TURNIP-MOSAIC.

New Zealand Journal of Agriculture, Volume 53, Issue 6, 21 December 1936, Page 321

TURNIP-MOSAIC.

A VIRUS DISEASE OF CRUCIFERS.

E. E. Chamberlain,

Plant Diseases Division, Plant Research Bureau, Palmerston North.

Turnip-mosaic was first recognized in New Zealand in 1932, when it was observed on rape-plants being grown for seeding purposes at the Plant Research Station Area, Palmerston North. Since then it has become a serious disease of swedes, rape, and turnips at the Station Area, and has been found also in a number, of districts throughout the Dominion. The first accounts of turnip-mosaic are from North America, where it was recorded simultaneously by Gardner and Kendrick (1921) and Schultz (1921). It has also been recorded from Denmark (Gram and Rostrup, 1924), Germany (Pape, 1935), England (Smith, 1935), and Australia (Samuel, 1931). Symptoms. On swedes the characteristic symptom of the disease is a mottling and crinkling of the leaves. The mottling is diffuse, the difference between the light and the dark areas being slight (Fig. 1). Less commonly, it occurs as dark-green blistered areas. Symptoms appear only on those leaves which develop after infection has taken place. Infected plants soon become stunted in appearance (Fig. 2), and the “bulbs” (the bulbous portion of the roots), although, they continue to grow, are much smaller than those of healthy plants. ' The leaves of infected plants tend to die prematurely (Fig. 3), and under certain conditions the “ bulbs ” become susceptible to soft-rot. In stecklings* during the winter and early spring the symptoms appear as a pronounced mottling of the leaves with little or no crinkling. Infected plants remain stunted and produce a light crop of seed. In the glasshouse mottling and crinkling are preceded by a clearing .of the veins on the developing leaves. This vein clearing, which is a loss of the green colour along the veins, is rarely seen under field conditions. * Plants of autumn sowing used for seed-production.

On turnips, symptoms, although much the same as for swedes, tend to be more pronounced. Thus leaf-mottling is usually more clearly defined (Fig. 4) and the distortion of leaves and the stunting of plants, greater (Fig. 5). Infected plants are also much more susceptible to soft-rot.

On rape the disease produces a definite mottling of the leaves, but leaf distortion and stunting is not as pronounced as with swedes or turnips. There does not appear to be any premature death of leaves, and, after “ feeding-off,” the plants produce new growth which is, however, still mottled and stunted.

Incidence. The disease was first observed in 1932 on a few rape plants at the' Plant Research Station. In the following year it was found to be common on swedes, . rape, and turnips at this area, and was also prevalent on the two former hosts in varietal trial plots at Marton. At the Station Area the amount of infection has increased each year until in the past season practically all swedes and rape were infected. ... An inspection of several rape crops in the Marlborough district made during the past season showed a small percentage of mosaic

to be present in each. In a recent survey of swede crops a small amount of turnip-mosaic was found in the Otago and Rangitikei districts.

The disease has also occurred during the past three seasons in crops (approximately 2 acres each season) of swede stecklings grown at Colyton, in the Manawa district. On this area, in the spring of 1935, from 2 per cent, to 3 per cent, of the plants showed infection early in the season. ' ■ ’■ Economic Importance. The only country in which turnip-mosaic has been reported to be of . economic significance is Germany (Pape, 1935). In that country Pape stated that infection on swedes in Schleswig-Holstein was found to range from 1 per cent, to 90 per cent., and that in two test plots the reduction in yield amounted to 63 per cent, and 57 per cent, respectively. • At the Plant Research Station Area at Palmerston North, where the crops are grown for experimental purposes, turnip-mosaic is a serious disease. It has been particularly severe on turnips, for not only have they become infected with mosaic but the “bulbs” have been subsequently attacked and destroyed by soft-rot. During the past season the effect 'on swedes was almost as severe, for in several plots the secondary attack by soft-rot destroyed a large percentage of “ bulbs.” In other plots soft-rot was not responsible for such heavy losses, but the effect of the mosaic was sufficient to render valueless experiments on the control of brown-heart. The effect of the disease on rape was not so pronounced, being limited to stunting of the plants. Attempts to carry out yield trials with healthy and infected swedes and turnips failed because control plants. could not be kept free from mosaic throughout the season. Trials with rape were more successful, since it was found possible to keep the control plants reasonably free from the disease until the time of cutting and weighing.* . During the 1934-35 season trials were carried out in which 100 each of healthy and mosaic-infected rape-plants were grown in ten replications of ten plants per plot. The weights of green leaf showed for healthy plants an average of 0-57 lb. per plant and for mosaic-infected' plants 0-42 lb. This represents a reduction in yield of 25-4 per cent., caused by the disease. In the 1935-36 season further trials were laid down, but despite frequent spraying to control insect vectors! a certain amount, of spread of mosaic occurred. All infected plants were, therefore, harvested and weighed separately. Eighty-eight healthy plants gave an average yield per plant of 1-65 lb., while 112 mosaic-infected plants gave an average yield, of 1-22 lb. In this case the reduction in yield was 26 ,T per cent. During both seasons the disease spread so rapidly after cutting that the trials were abandoned. * The crop was harvested and weighed when the plants had reached maturity—i.e., at a stage of growth when they were fit for “ feeding-off.” t The term “ vector ” is applied to any agent, usually an insect, which transmits a virus from one plant to another.

