Section 2 Virus Diseases of Vegetable Plants. 1. Virus Diseases of Tomato.
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Several virus diseases of tomato are not as prevalent as the wilt and foliar diseases. Three of the more common virus diseases are tomato mosaic (also referred to as tobacco mosaic), cucumber mosaic, and tomato spotted wilt. The tomato mosaic virus can attack a wide range of plants, including tomato, pepper, eggplant （茄子）, tobacco, spinach（菠菜）, petunia（矮牵牛花）, and marigold（金盏草）.
1.1 Sympotom • On tomato, virus infection causes light and dark green mottled areas on the leaves. The dark green areas tend to be somewhat thicker than the lighter portions of the leaf. The leaf mottling is seen more easily if the affected plant surface is partially shaded. Stunting of young plants is common, and often is accompanied by a distortion and fern-like appearance of the leaves. Older leaves curl downward and may be slightly distorted. Certain strains of the virus can cause a mottling, streaking, and necrosis of the fruits. Infected plants are not killed, but they produce poor quality fruit and low yields.
The cucumber mosaic virus has one of the broadest host ranges of any of the viruses. The disease affects a number of important vegetables and ornamentals including tomato, pepper, cucumber, melons, squash, spinach, celery, beets, and petunia. Tomatoes infected with the cucumber mosaic virus develop a slight yellowing and mottling of the older leaves. Expanding leaves typically become twisted, curl downward, and develop a 'shoestring' appearance as a result of a restriction of the leaf surface to a narrow band around the midrib of the leaf. Diseased plants are stunted and produce small quantities of fruit.
The tomato spotted wilt virus also has a wide host range and can affect a number of ornamental plants as well as tomato. Early symptoms of spotted wilt on tomato are difficult to diagnose. Young, infected plants may show an inward cupping of leaves, and the foliage may appear off-color or have a slight bronze cast. In some cases, leaves with exhibit dark purple flecks or small necrotic concentric rings.
Leaf symptoms of tomato mosaic virus. Fruit symptoms of tomato mosaic virus
Fruit symptoms of tomato spotted wilt Purple flecking of young leaves caused by tomato spotted wilt.
As the disease progresses, plants may develop dark brown to black streaks on the main stem. Occasionally the top portion of the plant wilts. • The most characteristic symptom of spotted wilt appears on the fruit. On young fruit, white to yellow concentric rings, one-half inch in diameter, develop on the fruit skin. a bumpy or warty appearance. The bright yellow rings on red, mature fruit are quite striking and are easily diagnosed as spotted wilt. The area within the ring typically is raised, which gives the fruit.
1.2 Causes • Tomato mosaic, cucumber mosaic, and spotted wilt are incited by viruses. The tomato mosaic virus is very stable and can persist in dry contaminated soil, in infected tomato debris, on or in the seed coat. The virus is transmitted readily from plant to plant by mechanical means. This may simply involve picking up the virus while working with infected plant material, then inoculating healthy plants by rubbing or brushing against them with contaminated tools, clothing, or hands. Aphids are not vectors of the tomato mosaic virus, although certain chewing insects may transmit the pathogen.
The cucumber mosaic virus overwinters in perennial weeds and may be transmitted to healthy plants by aphid vectors (although tomatoes are not the preferred host of aphids) or by mechanical means. • The cucumber mosaic virus cannot withstand drying, or persist in the soil. It also is more difficult than tobacco mosaic to transmit mechanically. Thus, cucumber mosaic tends to progress more slowly than tobacco mosaic in a field or garden.
The spotted wilt virus is transmitted from plant to plant by several species of small insects called thrips. Thrips are less than one-quarter inch in length, light green to brown, and are extremely difficult to find on the plants. Several weedy hosts and ornamental plants may serve as alternate hosts for the virus.
1.3 Control • Virus diseases cannot be controlled once the plant is infected. Therefore, every effort should be made to prevent introduction of virus diseases into the garden. Many varieties of tomato are available with resistance to tomato mosaic virus. Sanitation is the primary means of controlling the other two virus diseases. Infected plants should be removed immediately to prevent spread of the pathogens. Perennial weeds, which may serve as alternate hosts, should be controlled in and adjacent to the garden. Avoid planting tomatoes next to cucurbits, spinach, or other vegetables and flowers susceptible to these diseases. Control of insects, especially aphids and thrips, will help reduce the likelihood of cucumber mosaic and spotted wilt.
