Leafroll Disease is Spreading Rapidly in a Napa Valley Vineyard

7/30/2019 Leafroll Disease is Spreading Rapidly in a Napa Valley Vineyard

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http://escholarship.org/http://escholarship.org/uc/search?creator=Rowhani%2C%20Ahttp://escholarship.org/uc/search?creator=Sim%2C%20Sue%20T.http://escholarship.org/uc/search?creator=Weber%2C%20Edhttp://escholarship.org/uc/search?creator=Golino%2C%20Deborah%20Ahttp://escholarship.org/uc/item/4z04w88rhttp://escholarship.org/uc/search?creator=Rowhani%2C%20Ahttp://escholarship.org/uc/search?creator=Sim%2C%20Sue%20T.http://escholarship.org/uc/search?creator=Weber%2C%20Edhttp://escholarship.org/uc/search?creator=Golino%2C%20Deborah%20Ahttp://escholarship.org/uc/anrcs_californiaagriculture?volume=62;issue=4http://escholarship.org/uc/anrcs_californiaagriculturehttp://escholarship.org/http://escholarship.org/http://escholarship.org/http://escholarship.org/


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156 CALIFORNIA AGRICULTURE s 6/,5-% 62 .5-"%2 4

RESEARCH ARTICLE

W

Leafroll disease is spreading rapidly in a Napa Valley vineyard

byDeborah A. Golino, Ed Weber, Susan Sim

andAdib Rowhani

In the 1930s and 1940s, little wasknown about viruses, and information

on plant diseases caused by viruses

was just beginning to appear in the

scientific literature. Problems with

grapevines in California, first referred

to as red leaf, were initially attrib-

uted to inexperience in viticultural

techniques and poor growing condi-

tions. However, the problem was later

identified as leafroll disease, which

causes red leaves, and poor yields and

fruit quality. We evaluated its rate of

spread for 5 years in a Napa Valley

vineyard, and found an average rate

of more than 10% per year. Leafroll

disease can be vectored by low-level

populations of grape mealybugs,

and is now spreading rapidly in at

least one Napa Valley vineyard for

unknown reasons. Using stock for

planting vines that is certified as virus-free is a key strategy in preventing the

spread of grapevine leafroll disease.

ASCalifornias wine industry de-veloped and grapevine plant-ings expanded during the 1940s, newknowledge and methods of disease de-tection gradually made clear to scien-WLVWVMXVWKRZZLGHVSUHDGYLUXVGLVHDVHproblems were in the states vineyards.A classic case involved leafroll diseasein a red table-grape variety from Iran.Called Emperor, this variety was re-ported to be the third-most-importanttable grape in California in 1941. Often,growers reported low color develop-ment and sugar levels, which led to theidea that two varieties actually existed:the normal, red Emperor and theso-called White Emperor. However,in 1943, UC Davis viticulturist HaroldOlmo and his colleagues determinedthat this problem was perpetuated by

vegetative propagation and proposedthat a virus was involved (Olmo andRizzi 1943).

In 1946, U.S. Department ofAgriculture scientists in Fresno dem-onstrated that the White Emperorcondition was also transmissible viagrafts, a method still considered toprovide strong evidence that a virusis the causal organism (Alley andGolino 2000; Harmon and Snyder1946). Grafting a piece of a diseased,infected plant onto a healthy plant cantransmit a plant disease caused by aYLUXV9LUXVHVPRYHIURPRQHOLYLQJcell to another (they are obligate para-sites) and will move readily from thegrafted piece of the diseased plant intothe healthy plant to which it is grafted.7KLVZDVDQLPSRUWDQWSLHFHRIUHVHDUFKin grapevine virology because it alsolinked virus disease with poor vine-\DUGSHUIRUPDQFH7KHLPSRUWDQFHRIpropagation from healthy stock also be-came clear to researchers and industry(Alley and Golino 2000).