Identity of the Disease.

The disease as it occurs in New Zealand has every appearance of a virus of the mosaic type. A study of its symptoms, methods of transmission, and . host range has demonstrated not only its virus nature, but its similarity to turnip-mosaic described by overseas workers. Methods of Transmission. Artificial Transmission. — That infection of healthy plants may be brought about by inoculating them with juice expressed from mosaicinfected foliage has already been shown by workers in other countries (Gardner and Kendrick, 1921 ; Schultz, 1921 ; Gram, 1925 ; Clayton, 1930; Pape, 1935). Early in 1934 experiments were commenced in New Zealand to determine whether the disease could be transmitted in this manner. Experimental Method. The inoculum used was juice extracted from leaves of mosaic-infected plants obtained from the field. All healthy plants were raised in steam-disinfected soil in an insect-free glasshouse. Inoculations were carried out usually on plants in the four- to six-leaf stage by rubbing the leaves with muslin moistened with the above juice extract. Results. results of the artificial inoculations are summarized in Table I. . s

Control plants equal in number to those inoculated all remained healthy.

From these results it is evident that turnip-mosaic is readily transmitted from diseased to healthy plants by juice inoculations. Insect Transmission. — This was first demonstrated by Schultz (1921), who showed that the aphis Myzus persicae was a vector. Clayton (1930) found that the disease was readily transmitted by the aphis Brevicoryne brassicae. Working with what was probably the same disease Hoggan and Johnson (1935) showed that both these insects were vectors. Pape (1935) claimed that the disease was also transmitted by the capsid-bug Lygus pratensis. In New Zealand the sucking insects most commonly found on cruciferous crops are the aphides B. brassicae and M. persicae*, so transmission experiments were confined to these. Experimental Method.— healthy, plants were raised in. steamdisinfected soil in an insect-free glasshouse. The infected plants used as a source of inoculum were either transplants from the field or plants * The identification of these aphides was kindly made by Mr. W. Cottier, Assistant Entomologist at this Bureau.

artificially infected in the glasshouse. The aphides, after being allowed to feed on mosaic-infected plants for at least seven days, were transferred to healthy plants, which were then enclosed in muslin cages in the manner described in a previous article (Chamberlain, 1935). After the aphides had been allowed to feed on the plants for seven to ten days the cages were removed and the plants fumigated. Control plants were treated in a similar manner, except that no aphides were transferred to them. Results. — The results of the experiments on insect transmission are given in Table 11.

Control plants equal in number to those inoculated all remained healthy.

These results show that turnip-mosaic is readily transmitted by Toth B. brassicae and M. persicae. Seed Transmission. — Field trials carried out by Clayton (193,9) with seed from mosaic-infected swede-plants indicated that the disease was not seed-carried. ' ■ . • ■ : The following results, although the experiment was not sufficiently large for conclusive proof, support those of Clayton’s, for of 432 plants grown from seed of infected plants none developed mosaic. Experimental Method— Mosaic-infected swede stecklings were collected from the field and seeded in the glasshouse. Seed from these plants was sown in ; steam-disinfected soil and the seedlings pricked ■out into 4 in. pots. These were kept in an insect-free glasshouse until they had developed to the six-to-eight-leaf stage. - - Host Range. The following cruciferous plants have been recorded as hosts of turnip-mosaic: Turnips (Brassica rap a}, Chinese cabbage [Pe-tsai . (B. cornua)], Pot-herb mustard (B. japonica) — (Schultz, 1921) ; swedes (B. napobrassica), charlock (Sinapis arvensis}, Raphanus sp. —(Gram, 1925) ; Chinese cabbage [Pak-choi (B. napus var. chinensis)], white mustard (S. alba), black mustard (Bf nigra), rape (B. napus var. typica), Brussel’s sprouts (B. oleracea var. bullata), cauliflower (B. oleracea. var. — (Clayton, 1930) ; kale (B. oleracea var. acephala) — (Smith, 1935)In New Zealand the disease has been found only on swedes, turnips, and rape. In order to determine whether other cruciferous plants were

susceptible, an attempt was made in one experiment to transmit turnipmosaic, by means of artificial inoculations, to the following vegetables : cabbage (B. oleracea var. capitata), cauliflower, Brussels’ sprouts, broccoli (B. oleracia var. botrytis), and radish (Raphanus sativusp. No infection was secured. ‘ In a second experiment transmission to the same five crucifers was attempted by means of the aphis M.- persicae. The method used was the same as that already described for insect transmission of the disease between swedes and turnips. On 29th January, 1936, approximately twelve M. persicae were transferred from a mosaic-infected swede-plant to each of eight plants of the five above-mentioned species.

