84 5'43 infected

Drilledt 16 May Control 3'53 3.08

infected

Mean of infection dates

Experimentally 2.78 2.73

Mean of sowing dates

1st infection Control 5'33 4.62 Experimentally 4.23 3.02 infected

2nd infection Control 5'23 5'37 Experimentally 3'39 5.14 infected

S I "Sprayed 1 5 May 20 May 19 June S2 Sprayed - zoMay 19June S 3 Sprayed - - 19 June S 4 Unsprayed

6

S 3 S 4 difference Mean difference

5'45 4'30 o.,II 5'95 o.350 6.79 5.06 5'5 1

2'22 2.08 o.711 2'72 . o.350 2.67 2.09 2.5 I

3.67 3.02 o.711 4'14 o.350

4'05 3'37 o.711 4'49 o.350

4.47 2.80 3.63

4'94 4'34 4'45

tSprayed 5 June 19 June 7 July Sprayed - 19 June 7 July

7 July Sprayed - -

Unsprayed App. Bid. j3

82 MARION WATSON AND E. P. SERJEANT suggesting that aphids invaded more frequently and spread virus more often during May and early June than during June and early July. Spraying was more effective on the late-sown crops because alatae invaded less frequently, as in 1960. Both drillings became severely infested by aphids, but the late-drilled plants seemed to suffer more in the early stages of growth, and this may have contributed to the difference between early- and late-sprayed late-sown plots. However, it could not account for this and for greater loss of potential yield of early-sprayed early- sown carrots, which can have been caused only by virus. Had they not been virus- infected the early-sown carrots would probably have yielded several tons more than the late.

The yields of tops are discussed later.

Table 5 . Woburn, 1961. ESfect of spraying one, two or three times on crop early infested with Cavariella aegopodiae. Main plot comparisons tonslacre fresh weight

SI" s2 s3 s 4 S.E. Mean S.E.

Drilled 13 April Roots 7'' 6.9 6.8 6. I 0.60 6.7 0 3 0 Tops 2'0 2'0 2'0 2'0 0'12 1'9 0.23

Drilled 16 May Roots 6.6 5'9 46 3'4 0.60 5'2 0 3 0 Tops 3'6 3'7 2'9 2'3 0'12 3'1 0.23

* See Table 4.

Aphid invasion and infestation (Table 6). On 15 May, before spraying, IOO seedlings were taken from the first-sown carrots and examined in the laboratory. Twenty-one alate C. aegopodiae and 1 3 0 nymphs were found, but no adult apterae. On the same day winged adults and nymphs, found on willow trees by a lake near the experiment, were put on carrot seedlings but did not infect them. Some were cultured on young willow sprouts and others on healthy carrots; when tested later neither colony infected carrot but when fed on infected carrots both did. Similar results were obtained with aphids collected from willow on 23 May, and also some from weeping willow in Harpenden in 1962.

Aphids were counted on samples of plants taken on 23 May, 12 June and 5 July (Table 6). Half the plots were sampled on each occasion. The May-sown carrots were seedlings at the first June sampling and still very small at the second, so the numbers of aphids recorded represent severe infestation, but by then the infestation of some early-sown carrots was also severe and two unsprayed plots had deposits of aphid exuviae beneath the plants at the last time of sampling.

Spraying decreased numbers of apterae and older nymphs almost to zero on some plots but had much less effect on alatae, probably because these invaded frequently and moved from plot to plot. Spraying had little effect on virus-spread although it controlled apterae. This supports the hypothesis that when both alatae and apterae are numerous transmission of persistent viruses is usually by alatae (Broadbent & Tinsley, 1951; Watson & Healy, 1953).

Eflect of motley dwarf virus on yield of carrots 83

Woburn 1962 Table 7 (spraying treatments and Menazon seed-dressing ; var. James Scarlet

Intermediate). The experimental design was a plaid rectangle of thirty-two plots 14 ft. x 66 ft.,

using all combinations of early, intermediate and late spraying and no spray, on eight

Table 6. Woburn, 1961. Counts of Cavariella aegopodiae on plants taken from dijferently sprayed plots. Expressed as aphids1100 plants

23 May Winged Adults and nymphst

12 June Winged Adults Nymphs

Winged Adults Nymphs

5 July

Drilled 13 April ,

s I * s z s 3 s 4

23 53 I 1 2 53 299 1596 728 813

70 5 0 I I 0 60 0 0 520 1080

700 62 3190 4560

Drilled 16 May

4

5 0

21

8 146

s2

6 5

52

I 0

5 3'5

s 3 s4

5 3 22 4 204 83

* See Table 4. t Mainly nymphs.