6LJQLFDQWSURJUHVVZDVPDGHLQthe 1960s and 1970s to reduce the inci-dence of leafroll disease in CaliforniaYLQH\DUGV7KHJUDSHYLQHFHUWLFDWLRQSURJUDPUVWSURSRVHGE\2OPRin 1951 had become a reality, and thematerial produced in the programEHFDPHZLGHO\DYDL ODEOH7KLVVXFcessful approach is based on the useof disease-tested grapevine nurserystock, produced at Foundation Plant

Services (FPS) at UC Davis throughthe California Grapevine RegistrationDQG&HUWLFDWLRQ3URJUDPZKLFKLVoverseen by the California Departmenof Food and Agriculture (Olmo 1951;$OOH\DQG*ROLQR7KURXJKWKLVprogram, virus-contaminated stock incommercial propagation is replaced wigrape scion and rootstock varieties thatare disease-tested, professionally identiHGDQGPDGHDYDLODEOHWRJUDSHJURZers by participating grapevine nurserie7KHSURJUDPLVVWLOODFWLYHWRGD\

However, many California grapeJURZHUVFRQWLQXHWRXVHQRQFHUWLHGplanting stock, which is often infectedwith virus. Historically, where theabsence of soil-inhabiting pests suchas phylloxera or nematodes makes itpossible, growers of wine, table andraisin grapes have planted vines thatare simple rooted cuttings with norootstock (known as rootings). Mostoften obtained from local vineyards,the propagating wood may be heav-LO\LQIHFWHGZLWKYLUXVHV7KLVFDQVDYH

money at planting time but inevitablycosts growers money in the long termin reduced yields and quality of fruit. Inareas where the insect phylloxera willkill vines unless resistant rootstocksare used, wine-grape growers often buyFHUWLHGURRWVWRFNIUHHRIYLUXVWKDWLVHOGEXGGHGZLWKVFLRQZRRGWKHIUXLWproducing top portion of the vine) ob-tained either locally or from a vineyardwith a reputation for producing excelle

Grapevine leafroll, a viral disease that reduces fruit quality and yield, is diagnosedby the presence of red leaves (shown). The manager of this vineyard had observedlow levels of grape mealybug, an effective disease vector, since the early 1990s.


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http://CaliforniaAgriculture.ucop.edu s /#4/"%2n$%#%-"%22008 157

wines. Much of this scion wood is in-fected with virus, and even though theFHUWLHGURRWVWRFNLVIUHHRIYLUXVWKHinfected scion bud can transmit virus tothe entire vine, from top to bottom.

Unfortunately, the use of propagat-ing wood that is not checked for virusDOVRNQRZQDVFRPPRQVWRFNKDV

resulted in virus disease problems,ranging from mild to severe, in manyCalifornia vineyards. Growers continuethese practices, despite convincingevidence that modern vineyards donot perform optimally when a virusis present (D. Golino, in preparation).Furthermore, virus-borne diseases canUHVXOWLQVLJQLFDQWORVVHVRI\LHOGDQGfruit quality and may also lead to thedeath of vines (Martelli 2000). Morethan 50 different viruses are knownWRLQIHFWJUDSHYLQHVZRUOGZLGH7KH

most common, economically damagingviruses in California are grapevine fan-leaf virus, the grapevine leafroll virusesand the grapevine vitiviruses.

Virus epidemiology

In 1992, enzyme-linked immuno-sorbent assay (ELISA) tests were putinto use at FPS for grapevine leafrollviruses. ELISA is a simple laboratoryserological test that can in some casesVXEVWLWXWHIRUHOGWHVWVWDNLQJXSWR\HDUV:HEHUHWDO7KHVHWHVWVrevealed the presence of grapevineOHDIUROODVVRFLDWHGYLUXVHV*/5D9in previously healthy vines in a vine-yard block from the 1960s maintained

by FPS, indicating active virus spreadin recent years (Rowhani and Golino1995). FPS responded by removing the

block from the Grapevine RegistrationDQG&HUWLFDWLRQSURJUDPLQFUHDVLQJisolation distances from any grapevinesthat might have virus, and implement-ing a comprehensive virus-screeningprogram with the new methodology.Since that time, new and increasingly

sensitive laboratory tests for grapevineviruses have allowed regular testing ofall FPS vines.