1.4 Major Tomato Viruses 1.4.1Tobacco mosaic virus (TMV) is distributed worldwide and may cause significant losses in the field and greenhouse. TMV is one of the most stable viruses known, able to survive in dried plant debris as long as 100 years. Many strains of TMV have been reported and characterized. TMV can be seedborne in tomato, is readily transmitted mechanically by human activities, and may be present in tobacco products. The virus is not spread by insects commonly occurring in the greenhouse or field.
The symptoms in tomato vary greatly in intensity depending on the variety, virus strain, time of infection, light intensity, and temperature. High temperatures, for example, may mask foliar symptoms. The most characteristic symptom of the disease on leaves is a light- and dark-green mosaic pattern (fig. 1). Some strains (referred to as the acuba strains) may cause a striking yellow mosaic, whereas other strains may cause leaf malformation and “fernleafing“(蕨叶). With the use of TMV resistant or -tolerant varieties, plants may be infected by some strains whose symptoms are latent. Ordinarily the fruit from infected plants do not show mosaic symptoms, but may be reduced in size and number.
Seeds suspected of carrying TMV should be treated with a solution of trisodium phosphate (Na3PO4) for 15 minutes, rinsed in running tap water, dried, and treated with a seed fungicide. Control of TMV spread in the greenhouse is especially difficult because plants are handled more often than in the field. Hygienic measures must be taken to prevent the introduction and spread of this virus. Choose only varieties resistant to TMV and remove any suspicious plants as soon as they are observed.
Single and double virus streak and tomato leaf rolling are three separate diseases, which involve strains of TMV, mixed infections of TMV with other viruses, or interaction of TMV with a specific tomato gene. Single virus streak is caused by a strain of TMV that causes the same foliar symptoms as other TMV strains, but under certain environmental conditions the plant develops brown streaks（褐条病） on the stems and petioles(叶柄）. On the fruit the symptoms appear as slightly sunken brown rings (fig. 2) Double virus streak is the result of a mixed infection of TMV with potato virus X (PVX). Both viruses are readily transmitted mechanically and, when they occur together, result in brown streaks along the petioles and stems, and small, irregular brown areas on fruits. Leaf rolling is the result of TMV infection interacting with the wilt gene (wt) found in some tomato varieties.
1.4.2Cucumber mosaic virus (CMV) is the second most important virus disease of tomato. CMV has an extensive host range and is transmitted by aphids in a nonpersistent manner. Unlike TMV, CMV is not seedborne in tomato and does not persist in plant debris in the soil or on workers’ hands. CMV has been found in greenhouse plantings. Seedlings grown outdoors and left unprotected by isolation before moving indoors are one likely source of infection. Other sources of inoculum are the spread of CMV by aphids from infected plants in adjoining （邻近的）greenhouses (weeds under benches, ornamentals, or other vegetables) and by viruliferous （带病毒的）aphids entering through non-insect-proof vent windows.
The symptoms of CMV on tomato can be spectacular, with the most characteristic symptom being shoestringlike leaf blades (fig. 3) The symptoms of CMV can be transitory, and bottom leaves or newly developed top leaves may show a mottle similar to that caused by TMV (fig. 4). • Because of the wide host range for CMV, sources of inoculum for field plantings are numerous. Avoid planting near weedy border areas or isolate tomatoes from such areas by growing taller, nonsusceptible barrier crops such as corn. No CMV-resistant or -tolerant varieties are currently available.
1.5 Minor Tomato Viruses • 1.5.1Tobacco etch virus (TEV) and potato virus Y (PVY), are two common tomato viruses in some areas. The symptoms of TEV and PVY can be confused with TMV. Because some varieties carry partial resistance to TMV and none have been bred for resistance to either TEV or PVY, plants with a general mosaic suggest TEV or PVY infection. TEV symptoms are usually more severe (fig. 5), causing foliage distortion and severely stunted plants. • Because of the lack of virus resistance and because both viruses are transmitted by aphids in a nonpersistent manner, most control measures are developed around the removal of solanaceous weed reservoirs. In southern states where tomato viruses are particularly troublesome, commercial growers have successfully used mineral oil sprays to delay virus spread in their crops.