Table 7. Woburn, 1962. Efjrect of Menaxon and spraying on crop free from Cavariella aegopodiae. Fresh weight, tonslacre

Treatments

Spraying*

Menazon seed-dressing Early Intermediate Late & S.E. of & & c--- S.E. O f

Absent* Presentt difference Absent Present Absent Present Absent Present difference

Roots 24.9 25.5 0.41 25.0 25.4 24.9 25.5 25.2 25.2 0.29 Tops 12.1 12'0 0'43 11'9 12'2 12'0 12'1 11.9 12'2 0.3 I

* Spraying: early, 20 May and 8 June; intermediate, 23 June and 4 July; late, 16 and 28 July. t Means of all other treatments.

strips of four plots. Four strips were sown with untreated and four with Menazon- treated seed ; the effect of Menazon was therefore less accurately determined than that of the sprays. Each spray treatment consisted of spraying on two dates: early (E) 29 May and 8 June; intermediate (I) 22 June and 28 June; late (L) 8 July and 18 July. A headland was left beside each strip to accommodate a tractor for spraying.

Menazon seed treatment and spraying in June seemed to increase yield by about half a tonlacre, on a mean of nearly 25 tons/acre (Table 7). The differences were consistent but too small for statistical significance. The Menazon result agrees with that obtained in the cool glasshouse experiment, Table 9.

6 - 2

84 MARION WATSON AND E. P. SERJEANT The reason for the poor response to insecticide and the greatly increased yield

compared with other years was simply that the crop remained aphid-free and conse- quently free from CMDV almost for the whole season. Fewer than 0.01 yo of plants were infected on 3 September.

Rothamsted 1962 Table 8 (main treatment, experimental infection; var. Suttons Early Giant). The design was four randomized blocks of six plots, half sown with Menazon-

dressed and half with undressed seed. Menazon had no effect on yield and is not mentioned in the results. The crop was sown on 13 April with a hand drill and germi- nated around 5-7 May, so was first infected when seedlings were about a month old. The whole area was irrigated with about 8 in. water on 12 and 28 June. Main plots were split into four subplots one of which (A) was experimentally infected on one of three dates: 5 and 22 June and 1 1 July. The other three were given one of three treatments: (B) caged with about the same number of virus-free aphids as were used to infect, (C) caged but no aphids, (D) uncaged. Control B was omitted at the two later infection times because there were too few virus-free aphids. As a precaution against natural infection the cages were put in place I week before the infection date.

Table 8. Rothamsted, 1962. Effect of experimental infection of carrots free from Cavariella aegopodiae. Tonslame fresh weight

( I ) Mean of three infection-dates. Yield of infected plots compared with caged and uncaged controls.

Tons/acre fresh weight

Experi- Caged Caged mentally healthy no Healthy S.E. of infected aphids aphids Uncaged S.E. mean difference

Roots 15.60 23'45' 23'42 23.98 0.384 23.62 0.463 Tops 12.59 18.17' 18.61 17-94 0262 18-24 0 3 1 3

(2) Effect of date of infection. Tons/acre fresh weight

A f

Dates of infection

5 June 22 June 1 1 July S.E. Mean S.E. Roots

Caged infected 13.83 16.24 16.71 0.665 15.60 0.384 Caged no aphids" 25.20 2490 20'21 0665 2 3 . 4 0.384 Uncaged 24'37 24'09 23'49 0.665 23.98 0.384

Tops Caged infected 11.52 12.30 13'94 0.454 12.59 0.262 Caged no aphids' 18.40 18.82 17'95 0454 18.39 0262 U n c a g e d 18.06 18.00 17.96 0.454 17'94 0.262

* Healthy aphids 5 June treatment only.