7KHFULWLFDOUHPDLQLQJSUREOHPRQFHthese actions were taken was the lackof information on leafroll virus epi-demiology. All the previous work hadindicated that vine-to-vine spread ofleafroll rarely occurred in Californiavineyards, and no insect vectors had

been reported (Goheen 1989). However,

when the distribution of infected plantsin the old FPS vineyard was mapped,newly infected vines were frequentlyDGMDFHQWWRWKRVHNQRZQWREHGLVHDVHGAlso, contrary to common wisdom atWKHWLPHWKLVHOGVSUHDGDSSHDUHGWRKDYHRFFXUUHGIDLUO\UDSLGO\LQMXVWDfew years (Rowhani and Golino 1995).

7DNLQJRXUOHDGIURPZRUNGRQHin Europe and New Zealand, whichdemonstrated that mealybugs couldspread leafroll viruses (Martelli 2000),we attempted to transmit Californiastrains of leafroll virus with mealybugspecies found in California vineyards.At that time, we determined thatfour species of mealybug were ableWRWUDQVPLW*/5D9XQGHUH[SHUL-mental conditions. All four are com-monly found in California vineyards:the obscure mealybug, Pseudococcus

viburni (Signoret); the longtailedmealybug, Pseudococcus longi-spinus7DUJLRQL7R]]HWWLWKHFLWUXVPHDO\-

bug, Planococcus citri (Risso); and thegrape mealybug, Pseudococcus maritimus(Ehrhorn) (Golino et al. 2002).

7KLVZRUNDORQJZLWKWKHHYLGHQFHof spread in the FPS vineyard, raisedconcerns among nurseries and growersDERXWHOGVSUHDGRIOHDIUROOGLVHDVHLQCalifornia vineyards.

Leafroll in a Napa vineyard

In fall 2002, a viticulture researcherin Napa called to our attention a12-year-old Cabernet Sauvignon vine-yard in which leafroll disease appearedto be spreading. No leafroll disease had

been evident in the early years afterthe block was planted in 1998, and thevines had been propagated with certi-HGURRWVWRFNDQGHOGEXGGHGZLWK

Symptoms of grapevine leafroll disease includered, cupped leaves. Older leaves are the first toshow symptoms each summer, and symptomsare strongest just before leaf-fall.

Leafroll symptoms in some vines were mild andshowed reddening only at the leaf margin ona few leaves; this plant tested positive usingELISA for GLRaV-3.

scion wood from a nonsymptomaticvineyard source. At this time, it wasXQXVXDOIRUD1DSD9DOOH\JUDSHYLQH-\DUGWREHSURSDJDWHGZLWKFHUWLHGscion wood, but management had madecareful observations used to sourcethe scion wood and it appeared to befree of symptoms. By 2002, however,

many vines were showing characteristicsymptoms of leafroll disease, includ-ing dark-red, cupped leaves with greenveins and fruit that matures moreVORZO\WKDQRQKHDOWK\YLQHV7KHPD-MRULW\RIV\PSWRPDWLFYLQHVZHUHRQRQHedge of the vineyard, close to an oldervineyard that had leafroll disease.

Mapping leafroll. 7KDWIDOOZHEHJDQmapping disease incidence in a portionof the newly infected vineyard in aneffort to determine the rate of spreadof leafroll disease. Mapping continued

XQWLOIDOOJ7KHPDSSHGYLQH -yard (block 1) was Cabernet Sauvignonplanted in 1989 on several differentrootstocks with 6 feet (about 2 meters)

between rows and 3.3 feet (1 meter) be-WZHHQYLQHVLQWKHURZ7KHEORFNZDV

budded on several different rootstocksbecause the winery was beginning toreplace blocks planted with AXR-1 withDOWHUQDWLYHURRWVWRFNV7KHVRXUFHRIthe budwood is uncertain, but no leaf-roll symptoms were observed for theUVWRU\HDUVVXJJHVWLQJWKDWWKH

original stock was free of virus. Red-leaf symptoms of leafroll appeared inthis vineyard in 2000, primarily at theHDVWHUQHQGVRIWKHURZV7KHQXPEHURIsymptomatic vines was reported to beincreasing each year.

