Circular Circ-1184

Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean

Aphid-Transmitted Viruses of Cucurbits inFloridaTom Kucharek and Dan Purcifull, Respectively Professor and, Professor, PlantPathology Department, University of Florida, Gainesville, 32611. April 1997(Revised 2001)

Cucurbit crops (Cucurbitaceae, cucumber &gourd family) are of major importance to thestate of Florida and its agricultural communi-ties. For the 1994-1995 marketing year, water-melons, squash, and cucumbers were valuedat $146,079,000 in Florida. This was 9.9 percentof the total value of vegetables at the farm-gateand does not include other cucurbits such ascantaloupes and pumpkins. Florida producesmore watermelons than any other state. In ad-dition to several fungal diseases, viral diseaseshave strongly interfered with the establishmentof a pumpkin industry in Florida.

For at least the past three decades, the mostimportant impediment to consistent productionof squash, watermelons, and some other cucur-bits in Florida, has been the annual occurrenceof aphid-transmitted viruses. Both yield andfruit quality have been reduced significantly bythese viruses. Growers constantly ask about theavailability of useable control measures. Inter-estingly, cucumber has not incurred many vi-ral problems for the past three decades or so,except where some exotic varieties were usedin the greenhouse or in the field.

On a world-wide basis, more than 50 virusesand four viroids have been reported to infect atleast one member of the cucurbit family. At leasthalf of the viruses are economically importantpathogens of commercial cucurbits. Currently,five aphid-transmitted viruses of cucurbits have

been found in Florida. These viruses are com-posed of a ribonucleic acid core encapsulatedby a protein “coat”. Table 1 summarizes thecurrent nomenclature of these viruses.

Papaya ringspot virus type W (PRSV-W),previously known as watermelon mosaic virusone (WMV 1), and watermelon mosaic virustwo (WMV 2) have been the most commonlyoccurring aphid-transmitted viruses in Floridafor the past several decades. Zucchini yellowmosaic virus has occurred in Florida sporadi-cally since the fall of 1981. Cucumber mosaicvirus has been less prevalent during the past30 years when compared to earlier periods, butit has recently increased somewhat in peppersand tobacco in north central Florida. Little isknown at this time about the unnamedpotyvirus, except that it is distinct from the oth-ers.

Table 1. Aphid-transmitted viruses foundnaturally in Florida-produced cucurbits.

Virus Acronyms

Paypaya ringspot virustype W PRSV-W(WMV 1)

Watermelon mosaic virus2 WMV 2(WMV)

Zucchini yellow mosaicvirus

ZYMV

Cucumber mosaic virus CMV

Unnamed potyvirus none

Transmission

The predominant method of natural trans-mission of the viruses listed in this publicationin the field is by aphids (sometimes called plantlice, Figure 1) in a non-persistent manner. Thismeans the viral particles (virions) are acquiredby the aphid on its stylet (tubular mouthpartwhich is inserted into plant tissue for feedingpurposes) and are retained in association withthe stylet for “short” periods of time. Althoughan aphid is likely to lose its ability to transmitthe virus within a few minutes or a few hours,longer periods of retention can occur. For ex-ample, two species of aphids, Aphis gossypii andMyzus persicae, were able to transmit ZYMV atlevels of 12.5% and 5.6%, respectively, for 3hours at 68° F (21° C) but not at all after 5 hours.In contrast, at 46° F (8° C). A. gossypii could trans-mit ZYMV at a 1% level for up to 20 hours andM. persicae could transmit at a 1% level for upto 30 hours. The actual retention time for sur-vival of virions on stylets is likely to vary.