1.5.2Potato leafroll virus(PLRV) is more commonly associated with potato, but a tomato-specialized isolate of PLRV has been identified. Leaf rolling and marginal leaf chlorosis are the typical foliar symptoms seen with this virus (fig. 6). Because this virus is transmitted by aphids in a persistent manner, aphid control should reduce the amount of virus spread. 1.5.3Tomato spotted wilt virus (TSWV) can cause a severe disease of tomato, especially in tropical areas where thrips are plentiful. In addition to causing foliar necrosis and brown streak of the stems and petioles, fruit are severely mottled, making them unmarketable (fig. 7). Use of insecticides to control the vector reduces disease incidence.
1.5.4 Tomato big-bud mycoplasma (TBB) is a disorder of tomato more closely identified with subtropical areas of the world where this disease with the leafhopper as vector is more commonly found. As the name implies, the bud’s size is greatly accentuated by this mycoplasma-like organism (fig. 8). Leafhoppers are generally not difficult to control in temperate climates; thus, vector control should reduce the incidence of this disease.
Many viruses affect cucurbits and cause mosaic diseases. The most important of these viruses are cucumber mosaic virus (CMV), squash mosaic virus (SqMV，南瓜花叶病毒), watermelon mosaic virus (WMV，西瓜花叶病毒), zucchini yellow mosaic virus (ZYMV，小胡瓜黄花叶病毒), and papaya ring spot virus (PRSV，番木瓜环斑病毒). The viruses differ in the range of host plants they infect. • The symptoms caused by different cucurbit viruses are commonly very similar. It is impossible to identify these viruses with certainty based on symptoms alone. Usually special laboratory tests are required to correctly identify a cucurbit virus.
zucchini yellow mosaic virus (ZYMV，小胡瓜黄花叶病毒)
Leaves of virus-infected plants often appear mottled and distorted. Pumpkin plants infected early in their development (near or before the time of flowering) are severely affected and produce few fruit, and most of the pumpkins that are produced are likely to be misshapen（畸形的） or off-color. However, plants infected after fruit reach full size may not show any effect on yield or quality. Late-season pumpkins are especially prone to losses associated with virus disease.
watermelon mosaic virus (WMV，西瓜花叶病毒), squash mosaic virus (SqMV，南瓜花叶病毒) ， papaya ring spot virus (PRSV，番木瓜环斑病毒)
Viruses survive in infected weed hosts. SqMV is seed-borne. These pathogens are spread by insect vectors (especially aphids) and mechanical operations that disturb plants and bruise leaves and vines. • Virus diseases of cucurbits can be controlled by planting resistant varieties (wherever available), using virus-free seed, cultural practices, and insect control. Early planted field tend to have less damage than those that are planted later. Weed control within and around field is important. Avoiding mechanical transmission of viruses is important. Insect control for reducing incidence of virus diseases is effective. However, Attempts to control insects for virus disease control may be futile, because insects may transmit the virus before insecticides are effective.
Mosaic virus is caused by the cucumber mosaic virus. This virus is one of the most widespread and destructive pathogens on cucumbers, muskmelons(香瓜）, and squash（南瓜）. The virus is found worldwide and infects many different types of plants including various vegetables, flowers, and weeds. Much of the early infection on cucurbits comes from floral crops which harbor the virus. Ornamentals as asters（紫苑）, chrysanthemums（菊花）, geraniums（天竺葵）, gladiola（剑兰）, larkspur（翠雀）, marigolds（万寿菊）, petunias（矮牵牛花）, phlox（夹竹桃）, zinnias（鱼尾菊）. Weeds may also harbor the virus; examples include wild ground cherries（野生酸浆草）, milkweed（马利筋）, pokeweed（美洲商陆）, and spurge（大戟）.