Across an avenue from the easternend of this block was another CabernetSauvignon vineyard (block 2) that was


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Quantifying virus spread

Our 2002 mapping results showedthat leafroll symptoms were presentin 23.3% of the vines in the mappedDUHDRIEORFNJ7KHGLVWULEXWLRQof symptomatic vines suggested thatleafroll initially spread from block 2

WKHDGMDFHQWROGHULQIHVWHGEORFNLQWthe eastern end of block 1, and subse-quently spread down the rows towardthe west. Nearly all vines on the easterends of the rows were rated positive foleafroll, and only a handful were posi-tive on the western end.

7KHLQFLGHQFHRIV\PSWRPDWLFYLQHVin the mapped area increased to 41.2%in 2003 and to 45.8% in 2004, and thedistribution of diseased vines continuedto show evidence of spread from east towest. We also observed leafroll symptom

in the more recently planted vines inblock 2, suggesting that leafroll had nowspread back into the new vines in block from the diseased vines in block 1.

In 2004, there was such a difference fruit quality and ripening patterns thatthe vineyard was harvested twice astwo separate fruit loads with differentharvest dates. Fruit from the healthyvines ripened earlier, was better qualityand was used for reserve wines, whichcommand a higher price. Fruit from thediseased vines was picked several week

later and was not used for reserve wine7KHLQFLGHQFHRIOHDIUROONHSWLQFUHDVing, reaching 49.8% in 2005 and 66.1%DWWKHHQGRIRXUVWXG\LQJDue to the inferiority of fruit from theinfected vines, the vineyard owner isnow faced with the need to replant this

block after only 15 years. Most grapegrowers would expect a much longervineyard life for their initial investmenquite often twice this long. In 2003, UCcost studies estimated the cost of estab-lishing an acre of vineyard in the NorthCoast at about $25,000; no harvest woul

EHH[SHFWHGLQWKHUVW\HDUVZKLOHthe vineyard is being established. It iseasy to see that more-rapid replantingof vines due to leafroll spread in vine-yards could greatly increase the cost ofgrape-growing in any part of Californiaaffected by this problem.

Role of grape mealybug

7KHYLQH\DUGPDQDJHUZKRLVDQentomologist, had observed grape

ELISA tests were examples of vines thatwere already infected but not yet show-ing symptoms. Nonetheless, it was clearthat visual symptoms were highly cor-related with the presence of virus and

could be used for large-scale mapping.With this background information,

in subsequent years we eliminated theTXHVWLRQDEOHDQG FDQNHUFDWHJRULHV

from the rating system, so that each plantwas rated either negative for leafroll, posi-tive for leafroll, dead or missing.

In fall 2007, we repeated ELISA test-ing on 204 vines to access the accuracyof our visual ratings. Out of 101 vinesvisually rated as positive for leafrolldisease, all but one tested using ELISAZHUHSRVLWLYHIRU*/5D92XWRI

103 vines visually rated as negative,all but three also tested using ELISAZHUHQHJDWLYHIRU*/5D9,WLVKLJKO\probable that the three latter nonsymp-tomatic vines represented early stagesof infection when there were mild orno symptoms. In our experience, mildsymptoms are easily overlooked.

Each year, a larger proportion of the vineyard was diseased,

reducing yield and quality with each increment of spread.