In watermelon, no difference in susceptibil-ity to infection to WMV 2 existed in plants from1 1/2 to 7 1/2 weeks old in an experimentalsituation. However, symptoms develop at dif-ferent rates depending upon the age of the plantat the time of infection. Older plants have anuneven distribution of virus and lower levelsof virus when compared to the younger plants.However, it is common for plants much olderthan 7 1/2 weeks in commercial fields to be-come infected with WMV 2 or other viruses.Low relative humidity (e.g. 40%) has reducedtransmission of ZYMV by the aphid species A.gossypii but not by the aphid species M. persicae.The retention time of non-persistent viruses istypically further shortened as feeding periodslengthen. Thus, feeding on non-host tissue af-ter acquisition of the virus, and before the sus-ceptible crop is reached and probed by theaphid, is likely to deplete viral inoculum. Re-tention time is affected by so many variablesthat it is not advisable to provide generaliza-tions for a specific situation.

Aphids are efficient vectors of viruses. First,they are sometimes able to acquire viruses inless than eight seconds of probing and trans-mit them in less than four seconds. Probes of15 to 60 seconds can enhance transmission, butlonger probes tend to result in lower levels oftransmission. However, aphids may feed forhours at one site if the host is suitable. Second,aphids reproduce rapidly, resulting in millionsof aphids in a short period of time. Thirdly, alate(winged forms, Figure 1) aphids are capable ofmovement from one location to another eitherby flying or by passive movement with thewind. Winged forms tend to develop in re-sponse to crowding from other aphids or whenplants deteriorate. Fourth, aphids can feed ona number of plants within a short period of time.They have “personal” feeding preferences andwill move from plant to plant of the same ordifferent plant species until they find a satis-factory feeding site. Fifth, numerous species arecapable of vectoring viral diseases. For ex-ample, at least 60, 42, 25, and 12 species ofaphids are known to be capable of vectoringCMV, WMV 2, PRSV-W, and ZYMV, respec-tively, on a world-wide basis.

In Florida the number of aphid speciesknown to vector WMV 2, PRSV-W, and ZYMV,are 16, 9, and 13, respectively. Most of the spe-cies do not reproduce on watermelon, but theymay land and probe while searching for theirpreferred host. It is noteworthy that of thesespecies only a few species may account for amajority of the transmissions in any given situ-ation.

Aphid transmission of viruses is affected byan enormous number of variables, and the in-teractions between variables. For example, theefficiency of aphid transmission may or maynot be altered when the aphid is carrying morethan one virus. In one situation, maximumaphid transmission of CMV into cantaloupewas delayed by one week when aphids ac-quired a combination of CMV and WMV 2

when compared to CMV alone. Another ex-ample is the extensive variation that exists intransmission efficiency between aphid speciesand clones of aphids within species as they in-teract with other variables. For example, theamount of transmission of WMV 2 from 10 dif-ferent cucurbit species to cucumber with theaphid species Aphis gossypii varied between 8and 80% in one test.

The percentage of naturally occurring, mi-gratory, alate (winged) aphids that carry trans-missible virions. has been measured to be aslow as 1.5% for ZYMV and WMV 2 in one year.In another year, none of 15,122 aphids wasfound to be carrying transmissible virions. In asituation with Florida-produced watermelons,no aphids with WMV 2 could be detected atthe earliest time of epidemic development.However, when 4% of the plants in a commer-cial field had symptoms, 99% of the aphids inthe species Aphis middletonii were found to carryWMV 2.

Aphid populations and viral diseases com-monly progress over time in a sigmoidal fash-ion, as do many other diseases. Typically,flights of large populations of aphids occurduring dry periods accompanied by moderatetemperature. Such a situation occurs on a regu-lar basis in south Florida during the fall (Octo-ber-November) and in south, central and northcentral Florida during the spring (March-May).Within one to two weeks after these aphidflights, viral symptoms are likely to appear.Nearby fields will vary considerably with re-spect to intensities and times of influx of peakaphid flights.

Gradients of viral diseases are commonlyencountered in fields, particularly during theearly aspects of an epidemic. For example,higher levels of disease often occur near edgesof fields or near known sources of inoculum.These gradients become less evident over timeas the plants throughout the field become in-fected from secondary spread.