This proved unnecessary, as the exposed plots remained healthy until late in the season. As a control measure, not a treatment, early and late infected plots were sprayed on 13 June, early and intermediate infected ones on 29 June, and all on 23 July. The areas covered by cages and their controls were harvested on 29 and

Effect of motley dwavf virus on yield of carvots 85 30 August. On the average of all infection-dates (Table 8, I ) caging with infective aphids (treatment A) reduced the fresh weight of roots by 8 tons/acre. Yields were the same for plots caged on 5 June with healthy aphids (treatment B), caged without aphids (C) and uncaged (D). Plots infected on 5 June (Table 8, 2) yielded about 13 tons/acre, on 22 June 17 tons/acre, and on I I July 16 tons/acre. These correspond to losses of 44, 32 and 24% of the mean yields of the controls which were 25, 24'5 and 22 tonslacre, respectively. The decline of healthy yields with later infection seems to have been an effect of caging. Possibly shading affected plants when they started to compete for light in July but not when they were small. If proportional losses of yield are estimated from treatments A and C, they are 45, 35 and 17 yo respectively, for the three infection dates.

The loss caused by infection was less than might have been expected from results in 1959 and 1961. In those years seedlings were probably infected less than a month after germination, and probably by a more virulent virus than the one used experi- mentally. Loss of virulence of CMDV isolates in the glasshouse will be discussed in a later paper. The variety was different from the one used previously and this may also have affected loss.

Rothamsted I 962 Cool glasshouse experiment (Menazon and experimental infection on carrot

varieties, Table 9, Fig. 2).

Table 9. Rothamsted, 1962. Cool glasshouse experiment. Effect of Menazon seed treatment on two varieties. Total dry matter g./row

Early Giant James Scarlet Intermediate Times of 7- & sampling (-1 M (-) M S.E.

7 March8 2.8 1.09 2.41 1.30 0.415 2 May 6.27 5.84 10.64 10.16 I '03 28 May 27.80 30.01 32.16 35'62 3.16

* Time of singling.

Carrots, variety Early Giant and James Intermediate, were sown in yard-square wooden boxes on 30 January. Each of four boxes had six rows of carrots ( I g. seed/ row); three rows were sown with untreated seed and three with seed dressed with Menazon at 4 % by weight. Half-lengths of the rows were covered with a single cover during infection on 22 March for the Early Giant and 4 April for the James Inter- mediate. The centre rows of half-boxes were harvested on 2 May, and the remaining rows on 28 May. At each harvest the plants were weighed fresh and after oven drying. The dry weights in Table 9 show that Menazon affected both varieties alike, and infection seemed to do so, but the different dates of infection prevent proper compari- son. On 7 March when samples were taken there was a large decrease of yield caused by Menazon. There was still a loss on 2 May, when the tap roots were still small, but by 28 May when they were growing much faster than the tops the loss had been more than made up. Infected roots harvested on 28 May weighed only half as much as healthy ones. Between z and 28 May (Fig. 2) roots of uninfected plants increased

86 MARION WATSON AND E. P. SERJEANT in yield much more than the tops, but those of infected plants increased by about the same amounts as the tops. Independently of size, infected tops seemed less efficient at determining root increase than healthy ones.

For its size and the amount of labour involved this little experiment gave quite good results and the method looks promising. It showed that the initial set-back to seedlings caused by Menazon was not likely to affect yield seriously and that Menazon did not prevent viruliferous aphids from infecting plants, which was confirmed by the results of the field experiments.

2 8 May

Date of harvesting

Fig. 2. Yields of tops and roots of healthy and infected young carrots grown in boxes in a cool glasshouse. The roots of healthy plants increased greatly in 26 days but those of infected plants did not. L.S.D. Least Significant Difference; t (5 %) = 2.45.

E F F E C T O F I N F E C T I O N O N R A T I O O F TOPS T O R O O T S

The change in the weight of tops relative to that of roots, as assimilate increasingly moves to the roots, is seen clearly in the two harvests from the cool glasshouse experi- ment (Table 10). I n early May the tops were 5-8 times heavier than the roots and 34 weeks later only 1-2 times, depending on whether or not they were infected. Similarly in 1961 top :root ratios were larger for late than early-sown carrots harvested at the same time.

Ratios of tops to roots were larger in early-infected than in uninfected plants. The differences were small but usually significant, and probably indicate that the leaves of infected plants are not only smaller than those of healthy ones, but less efficient assimilatory organs, because infected plants seem to need a larger weight of tops than healthy ones to give an equivalent increase in root weight.