Results.— The numbers of plants which became infected in each species werecabbage, I ; cauliflower, 2; ■ Brussels’ sprouts, 8; broccoli, 7 ; and radish, o. The ten control plants remained healthy. These results show that cabbage, ' cauliflower, Brussels’ sprouts,, and broccoli are susceptible to turnip-mosaic. The symptoms which appeared on these plants were caused by turnip-mosaic, since by means of aphides the disease was transmitted back to swedes. (See Table II.) On cabbage, cauliflower, and Brussels’ sprouts turnip-mosaic produced a faint mottling in which the darker-green areas lay adjacent to the veins. On broccoli numerous pale-green areas on the leaves caused a distinct but not pronounced mottling. Under the conditionsof the experiments the disease did not adversely affect the growth of any of these four hosts ; and as the plants became older the symptoms were difficult to discern.

Hoggan and Johnson (1935) found that a mosaic of crucifers in North America, when transmitted to tobacco (Nicotiana Jabacum), caused conspicuous brown necrotic lesions confined to the points of infection. Similar results have been secured with turnip-mosaic -in New Zealand.

Varietal Resistance. Working with turnip-mosaic in Germany Pape (1935) recorded some relatively resistant varieties and others which , were severely attacked. Amongst the latter was the variety “ Yellow Wilhelmsburger.” ' • . During the past season a trial of seventy lines of different strains and varieties of swedes, including most ;of the commonly grown varieties, was undertaken to ascertain their varietal resistance to the

fungous disease dry-rot (Phoma lingam}. All plants were grown in one block, each line of seed being sown in a row 1 chain long. Plants throughout the block became infected with mosaic, thereby enabling observations on varietal resistance to be made. Counts of infected and healthy plants taken in mid-March, 1936, showed that some ten lines exhibited a degree of resistance (Table ITT).

Outstanding amongst the ten varieties showing resistance was Sutton's Sensation. Although this variety is highly resistant it is not immune, as mosaic symptoms appeared on a few plants later in the season. The seven strains of Wilhelmsburger grown in the trial all showed resistance to infection. Not only did this variety show resistance to turnip-mosaic, but it was also much less susceptible to a secondary attack by soft-rot. , ’ ' . . Control Measures. ■ ’ In New Zealand turnip-mosaic has become of economic importance only in areas of intensive cultivation. From this it would appear that the disease is most likely to become troublesome in seed-producing areas. The following recommendations are made for the control of turnipmosaic in crops grown for seed : (i) Dipping the leaves of plants, at the time of transplanting, in a solution of nicotine or nicotine sulphate to kill insect-vectors (concentration : Nicotine, i part to 2,000 parts water; nicotine sulphate, 1 part plus 4 parts soft-soap to 800 parts water). (2) Regular inspection of the crop and rogueing of all infected plants. (3) The avoidance ‘of other cruciferous . crops in the vicinity. (4) Keeping the area as free as possible from volunteer seedlings. (5) When mosaic has appeared, spraying the plants with a nicotine spray (concentration as above) to destroy aphides..