Block 1 Block 2

Cabernet Sauvignonplanted 1989

Mapped area = 7.2 acres

OldCabernet Sauvignon

planted 19701972

Heavy leafroll

Pulled 1994Replanted 1998Leafroll symptoms

first noticed

North

South

West

East

Fig. 1. Leafroll spread was mapped in a NapaValley Cabernet Sauvignon vineyard planted in1989, west of an older block heavily infectedwith leafroll. The two blocks were side byside for 5 years, providing the opportunity forleafroll disease to move from the old block tothe new. The red section shows where leafrollsymptoms were first noticed. Mapping wasconducted each fall from 2002 to 2006 on avine-by-vine basis in the 7.2 acres indicated bythe orange area.

SODQWHGIURPWRJ7KLVplanting was heavily infested with lea-froll, as evidenced by the red-leaf symp-toms throughout the block reported byWKHYLQH\DUGPDQDJHU7KLVYLQH\DUG

was pulled in 1994 due to leafroll diseaseDQGWKHHOGZDVUHSODQWHGLQDIWHUa 4-year fallow. Grape mealybugs wereobserved in both these blocks most years,

but never reached population levelswhere insecticide treatments were made.

In October 2002, we mapped partof the newly infected block 1 to assessthe incidence and pattern of vines withOHDIUROOV\PSWRPV7KHPDSSHGDUHDLQ-cluded 98 complete rows (approximatelyYLQHVDQGFRYHUHGDFUHVJorange section of block 1). When infected

with leafroll virus, Cabernet Sauvignonnormally produces strong, characteristicvisual symptoms that most notably in-clude dark-red cupped leaves with greenYHLQV9LQHVZHUHLQGLYLGXDOO\UDWHGIRUsymptoms of leafroll disease using ascoring system of: 0 = no symptoms; 1 =mild or severe symptoms; Q = question-DEOHXVXDOO\GLIFXOWWRGHWHUPLQHGXHto mite feeding on leaves, which can alsocause leaf-reddening); C = canker symp-toms masking possible leafroll symp-toms; and X = dead or missing vine.

Observations were made annually fromOctober 2002 through October 2006.ELISA testing.7RWHVWWKHDFFXUDF\RI

the 2002 visual-symptom ratings in block1, 75 petiole samples were tested usingELISA for four grapevine leafroll associ-DWHGYLUXVHV*/5D9*/5D9*/5D9DQG*/5D9:HEHUHWDO8VLQJour symptom scoring system, 35 of thesesamples were from vines rated stronglypositive for leafroll, 20 vines rated nega-tive and 20 vines rated questionable.

7KH(/,6$WHVWLQJIRXQGRQO\*/5D9LQVDPSOHVIURPV\PSWRPDWLF

YLQHV7KHYLVXDOV\PSWRPUDWLQJVZHUHvery accurate, although not in perfectagreement with the ELISA testing. All35 samples from vines visually rated aspositive for leafroll were also positiveIRU*/5D9E\(/,6$WHVWLQJ,QDGGL-tion, all of the vines rated as question-able tested negative for virus. However,2 of the 20 vines (10%) rated negativeDFWXDOO\WHVWHGSRVLWLYHIRU*/5D9It is possible that these two positive


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http://CaliforniaAgriculture.ucop.edu s /#4/"%2n$%#%-"%22008 159

2002 (23.3%)

2003 (41.2%)

2004 (45.8%)

2005 (49.8%)

2006 (66.1%)

No leafroll

No vine

Block 1 Block 2

Mappedarea =

7.2 acres

North

Fig. 3. Spread ofgrapevine leafroll virusin the mapped area ofblock 1 (see fig. 1) of aNapa County CabernetSauvignon vineyard,20022006.

70

60

50

40

30

20

10

0

2002 2003 2004 2005 2006

Year

Leafroll(%)

Fig. 2. From 2002 to 2006, the percentage ofleafroll-symptomatic vines in a Napa CountyCabernet Sauvignon vineyard increased from23.3% to 66.1%, an increase of more than10% per year.

mealybug in this and surroundingYLQH\DUGVRYHUPDQ\\HDUV7KHPHDO\-

bugs were at low populations that werenot considered problematic becausethey did not cause obvious economic

damage. However, a low populationcan still serve as an effective vectorfor virus diseases in many crop sys-WHPVZLWKRXWGRLQJGLUHFWVLJQLFDQWGDPDJHWRWKHSODQWV7KHUHIRUHZH

believe that the mealybugs were likelyUHVSRQVLEOHIRUWUDQVPLWWLQJ*/5D9

between blocks 1 and 2, and for theVSUHDGGRFXPHQWHGLQEORFN7KHrisk of leafroll spread from such smallpopulations had not previously been aconsideration when potential damagefrom grape mealybug was assessed.