EfJect of motley dwarf virus on yield of carrots 87

Table 10. Effect of experimental infection in 1961 and 1962, on ratio of tops to roots expressed on fresh-weight basis

Top : Root S.E.

Experimentally V----i

Control infected Horizontal Vertical

Woburn, 1961 Var. James Inter. Sown 13 April Infected 23 May 048 0.69 0.13 0.08

- -

- - Infected ~j June 0.46 0.57 Sown 16 May Infected I j June 1.20 I '42

Infected 4 July 1.19 1.31 - -

Rothamsted, 1962 (cool glasshouse) Var. James Scarlet Sample 22 May 5.2 5.6 0'45 0'45

Harvest 28 May 1.0 2'0 0.14 0.14

Harvest 28 May 1.6 2'2 0.14 0.14 Var. Early Giant Sample 2 May 7.5 8.6 0.45 0.45

Rothamsted, 1962 Var. Early Giant Infected 5 June 073 0.88 0.28 0.30

Infected 22 June 0.76 0.76 Infected 1 1 July 0.82 0.84

- - - -

Table I I . Ejfect of experimental infection in 1961 and 1962 on dry-matter

D I (sown 13 April) D z (sown 16 May) Dz-DI

Sample z May Harvest 28 May

Infected 5 JuneX Infected 22 June Infected I I July

content percentage of fresh weight

Dry matter "/o (roots)

A , Dry matter y o Experi- (tops) mentally r 7

Control infected S.E. Control Infected* S.E.

Woburn, 1961 13.0 13'3 0'12 17'7 17.6 0.30 13'9 14'3 0'12 18.5 18.8 030 0.9 I '0 018 08 1'2 0.37

Rothamsted, 1962t (cool glasshouse) 11.6 13.0 0.19 10.6 12.8 0.09 12'0 I 1.7 0.09 12.9 '3'5 0'1 j

Lamina Petiole &-

Control Infected Control Infected

Rothamsted, 1962

10.8 11.3 0'12 15.2 16.3 10.9 10.9 10.7 I 1.4 0.12 15.8 15.8 13'3 13'9 10'5 11.5 0' I 2 15'4 16.2 I 2 7 13'7

S.E. 0.27 S.E. 0'37

* Caged plots only. t Mean of two varieties.

88 MARION WATSON AND E. P. SERJEANT

EFFECT O F I N F E C T I O N O N D R Y M A T T E R P E R C E N T O F F R E S H

W E I G H T A N D W A T E R C O N T E N T

Sugar beet infected when young with beet yellows virus have more dry matter and less water percent of fresh weight than healthy controls (Watson & Watson, 1951). Our results have so far been given in tons/acre fresh weight but samples for dry- matter estimation were taken in 1961 and 1962, to measure the effect of virus and other treatments on total dry-matter production, which is not necessarily reflected in the fresh-weight yields.

Infection increased dry matter and decreased water content, especially when the time between infection and sampling was short. In the cool glasshouse experiment plants (mean of two varieties) sampled 4-5 weeks after infection had 1.4 % more dry matter than healthy plants, but 4 weeks later the difference had disappeared. Similarly, in 1961 late-sown infected carrots, and in 1962 late-infected carrots had more increase of dry matter than those from early-sown or early-infected plots. This suggests that there is a time when water content is optimally affected by infection, after which it increases again.

The water content of tops was less than that of the roots. When laminae and petioles were assessed separately in 1962, this was found to be mainly an effect of low water content of laminae; the petioles had about the same as the roots. The reduction caused by infection was similar in all parts of the plants.

T H E R E L A T I O N B E T W E E N A P H I D N U M B E R S A N D T H E E F F E C T S O F T R E A T M E N T S

Fig. 3 compares the numbers of C. aegopodiae, caught weekly on sticky traps at Woburn and Rothamsted from 1959 to 1962, with yields of differently treated plots on the experiments. There is obvious negative relationship between aphid numbers and yield, and there are differences in the effects of treatments. Spraying affected yield appreciably only in 1960 when aphid infestation was moderate. In 1959 and 1961 when they were numerous, and 1962 when they were few, spraying had in- significant effects.