Primary inoculum of virus comes from weedhosts, abandoned cucurbit fields, nearby cucur-bit fields, or volunteer cucurbits. Viruses areintroduced into a crop field by flights of aphidswhere often an extremely low number of aphidscontain transmissible virions. It is generallybelieved that because of the “short” retentiontimes of virions on stylets, sources of virus arelikely to be near the field. However, the reten-tion times of 20-30 hours in some situationssuggests that more distant sources of virusmight also be involved.

The apparent rapid increase in the incidenceof diseased crop plants that we commonly seeis the function of secondary spread of virus. Thepercentage of aphids that have acquired virusfrom infected crop plants increases as the inci-dence of disease increases. To exemplify howfast epidemics develop, several epidemics inwatermelon required only 20 days to progressfrom zero to 100% incidence.

Leafminers have been shown to transmitWMV-2 and PRSV-W in squash. While this typeof transmission can occur, it has not been con-sidered a major method of infection in Floridain cucurbits.

Mechanical movement of plant sap from oneplant to another is another method by whichthese viruses are spread. This can happen any-time people or equipment move within the fieldand make contact with infected plants. Such hashappened with CMV where the act of harvest-ing cucumbers increased the incidence (num-ber of infected plants) of disease three-fold.Also, for experimental work, rubbing plant sapfrom an infected plant to a healthy plant is acommon way of transmitting WMV-2, ZYMV,PRSV-W, and CMV (Figure 8). While we nor-mally think that aphid-transmission is highlyimportant for the spread of these viruses, wemay be underestimating spread of viral dis-eases by mechanical means—at least wheremultiple harvests occur or where movement ofplant sap occurs in other ways.

Seed transmission has not been demon-strated for PRSV-W or WMV 2. Seed transmis-sion of CMV has been demonstrated in someweed species, which could conceivably serveas a mechanism to perpetuate the virus in weedhosts. Also, strains of CMV were found to beseedborne in cowpea and bean. Seed transmis-sion has occurred with ZYMV at 0.048% in onestudy but in other studies seed transmissiondid not occur. Even though seed transmissionof ZYMV is infrequent and at low levels, it doesprovide a mechanism for ZYMV or any of itsrelated strains or any other virus to be distrib-uted to new locations on a world-wide basis.

A virus that is seed-transmitted in canta-loupe and squash and that has occurred rarelyin Florida is squash mosaic virus (SMV). SMVis also transmitted by cucumber beetles, but notby aphids. We mention SMV here becausesometimes individuals talk about mosaicsymptoms in squash as “squash mosaic”. Themosaic viruses commonly seen in squash inFlorida are WMV 2, PRSV-W, ZYMV, andCMV, not SMV.

Symptom Expression

Upon mechanical or aphid transmission ofviruses to susceptible plants, infection becomessystemic throughout the plant. Expression ofsymptoms can be modified when mixed infec-tions of two or more viruses occur within thesame plant. Mixed infections with PRSV-W,WMV 2, or ZYMV occur fairly often. Variationsin temperatures, light regimes, crop varieties,viral strains, host vigor, titer (concentration) ofvirions in host tissue, and many other factorsinteract with each other in complex and oftenunknown ways to cause variation in symptomexpression in plants. For example, with PRSV-W, WMV 2, and CMV, symptoms occurred 3 to7 days after inoculation at 105° F (40.6° C), com-pared to 8 to 11 days at 65° F (18.3° C). How-ever, progressive diminution of symptoms oc-curred with five weeks of exposure to tempera-tures between 86° F (30° C) and 105° F. At the

highest temperatures, symptoms were absentor barely visible. At 65° F, symptoms main-tained their original integrity.