Similar low populations of grapemealybug are regularly observed inPXFKRI1DSD9DOOH\.HQW'DDQHSHU -sonal communication).

7KHVWHDG\LQFUHDVHLQLQIHFWLRQUDWHVHHQLQRXUPDSSLQJVWXG\LVWKHUVWGRFXPHQWDWLRQRIVLJQLFDQWDQGUDSLGHOGVSUHDGRIOHDIUROOGLVHDVHLQD&DOLIRUQLDYLQH\DUG7KHVLJQLFDQFHRIthis spread to the grower is clear: eachyear, a larger proportion of the vine-yard was diseased, reducing yield andquality with each increment of spreadJVDQG

Possible causes for rapid spread

Several other vineyards in Californiahave been reported recently by growersat meetings to exhibit leafroll spread atrates similar to those documented here.We believe that if such high rates ofleafroll spread had occurred in the pastin California, the problem would have

been described by a number of excellentresearchers who have worked on grape-

vine virus diseases or by observantYLQH\DUGPDQDJHUV7KDWWKH\QHYHUdid so suggests that something funda-mental has changed in these vineyardsto allow such spread to occur, such as

vector epidemiology, grower rootstockpreferences and/or new leafroll strainsthat are more easily transmitted inCalifornia vineyards.

Vector epidemiology. While the pos-sibility always must be considered, noth-ing has been observed to suggest that anew vector is responsible for the spreadof grapevine leafroll disease observed inWKLVVWXG\7KHYLQH\DUGKDGDKLVWRU\of grape mealybug, which has been inCalifornia throughout its grape-growinghistory. In addition, while the vine

mealybug is blamed for rapid spread ofleafroll disease in South Africa (Martelli2000), it was only recently introduced toCalifornia and is not yet present in Napa9DOOH\,QWKHODWHVYLQHPHDO\EXJZDVWKHUVWVSHFLHVRIWKLVSHVWUHSRUWHGin the international literature to transmitleafroll virus (Martelli 2000).

Unfortunately, we expect vinemealybug to be an effective vectorof leafroll virus in Cal ifornia. FirstGLVFRYHUHGLQWKH&RDFKHOOD9DOOH\LQthe late 1990s, the vine mealybug hasslowly spread north throughout the

VWDWH'DDQHHWDO7KLVVHULRXVSHVWLVGLIFXOWWRFRQWURODQGFRQWUROis even more critical because the insectis known to be a vector of leafroll vi-UXVHV7KXVLIYLQHPHDO\EXJEHFRPHVestablished throughout California,leafroll could spread even more ag-gressively (see pages 167 and 174). Wedo not, however, believe it caused theleafroll disease spread that we saw inthe study vineyard.

7KHSRSXODWLRQG\QDPLFVRIWKHYHF -tor could also have been affected bychanging pest-management practicesin the vineyard, and possibly linked tochanges in the number of parasites and

predators of the leafroll virus vector.Detailed monitoring of many speciesof arthropods that inhabit vineyardswould be needed to determine whetherthis played a role in leafroll spread.