In 1959 and 1961 experimental infection had little effect on yield because it was swamped by natural infection. In 1962 yield appeared greatly affected because the crop was otherwise healthy. The year 1962 was not entirely free from virus spread. At Rothamsted carrots of four varieties, sown on 2 June, had up to 70% of plants infected when inspected on 3 October; similar plots sown on 22 June had about 35 yo and those sown on 17 July about 8 %. Apparently carrots germinating by the middle of July became infected but those germinating later did not, so most of the spread must have been in July as the trap-catch of aphids suggests.

EFFECT O F R A I N F A L L O N A P H I D N U M B E R S

The points joined by solid lines on Fig. 3 represent weekly rainfall in inches. There seems to be some indication that aphid-numbers are negatively correlated with rainfall, and this was examined further. Means for each week and year were fitted to the

Effect of motley dwarf virus on yield of carrots 89 logarithmically transformed counts and to the rainfall figures for the years 1959-61, and a regression analysis made on the residuals from these means. This had the effect of eliminating regular annual fluctuations in aphids and rainfall, The points represent- ing departures of aphid and rainfall figures from their averages are plotted in Fig. 4, The regression line, which has a correlation coefficient of b = - 0.248 0.1 10,

Woburn. 1959 Aphids per sq f t sticky trap

-Rainfall (in /week)

Yield of sprayed carrots

20

10

0 h

Woburn. 1960

s .L,

Woburn, 1961

-

- 2 -- 200 Woburn, 1962

5 1 0 20 30 9 19 29 9 19 29 8 18 28 May June July August

Fig. 3. Diagram showing the relation between numbers of C. aegopodiae caught weekly on sticky traps during May, June and July, and yields of differently treated carrots in experiments at Wobum and Rothamsted. The solid lines join points representing weekly rainfall in inches.

90 MARION WATSON AND E. P. SERJEANT suggests that aphid increase is halved by each additional inch of rainfall above the average. In 1960 3 weeks in June had additional rain, for June in the other 2 years was dry. This may explain the lack of increase of aphid population after the early and fairly large migration in May, and the consequent slow spread of virus. Data for 1962 were not used because there were too few aphids after the cold winter.

Y c. 3 0

a Y E c,

I 0 0

I -0.70 Residual weekly rainfall (in.).

Fig. 4. Relation between residual variation in log, weekly trap-count and weekly rainfall in inches for the years 1959-61. Correlation coefficient b = -0.248 fo.110; t (j %) = 2.26.

(I) Logs of mean weekly trap-counts (2) Means of logs of weekly trap-counts.

Fig. 5 . Relation between yields of unsprayed plots of April-sown carrots at Woburn and Rothamsted 1959-62 and numbers of C. aegopodiae caught on sticky traps. (I) Mean of logs of weekly trap-counts. Correlation coefficient b = - 26-40 f 8.00. (2) Log of average weekly trap-counts. Correlation coefficient b = - 13-34 f 2.31.

Effect of motley dwarf virus on yield of carrots

R E L A T I O N B E T W E E N Y I E L D S A N D A N N U A L V A R I A T I O N

I N A P H I D N U M B E R S

Fig. 5 shows the relation between average numbers of C. aegopodiae caught weekly on sticky traps each year and the yields of unsprayed plots on the experiments. The two diagrams show the aphid counts differently transformed. In ( I ) yields are plotted against the logs of the mean weekly trap-counts, and in (2) the same yields are plotted against the means of log. ( N + IO), where N was the number of aphids trapped each week. Log. ( N + a constant) is commonly used for assessing insect-population data (Williams, 1937) as it gives less weight to occasional large numbers compared with frequent smaller ones. However, the first transformation, using the log. of the means of the aphid numbers, more successfully accounted for variation in yield, perhaps because large concentrations of aphids, particularly if they are early, spread more virus than the same numbers distributed over several weeks. It is probably the more appropriate for relating aphid numbers to virus spread. A large proportion of the variance in yield is accounted for by both regression coefficients, the first (b = - 13.3 & 2.31) for 89 % and the second (b = - 26.4 ? 8.00) for 71 %.

D I S C U S S I O N

These results show that motley dwarf virus was the main cause of the small yields of carrots in 1959 and 1961 which were dry years after mild winters when infestation by C. aegopodiae was early and severe. The losses caused by the virus have been confused with those of direct aphid-damage, drought, carrot fly (PsyZla rosae) and eelworm. The different effects are not easy to distinguish because the causes may be interdependent. Virus is correlated with aphids because it is aphid-transmitted, drought with aphids because it favours their multiplication, and so on.