The first symptoms can appear as early asthree to four days after inoculation or as late as18 or more days. Typically, plants inoculatedwith infectious sap manually or via aphid trans-mission will display symptoms seven to 12days later. The absence of symptoms is no guar-antee that the plant is not infected. An asymp-tomatic (without symptoms) plant may be in-fected but the incubation period for the virusmay not have been completed or in some envi-ronmental conditions, the symptoms are notdisplayed (sometimes referred to as “masked”).Symptoms may be present in one portion of aplant but not in another portion of the sameplant. Symptomatic plants are more likely toserve as sources of inoculum.

One should never rely on symptoms aloneto identify specific viruses. Also, chemical dam-age from certain pesticides (usually herbicides)or nutrient imbalances can cause symptomssimilar to those caused by viruses. Table 2 listssymptoms that are caused by PRSV-W, WMV2, ZYMV, or CMV.

Variations of mosaic and mottle symptomsin leaves are common and are exemplified inFigures 2, 3, 4, 5, 7, 8, 9, 10, & 14. Shading theleaves on sunny days usually aids in contrast-ing the lighter and darker portions of mosaicpatterns in leaves (Figures 4 & 8). Vein band-ing is also common and is shown in Figures 3& 5. Leaf bronzing (Figures 4 & 5) has been com-mon in recent years in watermelon. Necrosesof margins or interior portions of leaves alsooccur (Figure 6). Crinkling (rugosity) of leavesis fairly common (Figure 6). Linear or stippledgreening of fruit is very common (Figures 2, 7,10, 11, 12, 14, 15, & 16). Bumpiness of fruit isfairly common (Figures 12 & 13). Ringspottingcan occur on leaves or fruit (Figures 9, 10, 11, &15). Chlorotic spotting is sometimes inducedin the foliage (Figure 8).

Papaya ringspot virus type W(PRSV-W, WMV 1)

It is not known when PRSV-W first occurredin Florida but it is likely to have been in Floridaduring the early 1930’s. Currently, PRSV-Wpredominates in the southern half of the penin-sula throughout the year. PRSV-W occurs in thenorthern half of the state but its appearancethere is usually during the late spring, summer,or fall months. It may occur early in the springin central or northern Florida if infected trans-plants from south Florida are used. PRSV-Whas been found as far north as New York, butoverall, PRSV-W is not found as commonly inother areas of the United States as is WMV 2.PRSV-W tends to occur in tropical climates.

PRSV-W infects 40 plant species with 38 ofthem being in the cucurbit family and two ofthem in the goosefoot family (Chenopodiaceae).Type W strains of PRSV do not infect papayawhereas type P strains (PRSV-P) do infect pa-paya. PRSV-W and PRSV-P are very closelyrelated serologically.

PRSV-W is commonly found in a perennial,cucurbitaceous weed, balsam apple (Momordicacharantia, Figure 17) in south Florida. Balsamapple is a vine that is an excellent carry-overhost for PRSV-W and is commonly infected withthis virus. This weed is cold-sensitive and doesnot grow in north Florida. Thus, it is not a di-rect source plant for PRSV-W in north Florida.It is particularly common in Palm BeachCounty. It is found growing along irrigationcanals, hedge rows, wind breaks, old cultivatedfields, disturbed land, groves, or fences.

Table 2. Some symptoms that are caused byPSRV-W,WMV 2, ZYMV OR CMV.