2.1.1 Symptoms • Plants can be infected at any stage of growth. When seedlings become infected they remain small, turn yellow, and die. Symptoms on older plants include dwarfing of plants, mottling, distortion, and downward curling leaves. Infected cucumber fruit reveals patterns of white blotches interspersed with dark green spots that are raised into conspicuous blisters. Infected fruit has a bitter taste（苦味） when eaten or becomes soggy （黏湿的）when pickled（腌制）. The fruit of infected summer squash plants is knobby（多节的） and distorted. Muskmelon and winter squash fruit rarely shows symptoms but usually is off-color and bitter-tasting. Watermelons infected with the virus usually show less stunting and mottling than cucumbers and muskmelons.
2.1.2 Disease Cycle The virus survives the winter in roots of susceptible plants, in greenhouses, and possibly in seeds of wild cucumbers. Aphids are the principle means of dissemination of the virus. Mosaic also can be transmitted by the cucumber beetle and by workers picking cucumbers. As aphids feed on virus-infected hosts, they become contaminated with the virus and subsequently spread it to healthy plants.
2.1.3 Control Weed control around and within the planting is important, since the virus can survive in many weed plants. Most effective method of control of common mosaic on cucumber is planting resistant varieties. These types of cucumbers are resistant: Marketmore 70, Marketmore, Gemini F1, Sweet Slice, and Victory. Cucumber picking types that are resistant include Pioneer, Spartan, Valor, and Sulty.
Lettuce, endive (escarole，苣荬菜), spinach（菠菜）, and celery（芹菜）are members of different plant families, but share many of the same viral problems. most of these crops are produced on organic soils, but they may also be grown on mineral soils. Because these crops are often planted near one another, the viruses or other agents infecting the crops are the same, and they are transmitted by the same insect vectors.
The viruses infecting lettuce （莴苣/生菜） or endive(苣荬菜） include lettuce mosaic virus (LMV)，cucumber mosaic virus (CMV), turnip（芜菁） mosaic virus (TuMV), broadbean wilt virus (BBWV，蚕豆萎蔫病毒), Bidens mottle virus (BiMV，鬼针草斑驳病毒), and beet western yellows virus (BWYV，西部甜菜黄疸病毒). Additional organisms infecting lettuce are lettuce big vein agent (LBVA) and aster（紫苑） yellows mycoplasma (AY). • The two most important virus diseases of celery are CMV and celery mosaic virus (CeMV); the occurrence of aster（紫苑 ）yellows mycoplasma (AY) depends upon the prevalence of the leafhopper vector.
3.1 Lettuce and Endive Diseases • Lettuce mosaic virus (LMV) can infect all lettuce types (crisphead, cos, Boston, bibb, and leaf) as well as escarole and endive. LMV is seedborne in all lettuce types, but not in endive; and infected seed probably serves as the chief source of inoculum, although weed hosts are also recognized.
Plants infected through seed are referred to as seedborne "mother" plants because they can serve as virus reservoirs from which aphids can spread the virus to surrounding healthy plants. The mother plants show early mosaic symptoms, are stunted, and never develop marketable heads. Secondary infected plants show mosaic, leaf puckering, and deep or accentuated serration of the leaf margins in most lettuce types (fig. 1). Endive heads may be stunted, but generally show only mild mosaic symptoms.
Two methods for controlling LMV are currently in use. First, because of the prevalence of seedborne virus, lettuce seed is tested by three methods (direct reading of lettuce seedlings, inoculation of ground-up seed with a sensitive indexing host, or, more recently, a serological technique) to ensure that each lettuce seed lot contains no infected seeds in a sample of 30,000 seeds (MTO or mosaic tolerance 7ero). A more successful control measure is the incorporation of virus resistance into the principal lettuce types grown on both mineral and organic soils.
3.1.1Cucumber mosaic virus(CMV) is the most important virus disease of lettuce and the most difficult to control. Because the virus can infect more than 775 plant species, including many weed species (chickweed[繁缕], milkweed[马利筋], purslane[马齿苋], dayflower[鸭跖草], etc.), many reservoirs（蓄水池）exist to overwinter the virus near growers‘ fields. Winged aphids（有翅蚜）transmit the virus from bordering weedy areas to the crop to create primary infection sites.
Secondary infections account for most virus spread and occur when migrant aphids spread the virus from infected primary lettuce plants to surrounding healthy plants. When large migrant-aphid flights occur. an entire field may become almost totally infected in a matter of weeks. • Although the symptoms of CMV bear some resemblance to those of LMV, the mosaic is more intense with veinal chlorosis and, frequently, veinal browning and necrosis when temperatures in the mid-50s or lower occur (fig 2) .
