Plant Virus Diseases Becoming Increasingly Destructive

Press, Volume CVIII, Issue 31635, 22 March 1968, Page 6

Plant Virus Diseases Becoming Increasingly Destructive

Virus diseases plague man and plant alike and while considerable progress has bean made in the form of vaccine* to afford protection for man, there is nothing, as a rule, which can be done to restore the health of a plant infected with a virus disease. Once it has gained entry it is usually translocated in the sap stream and may eventually invade most of the plant. The increase in destructiveness of virus diseases in plants and in the number of known viruses can be attributed largely to the expansion of agricultural and horticultural enterprises and the Increased movement of plants and plant produce in recent years. Virus infection is actually more prevalent than is generally realised. All kinds and varieties of plants can be infected to a greater or lesser degree annuals, herbaceous, perennials, trees, shrubs, weeds and crops although some varieties of a host species may show immunity. The symptoms produced by virus infection vary widely as do the types of injury to plants, but with few exceptions, such as spotted wilt of tomatoes, plants are rarely killed. More often they cause lesser injuries that result in reduced yields and a lower quality of product. Subjects may be severely crippled and make little or no growth but frequently there is little loss of vigour and plant growth continues more or less normally in spite of prominent symptoms of the disease. One virus may attack a

wide range of plants, producing different symptoms. For instance, a particular virus which attacks cabbage causes a ring spot condition yet the Mme one will produce flower colour-breaking in stocks or wallflowers. Two Groups The symptoms produced by virus diseases are of two fairly distinctly defined S those that cause ig or spotting of the leaves and those that cause a dwarfing, yellowing, leaf curling or excessive branching, but little or no mottling or spotting. The most Common symptom seen on leave* is a mild mottling such as tobacco mosaic of tomatoes. The attractively variegated leavea of Abutilon striatum and those of the Japanese laurel, Aucuba japonlca varlegata are due to virus infection. Necrotic or dead patches may appear on the leaf. These can be extremely small or tend to become quite large as in the case of a virus which attacks dwarf beans. There is another which causes vein banding or clearing when the tissue alongside of the vein becomes pale in colour as exhibited by one of the rose viruses. With “yellows” viruses such as attack peaches and sugar beet the leaves assume a chlorotic appearance, the “yellows” symptom varying In intensity in different plants. Distortion of the leaf is common and may take the form of leaf thickening and rolling or crinkling, either up or down such as prominently displayed by many potato crops this year, leaf tissues may be so reduced that there is little more than veins apparent; and some viruses cause abnormal growths to appear on the leaf. Still others stimulate the production of clusters of thin wiry shoots giving rise to what is known as witch's broom. Flowers are occasionally affected by virus disease and the usual result is a break of the colour or variegation. Stocks and wallflowers have already been mentioned but tulips, violets and sweet peas are others which can become infected. Fruits, too, may exhibit symptoms and in consequence may be mishapen, mottled, spotted or smaller. For instance rings and mottling can occur In tomatoes infected by

spotted wilt while there is a condition known as stony pit Of pears which produces bard stone-like lumps in the flesh. Apples, tree tomatoes and passion fruit are yet other fruit* susceptible to the disease. Transmission The most important agents of transmission are insects and in fact some viruses can only be transmitted in this way. Aphides are the commonest vectors and induce a great variety of symptoms, the most important of which are the mosaics. One of the most efficient aphis vectors is the green peach aphis which transmits more than 50 different plant viruses. The disease is passed from an infected plant to a healthy one by means of the sucking mouth parts of the insect. Another important carrier of virus diseases amongst the Insect world is the leaf hopper while mealy bugs and white flies play a role of lesser importance in spread. Vegetative means provide the other major form of transmission for, as pointed out, viruses do not remain localised but invade all parts of the plant with the almost virtual exception of the seed. When propagating from infected stock spread can come from grafting, or bud -wood, cuttings, layers, bulb scales, division and tubers. Yet another form of transmission is by mechanical means. This can occur when, for instance, two leave* rub together causing slight damage to the hairs on the leaf so that there is an interchange of plant juices. Virus can thus spread from an infected plant to a healthy one. In like

manner some viruses can be spread by hands, clothes, tools and machinery. Tobacco mosaic, which attack* both tomato and tobacco crops, is probably the only plant virus which is extensively and chiefly spread in this way, and can cause terrific losses through carelessness. It may be of interest to note that tobacco mosaic in infected tobacco remains viable .even after processing and in consequence smokers are always a source of infection when working among glass house tomatoes. No Cure After a plant is Infected with a virus disease nothing can usually be done to restore its health. Methods of control must therefore be aimed completely st prevention of infection and also toward the development of disease-resis-tant varities. The use of insecticides to destroy vectors is only part of the answer to control as they will have to feed on the plant before absorbing the poison and in so doing the disease is transmitted. It is important to ensure that all propagating material comes from healthy plants, and as visual means are mostly negligible through leaf fall at such times all diseased Stock should be tagged or better still, in many cases, destroyed. Weeds should be kept down by regular suppression by means of hoe or weedkiller to prevent them becoming reservoirs of infection. Periodic washing of hands and dipping of tools in methylated spirits is recommended when working amongst plants that can be infected by mechanical means.