*Yellow or green; # for cantaloupe; ** Microscopicwithin cells

Leaf Fruit Other

Mosaic Mosaic Fewer flowers

Mottl ing Green mott l ing Plant stunting

Interveinalchlorosis B u m p s

FlowerClustering

Leaf s trapping S h a p edistortion

Greenishflowers

Rugosi ty(crinkled) Smaller s ize

ShortenedInternodes

Marginalnecros is Lower sugar Deformed seed

Interveinalnecros is Ringspots

Slower plantGrowth

Bronzing Green spotsPlant

deterioration

Reduced s ize Off color Shorter vines

Curling Fewer seedsDistortedStigmas

Vein banding* External cracksDistortedStamens

Deep serrat ions AbnormalNetting #

Flower necros is

Malformation Lower weightVariable f lower

Color

Ringspots Fewer fruit

Chlorot icspott ing Greenspott ing

Dark greenspott ing

InclusionBodies**

InclusionBodies**

Another weed host for PRSV-W is creepingcucumber (Melothria pendula, Figure 18). Thisweed grows as an annual or perennial in fence-rows, ditchbanks, abandoned fields, woods,and other non-crop sites. It occurs in north, cen-tral and south Florida. Creeping cucumber iscommonly infected with PRSV-W in southFlorida and sometimes in central Florida. It is amajor weed host for PRSV-W and WMV 2 insouthwestern Florida. Infected creeping cucum-ber along edges of watermelon fields has beenstrongly associated with outbreaks of PRSV-Win southwest Florida even before peak periodsof aphid flights. Although freezing weather cankill the vines in south Florida, regeneration ofnew vines allows the plant to be a continualsource of virus because of the systemic infec-tion. Because this weed is cold sensitive and ittypically does not grow in the northern portionsuntil after the spring crop is harvested, it is notlikely to be a source of PRSV-W or any othervirus for the spring crop unless an extremelymild winter occurs. It has not yet been identi-fied as a source plant for virus infections in thefall crop for north Florida.

Watermelon Mosaic Virus 2 (WMV 2)

WMV 2 was first distinguished from WMV I(PRSV-W) in 1965. It is serologically distinctfrom PRSV-W. As with most viruses, multiplestrains of WMV 2 exist. WMV 2 has been themost common aphid-transmitted virus of cu-curbit crops in north Florida, in the UnitedStates, and in other places in the world. For acouple of years in the early 1960s, WMV 2 wasfound commonly even in south Florida.

WMV 2 has a much broader host range thandoes PRSV-W. Over 160 dicotyledonous spe-cies in 23 families are susceptible. Many le-gumes are susceptible. In central Florida, WMV2 has been found in ornamental hibiscus plants,which are perennials and could serve as carry-over hosts. Annual plants found to be infectedwith WMV 2 in central Florida include citron,

showy crotalaria, hairy indigo, and one-leaf clo-ver. Balsam apple and creeping cucumber (seePRSV-W section about these two weeds), bothnatural carry-over hosts for PRSV-W, have notbeen found naturally infected with WMV 2 inthe field, even though both are susceptible. Oneof the probable reasons that WMV 2 has notbeen found in creeping cucumber or balsamapple is that WMV 2 is not found commonly insouth Florida where creeping cucumber andbalsam apple abound. Also, the growth ofcreeping cucumber in north and central Floridafollows the spring cucurbit crops.

Zucchini Yellow Mosaic Virus (ZYMV)

Zucchini yellow mosaic virus was first foundin northern Italy in 1973 and in Florida in thefall of 1981. ZYMV has occurred throughout themain cucurbit-growing areas of Florida and theUnited States, including Hawaii. ZYMV hasoccurred in Africa, the Middle East, Asia, Aus-tralia, and South America. It can be a problemin both tropical and temperate areas. Duringthe early 1980’s, ZYMV appeared in numerousplaces in the world in a short period of time(see an above section about seed transmissionof ZYMV). Recently, ZYMV remains as an im-portant virus, but it has not dominated the spec-trum of cucurbit viruses in Florida as do WMV2 and PRSV-W.

One of the initial concerns about ZYMV wasthe intensity of the resulting symptoms. ZYMVseems to be associated with severe symptoms,i.e. excessive leaf deformation, intense spotting,pronounced bumpiness of fruit, etc.

Although ZYMV has a large host range asdetermined by experimental inoculations, itseems to be a commercial problem only in cu-curbits. It has occurred naturally in creepingcucumber in central Florida. ZYMV has notbeen found naturally in balsam apple nor hasthis host been susceptible when inoculated withisolates from Florida. However, balsam applehas been infected by isolates in France.

