Journal of the Department of Agriculture, Western Australia, Series 4

Volume 16 Number 1 March, 1975 Article 2

1-1-1975

Virus diseases in agriculture

George McLean

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Recommended Citation McLean, George (1975) " diseases in agriculture," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 16 : No. 1 , Article 2. Available at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol16/iss1/2

This article is brought to you for free and open access by Research Library. It has been accepted for inclusion in Journal of the Department of Agriculture, Western Australia, Series 4 by an authorized administrator of Research Library. For more information, please contact [email protected]. Electron micrographs of virus particles. Photographs by courtesy of Dr. C. Bracker, Department of Botany and Pathology, Purdue University, Indiana, U.S.A. Straight rod-shaped virus Virus diseases in agriculture By George D. McLean, some have been shown to and viruses that are spread by mealy Plant Pathologist contain an enzyme. Viruses do not bugs are not spread by leafhoppers. reproduce by division, but the viral Some species of transmit Viruses are extremely small par­ nucleic acid "orders" the plant cell many viruses while other species ticles which can only be seen with to produce virus particles. transmit only a few. Aphids spread an electron microscope. A virus is How plant viruses are spread is , watermelon present in a diseased plant as huge important since a virus disease can­ mosaic virus and potato leaf roll numbers of tiny particles within not be "cured" and we must control virus and many other viruses. Thrips the cells of the plant. Viruses which the spread in a crop. Viruses are transmit tomato spotted wilt virus are infectious can cause disease, spread in various ways: by insects which causes disease in tomato and and reproduce only inside the cells and mites, by nematodes and soil crops. of a living plant. fungi, by vegetative propagation, There are two types of Preparations from virus-infected within and by handling of in­ transmission; non-persistent or sty­ leaves or from purified extracts fected in the field. In the let-borne and persistent or circula- made in the laboratory may be glasshouse many viruses are trans­ tive. Cucumber mosaic and water­ examined under the electron micro­ mitted by making a sap extract from melon mosaic viruses are transmit­ scope, and the particles may be infected plants and rubbing this ted in the non-persistent manner. viewed and photographed. Some extract on leaves of healthy plants The virus may be passed on to a particles are spherical; others are previously dusted with a mild abras­ healthy plant within minutes after rod-shaped while some are bacilli- ive such as finely divided carborun­ the aphid has fed on the diseased form. The simplest virus consists of dum powder. plant. The aphid only retains the a molecule of nucleic acid, usually Many insects transmit virus for a very short time. More ribonucleic acid, surrounded with a diseases—aphids, leafhoppers, simply, the virus adheres to an in­ protein coat. A small number of mealy bugs, thrips and beetles. sect's stylet after it feeds on an in­ viruses contain a molecule of deoxy­ Generally a virus is spread by only fected plant. ribonucleic acid plus a protein coat. one of these groups of insects. For Persistent or circulative trans­ Some viruses are more complex and example, those that are spread by mission means that there is a much contain lipid and in recent years aphids are not spread by thrips, longer period of time, from hours to 2 Journal of Agriculture Vol 16 No 1, 1975 . 1

