Department of Employment, Economic Development and InnovationAgri-Science QueenslandViruses in vegetable crops in AustraliaIntegrated virus disease managementViruses are a major cause of loss in many Australianvegetable crops. Often the intricate relationships betweenthe virus, host plants and the vector, or carrier, createproblems in developing effective management systems.This reference note provides information on plant virusesand how they are transmitted, and lists viruses ofimportance to the Australian vegetable industry.Summary most viruses infecting vegetables are transmitted bysap-sucking insects non-persistent transmission of viruses by insects is arapid process persistent transmission takes several hours weed and other hosts are crucial in the life cycle ofmany viruses and their vectors infected plants cannot be cured—control aims toprevent or delay infection using a combination of management options can besuccessful in preventing infection.What are viruses?Viruses are minute, non-cellular pathogens thatmultiply within the cells of their hosts. This is usuallyto the detriment of the host and results in thedevelopment of disease symptoms.Viruses are obligate parasites—that is, parasites thatmust live with their host or they die. Obligate parasitesdepend on the presence of a host to complete theirlife cycle.A virus particle consists of a nucleic acid core, whichcontains the genetic information necessary formultiplication, surrounded by a protective protein coat.How do plant viruses spread?Viruses are immobile and rely on other organismsfor dispersal.Most plant viruses are transmitted from plant to plantby a living organism called a vector or carrier. The mostimportant vectors are sap-sucking insects—aphids,whiteflies, thrips and leafhoppers.Plant viruses can also be spread by: other insects (e.g. mealybugs, leaf chewing-beetles) mites (e.g. the eriophyid mite Aceria tosichellatransmits Wheat streak mosaic virus) nematodes (e.g. the dagger nematode transmitsTobacco ringspot virus) fungi (e.g. Olpidium virulentus transmits Mirafiorilettuce virus) infected or contaminated seeds (e.g. Lettuce mosaicvirus, Tomato mosaic virus)Tomorrow’s Queensland: strong, green, smart, healthy and fair
(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.
Important vegetable crops and the viruses infecting themCrop/family Virus Means of transmissionBean—FabaceaeBrassicas—BrassicaceaeCapsicum—SolanaceaeBean common mosaic virus (BCMV)Bean yellow mosaic virus (BYMV)Subterranean clover stunt virus (SCSV)Tobacco yellow dwarf virus (TYDV)(Bean summer death disease )Beet western yellows virus (BWYV)Turnip mosaic virus (TuMV)Capsicum chlorosis virus (CaCV)Cucumber mosaic virus (CMV)Pepper mild mottle virus (PMMV)Potato virus Y (PVY)Tomato spotted wilt virus (TSWV)Seed, aphids (non-persistent)Aphids (non-persistent)Aphids (persistent)Leafhopper (Orosius argentatus),(persistent)Aphids (persistent)Aphids (non-persistent)Thrips (persistent)Aphids (non-persistent)Seed, contactAphids (non-persistent)Thrips (persistent)Carrot—Apiaceae Carrot virus Y (CaVY) Aphids (non-persistent)Celery—Apiaceae Celery mosaic virus (CeMV) Aphids (non-persistent)Cucurbits—CucurbitaceaeBeet pseudoyellows virus (BSYV)Cucumber mosaic virus (CMV)Papaya ringspot virus—type W (PRSV-W)Squash mosaic virus (SqMV)Watermelon mosaic virus (WMV)Zucchini yellow mosaic virus (ZYMV)Whiteflies (Trialeurodes vaporariorum)(semi-persistent)Aphids (non-persistent)Aphids (non-persistent)Seed, beetleAphids (non-persistent)Aphids (non-persistent), contactEggplant—Solanaceae Tomato spotted wilt virus (TSWV) Thrips (persistent)Lettuce—AsteraceaeOnion and related species—AlliaaceaePea—FabaceaePotato—SolanaceaeCucumber mosaic virus (CMV)Lettuce mosaic virus (LMV)Mirafiori lettuce virus (MiLV); Lettuce big-veinvirus (LBVV) – (Lettuce big-vein disease)Lettuce necrotic yellows virus (LNYV)Tomato spotted wilt virus (TSWV)Turnip mosaic virus (TuMV)Iris yellow spot virus (IYSV)Onion yellow dwarf; Leek yellow stripe virus(OYDV; LYSV)Bean yellow mosaic virus (BYMV)Pea seed-borne mosaic virus (PSbMV)Subterranean clover stunt virus (SCSV)Potato leafroll virus (PLRV)Potato virus X (PVX)Potato virus Y (PVY)Potato virus S (PVS)Tomato spotted wilt virus (TSWV)Aphids (non-persistent)Lettuce seed, aphids (non-persistent)Soil-borne fungus Olpidium; infestedtransplants, contaminated soil andhydroponic systemsAphids (persistent)Thrips (persistent)Aphids (non-persistent)Thrips (Thrips tabaci), (persistent)Aphids (non-persistent), infected garliccloves and onion bulbsAphids (non-persistent)Seed, aphids (non-persistent)Aphids (persistent)Aphids (persistent), infected seed tubersContactAphids (non-persistent)Infected ‘seed’ tubers, contactThrips (persistent)Sweet corn—Poaceae Johnson grass mosaic virus (JGMV) Aphids (non-persistent)Sweet potato—ConvolvolaceaeSweet potato feathery mottle virus (SPFMV)Aphids (non-persistent)continued on back page
(left, middle) Capsicum leaf and fruit affected by Tomato spotted wilt virus. (right) Zucchini plants severely affected byPapaya ringspot virus (typeW).Persistent transmission: it takes several hours of feeding for an insect toacquire the virus the virus must circulate through the insect’sbody to the salivary glands before transmissioncan occur there is a latent period during which transmissioncannot occur (while the virus particles travelthrough the insect’s body) when the latent period is completed, the insectcan then transmit the virus for many weeks orthe rest of its life without needing to obtain moreviruses from an infected plant.There are two types of persistent transmission.Non-persistent transmission:it takes less than one minute of feeding for an insectto acquire the virus virus particles remain on insect mouthparts for afew hours insect needs to re-feed on another infected plantif further transmission is to occur.Among important vegetable viruses transmitted inthis way are: Cucumber mosaic virus Celery mosaic virus Potato virus Y Papaya ringspot virus.1. Non-propagative—virus circulates throughthe vector’s body, but does not multiply.Examples are Potato leaf roll and Tomato yellowleaf curl viruses.2. Propagative—virus needs to multiply in the cellsof the insect vector before transmission can occur.The transmission latent period is normally longerwith these viruses. Examples are Tomato spottedwilt and Lettuce necrotic yellows viruses.The silverleaf whitefly transmits Tomato yellow leaf curl virus (left). The greenhouse whitefly transmitsBeet pseudoyellows virus (cucumber yellows).
Crop/family Virus Means of transmissionTomato—Solanaceae Capsicum chlorosis virus (CaCV) Thrips (persistent)Cucumber mosaic virus (CMV)Aphids (non-persistent)Potato virus Y (PVY)Aphids (non-persistent)Potato leafroll virus (Tomato yellow top virus) Aphids (persistent)(PLRV)Tomato spotted wilt virus (TSWV)Thrips (persistent)Tomato mosaic virus (TMV)Contact, contaminated seed andequipment, tomato crop debrisTomato leaf curl virus (Australia) (TLCV);Tomato yellow leaf curl virus (TYLCV)Whiteflies (Bemisia tabaci)For more information contact:Agri-Science Queensland principal plant pathologistDenis Persley on 07 3255 4388 .New South Wales Department of Primary Industriesprincipal plant pathologist Len Tesoriero on02 4640 6406.Victorian Department of Primary Industries seniorvirologist Brendan Rodoni on 03 9210 9222.This technical reference note has been produced byDenis Persley and Cherie Gambley (Agri-ScienceQueensland) as part of the Horticulture AustraliaLimited project VGO 7128-Integrated management ofviral diseases in vegetables.© The State of Queensland, Department of Employment,Economic Development and Innovation, 2010.Western Australia Department of Agriculture and Foodplant virologist Brenda Coutts on 08 9368 3266.University of Tasmania associate professorCalum Wilson on 03 6233 6841.PR09_5397