TOMATOES INJURED BY HORMONE WEEDKILLERS

New Zealand Journal of Agriculture, Volume 75, Issue 4, 15 October 1947, Page 349

TOMATOES INJURED BY HORMONE WEEDKILLERS

Other Cultivated Plants Susceptible

By

J. D. ATKINSON,

Plant Physiologist, Plant Diseases Division,

Department of Scientific and Industrial Research.

AMONG new hormone weedkillers developed during the recent A* war is a substance known commercially as 2.4 D, which is not strictly one compound, but a group of materials. Some of these have reached the New Zealand market during the past two years, and have already demonstrated both their usefulness for checking certain weeds , and their danger to tomatoes , and other cultivated plants. .

TN November, 1946, curious distorted * . foliage ' appeared throughout an early crop of. tomatoes - in a heated glasshouse' near Hastings. Although’ symptoms on individual plants re-

sembled those, of cucumber mosaic, the widespread nature of the trouble and its uniform onset were not consistent with characteristics of this, virus. Inoculation "tests, showed that the disease was not cucumber mosaic. It was suspected that traces of a weed-' killer containing Z 2.4 D might have caused the damage, as similar injury had already been reported from England (Swarbrick, 1944). Tests carried out under glass in Auckland showed that traces of this weedkiller produced symptoms on tomatoes identical with those found' at Hastings. During the

summer of 1946-47 other specimens showing similar injury were sent in

from five widely-separated localities in the North Island and two in the South. In each case it was found that hormone weedkillers had caused the distortion. ' . / •.' ■

Symptoms on Tomatoes Symptoms of 2.4 D injury on tomatoes are as follows: —When a spray of weedkilling strength (1:100) reaches a plant the main plant starts to droop within a few hours. Translucent spots develop on some leaflets, and these soon die. Brown necrotic blisters appear .on the stem. Leaf, stalks curl downward, while leaflet edges roll up. Twenty-four hours after treatment the plants are markedly twisted and beginning to wilt. _ Nine days later they are dead. M '.. ■

At. greater dilutions the effect of 2.4 D less drastic. With a dosage of

1:100,000 the only symptom shown .by fully-developed leaves is a slight drooping which disappears within a few days. . Stem tips start ' to twist spirally soon after. the spray reaches them. Growth does not stop, but a permanent bend forms at this point. New foliage developing after treatment shows a pronounced narrowing of the leaflets, which are often less than a third normal width, though only slightly reduced in length. - Enlarged leaf veins stand out prominently, against the darker green of affected foliage, being made still more conspicuous by the fact that they often run parallel to the midrib. Thin, misshapen lobes are usually present along leaf edges and the end of the leaf .becomes a long twisted point (Fig. 2). At this dosage narrow foliage is produced throughout the remaining life of the plant. If-fruit forms, it is generally elongated, beaked, and seedless, and may have internal cavities (Fig. 5). The whole plant appears spindly and - unthrifty and is useless commercially. , -

Where only a trace of 2.4 D reaches tomato plants the immediate reaction is . a slight twisting of the main , stem and drooping of some leaves. These effects soon . disappear,, but typical narrowing and distortion appears in the next two or three leaves formed. If the dosage has been very low (1:1,000,000 or less), later leaves may develop normally, while some normal fruit may form on the upper trusses.

Identification

Malformations of the type described closely resemble those produced in tomatoes by cucumber mosaic. There

are, . however, certain differences. Where virus is the cause leaflet lobes are long, thin, , and pointed; leaf veins are ; relatively s normal in appearance and any fruit formed is small and flattened (Fig. 1). : Taken individually these differences may not be conclusive, but the manner in which trouble appears in the field usually provides clear evidence of the cause. Infection with cucumber mosaic appears first in odd plants, spreading from these during a considerable period; where 2.4 D is involved a large number of plants show symptoms at the same time, usually 10 to 14 days after contamination. .' .

Accidental Injury in Field

Where 2.4 D is used as a weedkiller it may accidentally reach tomato plants in three ways—as residue in spray pumps, as spray drift, or as vapour from treated weeds adjacent to a crop. The first of these is the most common cause of trouble.

In some preparations the active agent is dissolved in oil. For normal weedkilling this is emulsified with water and applied through a spray pump. Oily residues are extremely difficult to remove from spray equipment and no ordinary method of washing will completely eliminate them. When some other spray is applied through the same pump there is enough 2.4 D. left to damage a large number of tomato plants. Rinsing with hot or cold water is almost useless and one thorough washing with kerosene does not remove all

INJURY BY HORMONE WEEDKILLERS

traces. Various attempts, made both here and abroad, to find some practical way of cleaning, contaminated equipment have been unsuc-

cessful.

Whenever 2.4 D is applied to weeds spray drift may cause injury to cultivated plants in the vicinity. This has occurred to tomatoes and the results are severe.

Some of the materials in this group are volatile and vapour given off by them produces typical injury on tomatoes. Staten (1946) has shown that vapour will cause injury on cotton plants 30ft. away from a dish containing a volatile solution. No figures are available for the distance at which this vapour damages tomato plants in the open. Under glass it has been found that vapour from plants sprayed with a trace of 2.4 D will cause distortion in tomato plants ft. away.

Injury to Other Cultivated Plants

Officers of the Department of Agriculture have observed various forms of injury produced by 2.4 D on cabbage, cauliflower, brussels sprouts, radishes, silver beet, swedes, turnips, onions, leeks, carrots, and beans. They have also noted damage on many flowering, plants and shrubs, one striking case being malformation produced on grape leaves (Fig. 4). In glasshouse tests with tobacco it was found that traces of weedkiller caused puckering and J stunting of young leaves, and malformation of four to five leaves developed after application (Fig. 3). /

Prevention of Injury

Weedkillers containing 2.4 D have been found useful in checking some troublesome weeds such as Californian thistle and convolvulus (Greenwood and Doak, 1946). As a result they may

become part of accepted agricultural and . horticultural practice in this country. When used in pasture country there is little risk, but in cultivated areas great care must be taken to see that neither spray drift nor vapour reaches susceptible plants. Where tomatoes are to be sprayed the only safe course at present is to have two pumps, one Tor weedkiller only, the other for all normal sprays. From the

nature of these hormone weedkillers it is apparent that they will kill or injure many cultivated plants. To avoid serious loss they, must be used even more carefully than well-known materials . such as arsenic pentoxide- or sodium chlorate. < LITERATURE CITED

- Greenwood, R. M., & Doak, B. W. (1946): N.Z. J. Sci. & Tech., Vol 28 (Agr. Sect.), pp. 70-79. Staten, G. (1946): J. Amer. Soc. Agr., Vol 38, pp. 536-544. Swarbrick, T. (1944): Ann. Rep. Long Ashton Agric. Hort. Res. Stat., pp. 36-48.