Spherical-shaped virus rod-shaped virus

days, between the aphid feeding on they have already infected the crop. programmes take many years to an infected plant and transmitting The symptoms of the disease will produce healthy stock. Virus-tested the disease to a healthy plant. appear some four to six weeks later. material is released to growers as Viruses transmitted in a persistent If the crop is infected when it is soon as it is available. manner have been observed in many young, the symptoms will be severe. In Western Australia, there are parts of the insect by electron There are a number of virus dis­ several viruses of fruit. Many grape­ microscopy. The insect retains the eases of subterranean clover in Wes­ vines are infected with grapevine virus for a long period of time from tern Australia. One of the most leafroll virus. Possibly the most days to months. Potato leaf roll recent diseases observed has been obvious symptom of this disease is virus is transmitted in this manner, red leaf virus, which is transmitted the failure of the red table grape, the aphid retaining the virus for its by an aphid. This virus also multi­ Red Emperor, to achieve its normal whole life after infection. plies in the aphid. Yarloop seems to red colour when infected with Efficient control will depend on be most severely affected, but other grapevine leafroll virus. The grower whether the aphids are carrying the cultivars are also affected. will also observe symptoms on the virus from an infected plant several Another important means of leaves: a scarlet or a red colour be­ hundred metres from the healthy virus spread is by vegetative pro­ tween the veins of the leaves of red crop, whether they are carrying the pagation in the orchard or the vine­ coloured fruit and a downward virus from an infected crop to a yard. If a healthy scion is budded rolling of the leaves. In other States bordering healthy crop, or whether onto an infected rootstock that and overseas, experiments have they are spreading the disease from healthy scion soon becomes infected shown that infected vines have plant to plant within a crop. with virus and the plant will then be smaller clusters, fewer clusters per It is important to spray with in­ diseased. Fruit growers should use vine and a reduced sugar content. secticides to control aphids as a the healthiest rootstock and scion Thus if the vines are severely affec­ preventative measure, rather than to material available. Scientists are ted by the disease there may be con­ await the appearance of the insects. continually working on plant im­ siderable yield losses for the grower. Once a grower sees aphids on his provement programmes to produce In apple orchards many growers crop of rockmelons, and if these virus-free selections of grapevines, have probably seen cream to yel­ aphids have carried a virus, then apples, stone fruit and citrus. Such low spots on the leaves of Jona- 3 Journal of Agriculture Vol 16 No 1, 1975 Plum line pattern virus on Beauty plums Grapevine leafroll disease on Red Emperor grapes

Cucumber mosaic virus on cucumber. This aphid-transmitted virus affects many plants Passionfruit woodiness virus

Stony pit disease on Josephine and Packham pears Lettuce big vein on lettuce

Barley yellow dwarf virus on oats. This virus is aphid Tomato spotted wilt virus on lettuce, aphid transmitted transmitted

Journal of Agriculture Vol 16 No 1, 1975 Red leaf virus of subterranean clover. This virus is Apple mosaic virus on Jonathan apple leaves transmitted by the potato aphid

Maize dwarf mosaic virus on maize Rose mosaic virus on rose

Subterranean clover stunt virus, aphid transmitted Camellia virus—flower break and leaf mottle

Tomato big bud disease, caused by a mycoplasma and transmitted by leaf hoppers Apple green crinkle on Granny Smith apple • r 1 •