Rootstock preferences.7RGD\VCalifornia rootstocks are less tolerantof leafroll infection than own-rootedvines or vines grafted on the rootstock$;5ZKLFKZDVXVHGE\WKHPDMRU -ity of grape growers until it succumbedto an epidemic of type B phylloxera inthe early 1980s. AXR-1 rootstock (also

known as Ganzin 1) is a cross betweenthe variety Aramon and Vitis rupestris;it is generally believed that the Vitis vin-ifera in Aramon led to its failure. (Golino*ROLQRHWDO7KHSUHYLRXVJHQHUDWLRQRI1DSD9DOOH\YLQH\DUGVZDVplanted primarily on AXR-1, which ismuch more tolerant of leafroll infectionsthan many rootstocks currently in use(D. Golino, in preparation). A vine prop-agated on AXR-1 rootstock and infectedwith leafroll disease can show mild orOLWWOHV\PSWRPV7RGD\WRGLIIHU-ent rootstocks are commonly in use and

many of them are extremely sensitive toviruses.

Perhaps leafroll viruses have alwaysspread among vines in our vineyards,

but the symptoms simply were notevident in most cases because the root-stocks were more disease-tolerant andVKRZHGIHZHUV\PSWRPV7KLVFRXOGaccount for at least part of the appar-ent change in epidemiology. Given thegreater susceptibility of todays root-


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ReferencesAlley L, Golino D. 2000. The origins of the grape pro-gram at Foundation Plant Materials Service. Proc 50thAnnual ASEV Meeting. Seattle, WA. Am J Enol Vitic51:22230.

Daane K, Bentley W, Walton V, et al. 2006. New con-trols investigated for vine mealybug. Cal Ag 60:318.

Goheen AC. 1989. Virus diseases and grapevine selec-tion. Am J Enol Vitic 40:6772.

Golino D. 1993. Potential interactions between root-stocks and grapevine latent viruses. Am J Enol Vitic44:14852.

Golino DA, Sim ST, Gill R, Rowhani A. 2002. Grapevineleafroll disease can be spread by California mealybugs.Cal Ag 56(6):196201.

Golino DA, Sim ST, Rowhani A. 2003. The role of root-stock genotype in the effects of single and mixed in-fections of grapevine viruses. Proc 14th Int Council forthe Study of Virus and Virus-like Diseases of Grapevine(ICVG) Conference. Bari, Italy. p 2467.

Harmon FN, Snyder E. 1946. Investigations on the oc-currence, transmission, spread, and effect of whitefruit color in the Emperor grape. Proc Am Soc HortSci 47:1904.

Martelli GP. 2000. Major graft-transmissible diseasesof grapevines: Nature, diagnosis, and sanitation. Proc50th Ann ASEV Meeting. Seattle, WA. Am J Enol Vitic51:2316.

Olmo HP. 1951. A proposed program for the intro-duction, improvement, and certification of healthygrape varieties. Wines Vines 32 (7):79.

Olmo HP, Rizzi AD. 1943. Selection for fruit color inthe Emperor grape. Proc Am Soc Hort Sci 42:395400.

Rowhani A, Golino D. 1995. ELISA test reveals newinformation about leafroll disease. Cal Ag 49(1):269.

Weber E, Golino DA, Rowhani A. 2002. Laboratorytesting for grapevine virus diseases. Practical WineryVineyard 22(2):1326.

of the vines that became symptomatic ofleafroll disease and were tested had thistype of leafroll. We believe some strainsRI*/5D9ZHUHSUHVHQWLQ&DOLIRUQLDZKHQJUDSHYLQHVZHUHUVWLQWUR -duced it is often found in our oldestvineyards, even in isolated locations.However, plant viruses are highly mu-

table and within-species severity mayvary greatly. Is it possible that a strain of*/5D9ZDVLQDGYHUWHQWO\LQWURGXFHGwith grape cuttings smuggled into&DOLIRUQLDIURP(XURSHRUHOVHZKHUH"With the number of illegal importationsknown to have occurred in the past20 years, this is a distinct possibility.