Cucumber Mosaic Virus

Cucumber mosaic virus (CMV) has an ex-tremely wide host range and is found world-wide. About 800 monocotyledonous and di-cotyledonous plant species in 40 families aresusceptible. Perennial ornamentals such asgladiolus, periwinkle, easter lily, and amaryl-lis are examples from the host range.

Like ZYMV, CMV is capable of causing ex-tremely severe symptoms. CMV was identifiedin numerous vegetables, ornamentals, andweeds in south and central Florida from the1930s to the 1950s, but it was particularly a prob-lem in celery (then called southern celery mo-saic virus on celery and CMV on other crops)at that time. By the late 1950s and early 1960s,progressively fewer reports about CMV wereavailable. From the mid- 1960s till the late 1980s,CMV was present but of no economic conse-quence to cucurbits, vegetables, or agronomiccrops in Florida.

CMV had a negative economic impact on thegladiolus industry which was solved in partwith the continual renewal of plant materialswith tissue culture-generated plants that werefree of CMV. CMV is considered to be the causeof the demise of the Easter lily business inFlorida

During the 1990s, the incidence of CMV hasincreased considerably in squash, peppers, andtobacco in the Alachua County area. Interest-ingly, dayflower (Commelina communis, Figures19 & 20) has been found commonly in AlachuaCounty during this time in lawns, gardens,open woods, fields, and hedgerows, often inwet or shaded sites. It can grow as an annual orperennial. A different species of dayflower(Commelina nudiflora) was found to be associatedstrongly with epidemics of CMV in celery inthe 1930s. In recent years, C. benghalensis hasbeen infected with CMV in Hamilton Countyand associated with severe epidemics of CMV

in tobacco. Over the past 30 or so years, CMVhas not been a problem in cucurbits in Florida,including cucumber. Some consider CMV tospread more slowly than the other viruses, butif infected Commelina spp. are in the vicinity,CMV can be devastating in a short period oftime. Strains of CMV vary in their ability to in-fect watermelon, but for at least the past threedecades, CMV has not been a problem in wa-termelon in Florida.

Control

Resistant varieties are the best controls forviral diseases. Resistance to CMV became avail-able in 1928. Since that time, breeding programshave successfully incorporated resistance toCMV, other viruses, and fungal diseases inpickling and slicing types of cucumbers. Thisis the primary reason why viral problems incucumber are not common in Florida.

Recently, useable and stable resistance toviral diseases in varieties of yellow and zuc-chini summer squashes has become available.The sources of resistance have been of twotypes, host resistance and pathogen-derivedtransgenic resistance. Genes from breedinglines of squash with host resistance to WMV 2,ZYMV, and CMV have been incorporated intohorticulturally acceptable varieties (e.g. ‘Divi-dend’& ‘Revenue’, zucchini types). Near 100%control of WMV 2 was measured in a test inFlorida. Additional varieties will be availablein the future.

With another form of host resistance, a mask-ing of greening in fruit occurs in infected plants.Thus viral symptoms occur in the leaves butare delayed in the fruit. This form of resistancehas been noted as having the “precocious-yel-low-character” (e.g. varieties ‘Multipik’,‘Supersett’, ‘and Meigs’) and has been effectivefor WMV 2. In the future, more varieties of thistype may be available.

Another source of resistance has been de-

rived from the viruses themselves. It has beenknown for some time that introduction of coatprotein of some viruses into plants can conferan “immunogenic”-like response in plants. Byincorporating segments of nucleic acid from thevirus that encode for the protein coat into “mes-senger” bacteria which in turn deliver a newform of the gene into plant cells, plants are pro-duced, via -tissue culture, that carry these newgenes that confer resistance. This new type ofresistance is referred to as pathogen-derived,“transgenic resistance” (e.g. as developed in theyellow summer squash varieties ‘Prelude II’,‘Liberator III’, and ‘Destiny III’). In a test inFlorida 100% control of WMV 2 was obtainedwith all of these varieties. Additional varietiesare likely to be available in the future.