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45706—2 Journal of Agriculture Vol 16 No 1, 1975 thans. This disease is caused by one of which, lettuce big vein dis­ and adopt the most effective control apple mosaic virus. Another apple ease, is present in Western Australia. measures, the spread of virus dis­ virus disease is green crinkle, which Tobacco mosaic virus and potato eases can be reduced. The possi­ affects Granny Smith fruit. The virus X are highly infectious and bility of epidemics will be low. symptoms are severely distored fruit are easily spread in the field. Dis­ with depressed creases and irregular eased plants may be broken with Virus disease diagnosis in swellings on the surface of the fruit. the tractor wheels or implements. If Western Australia These diseases are transmitted by these damaged plants are moved The Western Australian Department the use of infected scions or root- through the crop, they may contact of Agriculture is expanding the stocks. and infect healthy plants. Tobacco diagnostic service for the identi­ In the potato industry, many virus mosaic virus can be present in dis­ fication of plant viruses. Until diseases are transmitted by the use eased tobacco. Smokers may spread recently this service has been the of infected seed. Potato leafroll and tobacco mosaic virus from their testing of diseased plants by inocu­ potato viruses X and S and to a hands to tomato plants. lating sap extracts on to indicator lesser extent potato virus Y are all Growers should attempt to plants in the glasshouse. Facilities transmitted through infected tubers. become aware of virus diseases on for the handling of aphids will be This is a very efficient means of "self-sown" plants. Any plants developed for vector transmission spread of viral diseases. It is there­ with disease symptoms should be studies. Some viruses are not sap- fore vital for seed growers to try to destroyed. If infected self-sown transmitted and aphids must be used keep their crops free of disease. plants are not destroyed, they will be to transmit the diseases from plant a source of infection for aphids to plant. Diagnostic tests will also Since potato leafroll is spread by which might spread the disease to be carried out in the laboratory. aphids, it is important that growers a healthy crop. Virus-infected Viruses will be purified and charac­ spray with insecticides to control plants in vegetable crops should be terized so that the virus causing the aphids. In experiments overseas, rogued out. These plants could act disease may be identified. Serolog­ total leafroll infection of crops as a source of virus for insects ical techniques will also be used for caused a 40 per cent reduction in which could then infect other identifying viruses when a range of the yield of potatoes. healthy plants. Some viruses infect antisera to various viruses is assem­ Seed transmission occurs with a a range of plants. bled. Through co-operation with small number of viruses, and is a Subterranean clover stunt virus other scientific institutions, both sap very efficient means of spread when infects clover, beans, and other extracts and purified virus will be it does occur. Lettuce mosaic virus plants, so it may be difficult for the examined with the electron micro­ is spread in this way. Where a virus grower to control this virus with a scope. is known to spread in the seed the number of alternate hosts present. grower should attempt to obtain Similarly bean yellow mosaic virus Strong emphasis is being placed virus-free seed if it is available. infects beans, peas, clover and on the concept of plant improve­ Growers who intend to use their lupins. Both of these viruses are ment schemes. Since viral diseases own seed should select mosaic-free transmitted by aphids. can not be cured, efforts must be lettuce for this purpose. This Before 1967 about 50 plant dis­ made to obtain fruit varieties free disease produces mottled leaves, eases which were characterized by of known virus diseases. When usually a yellow mottling, sometimes yellowing of the leaves and growth varieties are located which are suit­ with the veins also affected. Plants abnormalities were considered to be able for Western Australian condit­ may be dwarfed and hearts fail to caused by viruses. These diseases ions such as climate and soil types develop. Since this disease can resembled a group of virus diseases they will be imported if they are also spread by aphids once intro­ in that they both produced similar free of known viral diseases. How­ duced into a crop, it is important to symptoms on plants and had a simi­ ever, these schemes will take some plant healthy seed. lar type of insect transmission. Then years before the grower receives the Viruses may spread in the soil by in 1967, a group of Japanese scien­ bud wood or cuttings of the new several methods. The simplest in­ tists observed with the electron material. Importations of some volves the persistence of stable vir­ microscope, mycoplasma-like organ­ fruit must be held for a year in uses in fragments of infected plant isms in infected plants. Mycoplasma quarantine. The imported bud material. When a new crop is differ from viruses in that they can wood must then be multiplied and planted these viruses may mechan­ be grown on cell-free media, they later compared in variety trials with ically infect the roots of the young are small and variable in shape and the present varieties. plants. Nematodes and fungi may are bounded by a triple-layered unit Plant pathologists are attempting also transmit viruses in soil. Viral membrane without a rigid cell wall. to elucidate the nature of disease diseases of potatoes and grapes are Some plant diseases caused by agents. New frontiers are being transmitted by nematodes, but there mycoplasmas are lucerne witches expanded each year—in 1967 it was broom, apple rubbery wood, paw­ has been little evidence of nema­ mycoplasmas, a couple of years paw yellow crinkle and tomato big tode transmission of viruses in Aus­ later it was viroids and in 1972, it tralia. A few plant viruses are bud. Tomato big bud disease is spread by a leafhopper. was rickettsial diseases of plants. transmitted by fungi. One fungus, Likewise in plant virology, scien­ Olpidium brassicae has been shown If growers become aware of the tists are discovering more about the to transmit three virus diseases, viruses which may infect their crop function and spread of viruses. 6 Journal of Agriculture Vol 16 No 1, 1975