Viruses in vegetable crops in Australia

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(from left to right) Common carriers of plant viruses—whiteflies, aphids and thrips. infected pollen (e.g. Tobacco streak virus) infected vegetative propagating material,(e.g. Potato leaf roll virus is transmitted in tubers) contact between plants (e.g. Tomato mosaic virus).Host plantsA virus has specific host plants and cannot infect allplant species with which it comes into contact.Several viruses have a wide range of hosts—forexample Tomato spotted wilt virus and Cucumbermosaic virus infect hundreds of different plantspecies. Papaya ringspot virus-type W, however,infects only cucurbits.Plant viruses are generally named after the first host inwhich they were found, though this may not give a trueindication of the importance of the virus to that host.Celery mosaic virus.Insect transmissionInsect transmission of a virus is a specific biologicalprocess. A particular virus is transmitted by onevector type only—for example an aphid or a whitefly,not both.The most significant insect vectors of plant viruses areaphids, leafhoppers, whiteflies and thrips. All havepiercing-sucking mouthparts that include a needlelikestylet that allows the insects to access and feedon the contents of plant cells.During this feeding there is an exchange of insectsaliva and plant cell contents.As the insect stylet isinserted, saliva and virusparticles can enter theplant cell. As the insectwithdraws the cell sap,virus particles presentwithin those cells are alsoacquired by the insect.Thus, if an insect hasacquired virus particlesfrom one plant, it can thendeposit them in the nextplant it feeds on, whichleads to subsequentinfection.There are two broadcategories of insecttransmission: persistent non-persistent.Persistent transmissionInsect needs to feed forseveral hours, often in foodconducting tissues of plants,to obtain the virus.Non-persistent transmissionInsect needs only very shortfeeding times, usually fromtissues near the surface ofleaves, to obtain the virus.The terms relate to thelength of time an insecttakes to acquire and totransmit a virus and the length of time the insectremains capable of transmitting the virus.
How do viruses survive?With very few exceptions, viruses cannot surviveoutside living host plants or insects.Viruses survive adverse conditions and intervalsbetween crop cycles in alternative annual andperennial weed hosts, volunteer crop plants,abandoned crops, infected seeds and vegetativeplant parts. Persistently-transmitted viruses may alsosurvive in the insect vector.How can you manage virus diseases?Plants cannot be cured once infected by a virus.Instead, disease control aims to prevent or delay theinfection of plants. destroy old crops promptly separate new crops from maturing crops and avoidoverlapping crops, especially continuous yearroundcropping.Protection of the host plant virus-resistant or virus-tolerant varieties use highly reflective mulches and oil sprays todeter insects use barrier crops and bare land to reducevector activity use insecticides strategically to protect plantsfrom insects.No single method is likely to provide perfect control.Nevertheless, by using a combination of the followingmanagement options disease control can besuccessfully implemented.Virus management can be achieved by plantingresistant varieties. The zucchini variety on the left isvirus susceptible; the variety on the right is resistant.Tomato mosaic is easily spread by contact and oncontaminated seed.Exclusion/avoidance plant virus-free seed and seedling transplants grow crops in regions where the disease seldomoccurs or during periods when the virus or its vectorare at a low level quarantine (international, state and regional).Reduction in virus inoculum levels control weeds and other virus hosts andinsect vectorsInsecticides are more effective against persistentlytransmitted viruses because insects are killed beforethey have time to acquire and transmit the virus.Vectors of non-persistent viruses will eventually bekilled after feeding on plants sprayed with systemicinsecticide. However, because these viruses can betransmitted within seconds, many plants becomeinfected before the insect dies or moves out of the crop.In fact, some insecticides agitate the insects andencourage movement and feeding of greater numbersof plants, resulting in increased transmission rates.A key aspect of virus disease management is toaccurately identify the virus causing the disease andthen implement appropriate management strategies.
Page 1: Department of Employment, Economic
Page 5 and 6: (left, middle) Capsicum leaf and fr

Viruses in vegetable crops in Australia

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