Research needs

Additional research is urgentlyneeded to help us better understandour observation of rapid spread of leaf-

roll disease in the vineyard. Related on-JRLQJ8&SURMHFWVLQFOXGHZRUNRQWKHtransmission biology of leafroll viruses

by mealybugs, mealybug managementand the impacts of 10 known speciesof leafroll viruses on grape scions andURRWVWRFNV$WWKLVWLPHVSHFLFUHF -ommendations for mealybug controlsto prevent virus spread are still underdevelopment. If leafroll virus spreadalso occurs across other grape-growingregions of California, as has been ob-VHUYHGLQWKLVVWXG\RID1DSD9DOOH\vineyard, the disease will have a fargreater impact than ever on vineyardSURGXFWLYLW\7KHUHIRUHEUHHGLQJSUR-grams should also be initiated to de-

stocks, choosing both rootstocks andscion wood free from leafroll infectionis far more critical than in the past.

New leafroll virus strains. At this writ-ing, there are at least 10 different speciesof leafroll virus, each in a taxonomicallydistinct group. Most have the geneticQJHUSULQWRIDJURXSRIYLUXVHVNQRZQ

to be transmitted by mealybugs. In fall2006, we tested for all these leafroll spe-cies in our study vineyard, but only*/5D9ZDVIRXQGDQGWKLVVSHFLHVLVknown to be mealybug-transmitted. All

In memory of Ed Weber

Ed Weber, a co-

author of this article,

was a longtime Napa

County Cooperative

Extension Advisor. He

knew Napa vineyards

well and was among

the first viticultur-

ists to observe an

apparent increase in

the grapevine leafroll infection rate.

7EBER DIED UNEXPECTEDLY ON $EC

2007, at the age of 51. This article is

dedicated to him.

7EBER ATTENDED 5# $AVIS WHERE HE

EARNED A "3 IN PLANT SCIENCES AND AN

-3 IN HORTICULTURE WITH AN EMPHASIS

on viticulture. He worked as viticultur-

IST FOR *OSEPH 0HELPS 6INEYARDS FROM

UNTIL WHEN HE JOINED 5#

Cooperative Extension in Napa County.

Weber served as a liaison between the

University and its researchers, wine-

makers, and state and local regulatory

agencies.

In the wine industry, Weber was

known as a great educator, communica-

tor and problem-solver. His exceptional

talent for speaking and writing was ac-

companied by an ability to take compli-

cated data and information and convey itin a straightforward and logical way that

could be understood by industry profes-

sionals as well as the general public.

In addition to his research and ex-

pertise in viticulture, Weber provided

administrative leadership as Napa

#OUNTY $IRECTOR AND SERVED AS CHAIR

of the ANR Communications Advisory

"OARD

velop resistance in both scion varietiesand rootstocks using traditional breeding strategies and possibly molecularELRORJ\WHFKQLTXHV7KLVZRXOGEHWKHultimate sustainable approach to con-trolling this disease.

In the meantime, grape growers arestrongly advised to plant their vineyard

XVLQJRQO\FHUWLHGSODQWLQJPDWHULDOthat has been screened for virus. Wherethat is not possible due to winemakerpreferences or other factors, propagatinstock should be carefully screened forvirus using laboratory tests (Weber et a2002), and only the healthiest possiblestock should be used for propagation.Maximizing the distance between newplantings and virus-infected old plant-ings should reduce the rate of spread.Care should be taken to ensure thatequipment, personnel and pomace mov

ing between vineyards (see page 172) arnot contaminated with mealybugs thatmight be carrying leafroll virus.

D. Golino is Cooperative Extension Plant Pathol-

ogy Specialist, Department of Plant Pathology, U

Davis; E. Weber was County Director and FarmAdvisor, UC Cooperative Extension Napa County

(deceased; see box); and S. Sim is Staff Research

Associate, and A. Rowhani is Plant Pathology Sp

cialist, Department of Plant Pathology, UC DavisThis project was initiated with funding from the

American Vineyard Foundation and the Californ

Competitive Grant Program for Research in Viti-

culture and Enology. We acknowledge the invalable help of Joshua Chase, Justin Jacobs, Judy Le

Laurel Leon and Yvonne Rasmussen in the survework and laboratory testing.

Leafroll Disease is Spreading Rapidly in a Napa Valley Vineyard

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