At Rothamsted in 1962 the effect of virus was measured in the absence of aphid infestation and, as the crop was irrigated, of drought; there was no PsyZZa msae damage. Plots inoculated by aphids in early June lost I I tons of rootslacre, about half their yield; those infected in late June lost 8 tons and those in early July about 5 tons/acre. In 1960, also in the absence of aphid damage and drought, it can be calculated that July infection would have caused a loss of about 40% of yield, had all the carrots become infected.

In 1959 and 1961 carrots at Woburn became infected in May, earlier than the experi- mentally infected 1962 plots which lost half their yield, and probably with a more virulent strain of motley dwarf virus. At a reasonable estimate the loss caused by virus in 1959 and 1961 was about 60%. The crops then yielded about 75 yo less than the healthy unsprayed plots in 1962, so the loss that remains to be accounted for by other causes is less than half that attributable to virus alone.

The amount of aphid-infestation, date of sowing, date of infection and other factors determine the rate of spread of virus and the success of insecticides in con- trolling it. In 1960 spraying was effective because there were comparatively few aphids, but in 1961 earlier and more frequent spraying had little effect because large numbers of winged aphids invaded and infected plants before insecticide could kill them. To be effective, sprays must be applied as soon as, or preferably before, aphids

92 MARION WATSON AND E. P. SERJEANT appear on the crop, and continued with a frequency related to the development of the infestation, which will vary not only with the numbers of aphids invading, but with weather and other factors during their colonization of the plants. Prediction based on the relation of aphid migration and multiplication to weather and other phenological phenomena is needed for safe and economical use of insecticides against spread of virus. Also it is necessary to understand fully what is successful control of virus. Increasing yield from 3 to 6 tons/acre, which was one of the effects of spraying in 1961 when aphids were numerous, may be 100% increase of yield but did not prevent the spread of virus. Preventing it would have increased yield to about 20 tons/acre. That spraying can prevent multiplication of apterae on the plants without causing this increase of yield is added evidence that aphids themselves are not responsible for most of the damage.

Possible sources of motley dwarf virus are overwintering carrot crops and wild hosts. A virus (red-leaf; Watson, 1961), associated with the transmission of motley dwarf, may be seed-transmitted although the evidence is uncertain. It causes red discoloration of the foliage but probably does little damage. It is occasionally found alone in carrots in nature.

There is no evidence that the motley dwarf complex is seed-transmitted and it has not been carried from willow trees by aphids hatching from eggs (see p. 82) . We have isolated it from perennial and biennial umbelliferous weeds, e.g. Anthriscus, Heracleum and Daucus spp. After mild winters it may be carried to carrots in April or May by aphids that overwintered on these or on carrot crops. The epidemiology of Cavariella spp. and motley dwarf virus would obviously repay further study.

The physiological causes of loss of yield have only been touched on in the present work. The effects of motley dwarf virus on water content and on distribution of weight among different parts of infected and healthy carrots suggest that the virus resembles beet yellows (Watson & Watson, 1951) and probably barley yellow dwarf (Orlob & Amy, 1961), which interfere with carbohydrate metabolism and with distribution of sugars. Probably the virus causes most loss a few weeks after infection when decrease in water content and increase of top: root ratio are greatest, but the implied recovery is likely to be very limited.

We gratefully acknowledge the help of Mrs Elizabeth Lennon and Miss Sheila Bunyan during the experiments, and of M r J. Dunwoody in the statistical analyses of the results.

R E F E R E N C E S

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BURT, P. E., BROADBENT, L. & HEATHCOTE, G. D. (1960). The use of soil insecticides to control potato aphids and virus diseases. Ann. appl. Biol. 48, 580.

ORLOB, G. B. & ARNY, D. C. (1961). Some metabolic changes accompanying infection by barley yellow dwarf virus. Phytopathology, 51, 768.

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WATSON, M. A. & WATSON, D. J. (1951). The effect of infection with beet yellows and beet mosaic viruses on the carbohydrate content of sugar beet leaves and on translocation. Ann. appl. Biol. 38, 276.

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