Although these forms of resistance have beensuccessful in Florida, one should remember thatthe incorporation of resistance is subject to“overthrow” by new strains of the virus forwhich resistance was not incorporated origi-nally. Also, resistance to one virus does nottypically confer resistance to other viruses. In-terestingly, some coat protein-mediated resis-tance to one virus has conferred some resistanceto other viruses. A lot of uncertainty still existsabout transgenic resistance.

With the current resistance to WMV 2,ZYMV, and CMV in summer squash, geo-graphical areas that commonly have these vi-ruses will have some control. Such is the casein north Florida in the spring. However, in thefall in north Florida, when PRSV-W is likely tobe present, the resistance to the three other vi-ruses will not control PRSV-W. Likewise, insouth Florida where PRSV-W abounds all year,the varieties with resistance to WMV 2, ZYMV,and CMV will not suppress PRSV-W. Resis-tance to PRSV-W in squash is being pursued.

Some resistance to WMV 2 and ZYMV hasbeen found in breeding lines of watermelon.At the present time, these lines lack horticul-tural acceptance, but work is being done to de-

velop varieties of watermelon with resistanceto WMV 2.

Plow down abandoned fields as soon asharvesting is complete. This will eliminate po-tential sources of inocula for viruses and otherpathogens.

Destroy weed hosts (primarily creeping cu-cumber and balsam apple) in the near vicinityof the field. Weed hosts can be major sourcesof inocula for aphid transmission.

Destroy volunteer cucurbits in the vicinityof your production fields or on the farm as theycan serve as reservoirs for viral inocula.

Windbreaks have been used with partialsuccess to reduce spread of viral diseases fromone site to another. Windbreaks are particularlyuseful where sequential plantings occur. Whenaphids move from their original site they maybe “trapped” by the windbreak plants wheretheir feeding might deplete the virus from theirstylets before the aphids move to the suscep-tible crop. This technique has been used suc-cessfully in southeastern Florida where spacedwindbreaks of tall grasses, sugarcane, palmtrees, melaleuca (naturally occurring), etc. run-ning north and south trap some aphids thatmigrate in conjunction with predominate east-erly winds. It would be best not to usemelaleuca as a windbreak because it is a goodhost for the aphid, Aphis gossypii.

Roguing infected plants at first occurrenceof disease can be attempted, but generally thistechnique does not keep pace with an ongoingepidemic. Also, the risk of spreading diseaseby shaking off aphids or by mechanical spreadexists.

Insecticidal sprays are not effective for theviruses mentioned herein. By the time the aphidhas received a lethal dose, the virus has beentransmitted. Most studies have demonstratedeither no control, extremely slight control, or

increased disease. The latter occurs when theaphids are agitated by the spray and thus moveto a new site where they inoculate other plantsbefore dying.

Oil sprays have been known for some timeto reduce progress of aphid-transmitted dis-eases. Currently, the best such oil is one knownas ‘JMS Stylet Oil’. It is effective if multiplesprays are applied at a spray pressure of 400psi with the specific nozzles indicated on thelabel. Reducing primary infections by startingthe spray program early will be beneficialwhereas waiting until the incidence of diseasebecomes 10% or more will not provide control.Whenever oils are used, risk of phytotoxicityexists from the oil or its interaction with othersprayed materials.