Because weed hosts harboring CMV are common, a concerted effort must be made by adjoining lettuce growers to make weed sanitation and other programs effective. Weed control must be done before or soon after the crop is planted. Because migrant aphids account for much of the virus spread and, normally, do not colonize lettuce, insecticidal sprays are only minimally effective in reducing spread. Reducing primary inoculum in the crop is so critical in delaying virus epidemics that major emphasis should be applied at this point. Make earliest plantings downwind from weedy border areas, so that older lettuce plantings do not serve as reservoirs of virus for subsequently planted crops.
Although not extensively tested on lettuce, mineral oil sprays are known to significantly reduce secondary virus spread in many nonpersistent, aphid-transmitted virus diseases such as CMV, provided the treatment is begun before primary inoculum exceeds 8-10%, and heavy virus pressure is not present in adjoining fields. Spraying border areas with mineral oils is also beneficial. The technique used is sophisticated; consult extension personnel for complete details.
3.1.2Turnip mosaic virus(TuMV) is not a common disease of lettuce varieties, but escarole and endive are very susceptible. Susceptibility to TuMV was inadvertently transferred to some lettuce varieties by breeding for resistance to downy mildew. TuMV typically causes veinal chlorosis and necrosis of lower leaves of both lettuce (fig. 3) and endive, severely stunting the plants and making the heads unmarketable. Cruciferous weed hosts (shepherdspurse, etc.) are the principal source of inoculum for this aphid-transmitted virus, and controls discussed under CMV are applicable.
3.1.3Broadbean wilt virus(BBWV), like CMV, can cause a serious disease of lettuce in susceptible varieties, resulting in large patches of infected plants (fig. 4). Symptoms are similar to those of LMV and CMV, but with low field temperatures the infected plants develop severe veinal necrosis. Some varieties are very susceptible whereas others display good tolerance to infection. BBWV is transmitted by aphids in a nonpersistent manner. The major weed hosts in New York have been identified as broadleaf plantain (Plantago ma/or) and buckhorn plantain (P. Ianceolata).
3.1.4 Bidens mottle virus (BiMV) has been recovered from lettuce and endive in Orange County, but is not widely distributed in the state. Symptoms caused by BiMV are similar to those of LMV in both lettuce and endive. The lettuce variety Valmaine (a romaine or cos type) is naturally resistant to BiMV and has been used in breeding programs for joint resistance to BiMV and LMV. BiMV is not seedborne in either lettuce or endive, and principal weed hosts for this aphid-transmitted virus are beggarticks, pepperweed, and others.
3.1.5 Beet western yellows virus(BWYV) is an important virus disease of lettuce and endive in California, Arizona, and Florida. Although not formally recognized in this state, weed hosts such as shepherdspurse are present. BWYV causes a pronounced chlorosis of the outer leaves of lettuce (fig. 5) and escarole. Although spread by many of the same aphid species that are vectors for the other common lettuce viruses, BWYV is transmitted in a persistent manner, meaning that a longer time is required to acquire and then transmit the virus. Insecticidal sprays would presumably help to delay the spread of this particular virus disease.
3.1.6 Lettuce big vein agent(LBVA) is probably caused by a virus, although the virus particle has not been seen with the electron microscope. This disease differs greatly from the other lettuce virus diseases; its vector in nature is a soilborne fungus (Olpidium brassicae). The virus is acquired by the fungus protoplast in virus-infected roots and is carried by fungal zoospores, which infect healthy roots and release the virus. Infected lettuce plants are striking, with leaves showing enlarged and lightened veins and petioles (fig. 6). Leaves are also thickened and distorted, and heads are smaller than normal. Because of the soilborne nature of the disease, no practical control measures are available.
3.1.7Aster yellows mycoplasma(AY), formerly thought to be caused by a virus, can infect a wide range of plants in addition to lettuce and celery. The disease results in strikingly yellowed and stunted plants (fig. 7), which are unmarketable. This disease may occur annually, its extent being determined by the preponderance of the aster leafhopper vector (Macrosteles J:ascifrons). Controls are based upon the use of insecticidal sprays to reduce the only known vector.