Summary. , (1) Turnip-mosaic, a virus disease of swedes, turnips, and rape, is recorded as occurring in a number of districts throughout New Zealand. (2) The symptoms of the disease are a mosiac mottling and crinkling of the leaves and a stunting of the plants. (3) Infected turnips and, under certain conditions, swedes become susceptible to a secondary attack by bacterial soft-rot. (4) Although not reported to be of economic importance in field crops, turnip-mosaic has been a serious problem on experimental areas at Palmerston North and Marton, and has been troublesome on a seed-producing area at Colyton. ■ : ' (5) Yield trials with rape have shown that it causes on this host approximately 25 per cent, reduction in yield. . (6) The disease has been transmitted artificially by the leaf-rubbing method. (7) The aphides Brevicoryne brassicas and Myzus persicae have been shown to be vectors. (8) No seed transmission was secured in a small trial involving 432 plants grown from seed of infected swedes. (9) The disease has been transmitted to cabbage, cauliflower, broccoli, and Brussel’s sprouts. It causes only mild symptoms on these hosts. (10) It may also be transmitted to. tobacco, where it produces brown necrotic lesions confined to the points of infection. (n) Field observations indicate that. Sensation swede (a Sutton’s selection of White-fleshed Purple Top) is highly resistant to . mosaic infection, while Wilhelmsburger, Webb’s No. 2 strain of Imperial, and Sharpe’s Ai are moderately so. (12) Recommendations are made for the control of turnip-mosaic .in seed crops. Literature cited. j Clayton, E. E. (1930) : Jour. Agric. Research, Vol. 40, pp. 263-270. Gardner, M. W., and Kendrick, J. B. (1921) : Jour. Agric. Research, Vol. 22, pp. 123-124. Gram, E. (1925) : Kohenhavn, Vol. 8, pp. 41-42. Gram, E., and Rostrup (Sofie) (1924) : Tidsskr. for Planteavl, Vol. 30, pp. 361-414. Hadfield, J. W., and Calder, R. A. (i 935 • N.Z. Jour, of Agric., Vol. - 51, . ' pp. 201—208. ; - < ' ? J? . Hoggan, Isme A., and Johnson, J. (1935) ; Phytopath, Vol. 25, pp. 640-644. Pape, H. (1935) : Dtsch. Landw. Pr., Vol. 62, pp... 319-320. .• Samuel, G. (1931) : Jour. Dept. Agric., S. Australia, Vol. 34, p. 746. Schultz, E. S. (1921) : Jour. Agric. Research, .Vol. 22, pp. 173-178.. Smith, K. M. (1935) : Ann. Appl. Biol., Vol. 22, pp. 239-242.

A development that may be expected to follow naturally from the unified thought and co-operative effort that are considered desirable in the work of the Department is a trend towards standardized practice, which, reflecting the best knowledge available, should lead to greater general efficiency, begetting both increased production and improved quality in our farm products. Annual Report, Minister of Agriculture. . ...

* Sutton’s Sensation is merely an improved selection of Vilmorin White-fleshed Purple Top (Hadfield and Calder, 1935)- The original Vilmorin strain was not included in the seventy lines under trial.

Date of Inoculation. Source of Inoculum. Species inoculated. , Number of Plants inoculated. Number of Plants infected. . 3/1/34 Swede . . Turnip 4 4 ; 3/1/34 Rape ,, 4 ' 4 -2 ; 2 3/1/34 Swede Swede . . . 4 3 8/IO/34 ■ Turnip ... 12 . 8 8/10/34 Swede . ... 12 5 21/11/35 ,, 8 2

Table I.—Artificial Transmission of Turnip-mosaic.

Date of Inoculation. Source of Inoculum. Species inoculated. Aphis Species. Number of Insects transferred. Number of Plants inoculated. Number of Plants infected. 13/10/33 •■ Swede Swede .'. B. brassicas . 36 IO . 7 IO/1/34 .. ,, * • • • ,, 20 20 5 5 3 . 3 17/7/36 •• ,, . • ,, • • • • 11 • ■. 20 8 8 10/1/34 .. Turnip. . I. • • >» ' • • Turnip. . 20 20 5 5 2 2 .' 8/3/35 •• >> * * Swede . . Swede . . M. pcrsicae. . M. peVsicae. . IO IO 8 8 8 8 12/5/36 .. ,, . • • . ,, • • •' 15 10 a • • 5 15 1° 5 12/5/36 ... Brussel's sprouts 20 10 ” • • 7 ,, ■ 20 IO'7 .12/5/36 . . Cabbage . . Ji • • 15 10 4 12/5/36 ... Broccoli . . , t • • ,, • • 12 10 2 J, 12 10 2 . 12/5/36 • . Cauliflower 20 To.. 6

Table II.—lnsect Transmission of Turnip-mosaic.

Row No. Variety. . Strain. .- Number of Plants. Percentage of Mosaic. 9 Wilhelmsburger Otofte. . Plant Research Station, 74 44-6 ■ Selection No. 1 .4 44'6 IO Plant Research Station, 87 • ■ 58-6 Selection No. 2 87 ■ 58-6 ii Selection No: 2. Canadian Strain . . 76 68-4 < 12 Johnson’s Benefactor . 77 -■ 57-i Strain 77 57’1 13 Strain >• Plant Research Station, Plant Research Station, 78 .73-1 Selection No. 3 78 73’i 26 . Imperial . . . . Selection No. 3 ; Webb’s No., 2 Strain. . 57 63-2 30 White-fleshed Purple Top Sutton’s ’ Sensation .76 o-o Strain* 76 o-o 41 Wilhelmsburger Otofte . . Strain* . .. Danish Strain . . . 64 , . . 78 • 1 52 • ‘ . .. • • • English Strain 63 . 73’0 (Sharpe’s) 63 73'0 ' 69 Sharpe’s Ai . . . . (Sharpe’s) \ . (Sharpe’s) . . ... . . 63 .71-4 . Average of the sixty other lines ' . 65-4 92-7 ;

Table III. Varietal Resistance to Turnip-mosaic.