Row covers have been used with partial suc-cess in reducing aphid-transmitted viruses incucurbits. They do so by being a mechanicalbarrier and by the repellency of aphids with thelight and somewhat shiny color of the rowcover. However, as soon as the covers are re-moved for accommodating bee visitation forpollination, one can expect infection to occurwith symptoms beginning in 1 to 2 weeks. In-terestingly, row covers left on zucchini squashfor up to 10-12 days after initial flowering re-sulted in higher yields compared to those with-out row covers because delaying disease over-rode the benefits of an earlier removal of thecovers for pollination. The benefit from rowcovers is that it delays the initial onset of dis-ease which could at least increase yields of theearly pickings. The use of closed systems thatexclude aphids but allow for activity of beeswould be an excellent control method for theseviral diseases.

Reflective mulches that have some bright-ness and shine are repellent to aphids and candelay the onset of viral diseases. Usually silveror aluminum mulches will be more repellentto aphids than white mulches. Once the mulch

is covered by plant growth, the effect is lost,but the delay in overall disease progress maybe worthwhile. Reduction of disease incidenceof WMV 2 has been as high as 97% at a giventime and averaged 63% in a test on squash inCalifornia with aluminum mulch. In tests inAlabama, delays of 10 to 13 days for onset ofviral epidemics from PRSV-W, WMV 2, ZYMV,and CMV occurred with aluminum mulch. InFlorida, reductions of viral disease by 72 and94% have been measured with use of alumi-num foil. In another test in Florida, no signifi-cant benefit was measured with white on blackplastic. Combining the use of row covers witheither white or aluminum mulches has signifi-cantly reduced viral diseases in cucurbits inFlorida and Oklahoma.

Protection (trap) crops, where a non-hostplant is inter-planted with the cucurbit crop totrap aphids, have reduced viral diseases. Thistechnique could be useful in specialty plantingssuch as in gardens or “organic” productionsites. It would be highly labor intensive and notcompatible with logistical operations andequipment.

Time of planting or transplanting can influ-ence levels of disease. In north Florida, earlierplanting or transplanting will allow the plantto be at a further stage of development beforeaphid flights begin. At least the earlier pickingsmight escape some damage. While transplant-ing rather than planting seed does not alwaysincrease yield, transplanted crops tend to haveearlier yields.

Healthy transplants should always be used.Failure to use disease-free transplants com-monly results in significant loss in yield andquality. Avoid purchasing plants from placesthat have had a history of producing diseasedtransplants.

Disease-free seed should always be used.Unfortunately, the grower has little recourse onthis matter except to ask questions of represen-

tatives from the seed companies. Except forSMV and ZYMV, seed transmission of mosaicviruses in cucurbits has been of little conse-

quence in Florida.Integration of the above tactics should be

done to the extent possible. Integrated diseasecontrol systems should

Figure 1. Winged aphid. Figure 2. Mosaic in squash leaf and fruit(ZYMV).

Figure 3. Green vein-banding in watermelonleaf (WMV-2).

Figure 4. Leaf mottling and bronzing inwatermelon leaf (Note: enhancement ofsymptoms with shading).

Figure 5. Bronzing and dark and light veinbanding in Watermelon leaves (WMV 2).

Figure 6. Necrosis and rugosity in pumpkinleaf (WMV 2 and PRSV-W).

Figure 7. Vein banding in leaf and mottlingin butternut squash leaves and fruit(ZYMV).

Figure 8. CMV-inoculated resistant andsusceptible squash.

Figure 9. Ringspotting in immature pump-kin fruit (WMV 2 and PRSV-W).

Figure 10.Ringspotting in mature pumpkinfruit (WMV 2 and PRSV-W).

Figure 11. Mosaic in watermelon fruit(WMV 2).

Figure 12. Mosaic and bumps in watermelonfruit (ZYMV + WMV 2).

Figure 13. Greening and bumps in yellowsummer squash fruit (ZYMV).

Figure 14. Mottling in yellow squash fruit(ZYMV).

Figure 15. Mottling in acorn squash fruit. Figure 16. Mottling in yellow summersquash fruit.

Figure 17. Balsam Apple. Figure 18. Creeping cucumber.

Figure 19. Small dayflower. Figre 20. Dayflower with flowers.