citrus tatterleaf virus in texas - aggie horticulture

Citrus Tatterleaf Virus in Texas

by Craig J. Kahlke

Citrus Tatterleaf Virus (CTLV)

‚a disease destructive when infectedbudwood is grafted onto trifoliate ortrifoliate hybrid rootstocks‚severe strains can cause a crease at thebud union which could cause the tree tosnap off during high winds‚less severe strains cause deep flutingand overgrowth of the rootstock at the budunion‚this can cause stunting and reducedvigor and yield of the tree

CTLV

‚very flexous filamentous particle•650 nm long•~ 10 nm wide‚In the capillovirus family•contains a ssRNA & a coat protein‚closely related to Apple Stem GroovingVirus (ASGV), another capillovirus•in fact some CTLV strains react via ELISA withsome antisera developed against ASGV

History-

PCTLV first described in 1962 by Wallaceand Drake in California as the tatter leafcitrange-stunt virus complexPthey grafted apparently healthy >>Meyer==lemon budwood onto Citrus excelsa, whichproduced yellowed blotches and leaf edgemalformation, thus AAtatterleaf@@Palso found stunting symptoms oncitrange and citremon

History (contd.)

‚CTLV is widespread in the far East, inChina and Japan, Taiwan, Korea, and thePhilippines‚considered endemic in these regions‚besides these areas and the US, CTLV isalso found in Australia, South Africa, andBrazil‚has done major economic damage wheretrifoliate and trifoliate hybrid rootstockshave been used

History (contd.)

‚most likely came the the US with infected>>Meyer== lemon trees imported from Japan‚these trees probably also containedcitrus tristeza virus (CTV), a much moreserious threat to citrus worldwide

Texas History

‚CTLV came to TX with the >>Meyer== lemonimports to the US from Japan in the early1900==s‚probably didn==t make it to TX and FL untilthe late 1920==s and early 1930==s‚original lemon strain wasn==t compatiblewith Sour orange rootstock‚so was rooted as cuttings‚budwood from these cuttings wascompatible with Sour orange

Texas History (contd.)

‚Testing in the 1950==s found CTV‚biological indexing in the 1960==s foundCTLV and CTV‚both viruses were symptomless as lemonscions on Sour orange rootstocks‚surveys in the 1990==s found CTLV inbudwood source trees in nurseries

Texas History (contd.)

‚>>Eureka== and >>Ponderosa== lemons alsoinfected‚one nursery had three source trees in arow infected, a >>Meyer== lemon, a >>Eureka==lemon an >>Algerian== tangerine‚CTLV probably spread from the Meyer tothe other 2 trees mechanically

Texas History (contd.)

‚Mechanical transmission•through infected cutting tools, leafrubbing, or roots•demonstrated in 1974 by Garnsey(Florida)•situation of CTLV spread in nurserysource trees in TX illustrates problem

Texas History (contd.)

‚The easy mechanical spread of CTLV‚symptomless carrier on a tolerantrootstock‚no symptoms on TX Sour orangerootstock‚no decline or economic loss seen in TX

So, What==s the problem?

‚There are ongoing trials in TX andelsewhere to find CTV-tolerant rootstocks‚over 99% of our rootstocks are on CTV-susceptible Sour orange‚because of excellent yield and growth inthe variety of soil types found in the Valley

CTV and Sour Orange

‚CTV has killed millions of trees worldwidethat have been grafted on Sour orangerootstocks‚severe strains•quick decline<fast tree decline and death•stem-pitting<small, unmarketable fruit

CTV in Texas

‚Extensive surveys by ELISA in TX havefound CTV infections ranging from 0.5% inthe LRGV up to 25% in east Texas•trees in East TX are grafted onto CTV-tolerant rootstocks‚no quick decline or stem-pittingsymptoms seen in field trees‚CTV spread by use of infected budwoodor by aphids

Aphids Vectoring CTV

‚Native aphids in TX spread CTV veryinefficiently‚natural movement of CTV in the field byaphids could take many years to become aproblem‚the Brown Citrus Aphid•extremely efficient at vectoring CTV•came from Southeast Asia, across Africa, theCaribbean•arrived in Florida in 1995

The Brown Citrus Aphid (BrCA)

‚Spread through entire citrus growingregion in FL (over 150 miles) in less than 4years‚FL previously had high incidence of CTVand quick-decline‚millions trees lost and pulled that were onSour orange rootstock‚FL has lost Sour orange as a rootstock‚BrCA confirmed in Southern Mexico in1999

BrCA (contd.)

‚No reason to think that the BrCA won==tarrive in TX‚question of when‚could arrive quicker if brought in illegallyon a tree, propagative material, etc.•unknowingly by collectors, etc.•this is probably how it arrived in FL

BrCA (contd.)

‚Natural spread of BrCA to TX•there is a lot of citrus between where BrCA isin Mexico and TX•large gaps, but could move quicker in storms,large air currents

CTV/BrCA control strategies

‚Virus-free Budwood program•production of certified trees tested free ofknown endemic viruses and other pathogens•VF trees produced through STG•micrografted apical meristems onto toothpick-sized seedlings under sterile conditions•tiny grafted tips can bypass theviruses/viroids

Virus-Free Budwood Program(cont==d.)‚Tiny STG plants transferred to pots withsoil-less media‚indexed to confirm their VF status, whenlarge enough‚lab tests- ELISA, PCR, PAGE‚biological indexing•inoculated tissue from STG plant onto abattery of indicator plants•observe over time for symptoms

Virus-Free Budwood program(cont==d.)

‚If no disease symptoms are seen, plantsare multiplied‚mother trees planted in the field•foundation block‚mother trees horticulturally evaluatedafter fruiting‚if trees true to type, budwood from thosetrees are budded onto rootstocks in amultiplication block

Virus-Free Budwood Program(contd.)

‚Budwood from multiplication or increaseblock cut and sold to nurserymen‚Increase Block trees planted in a cyclicalrotation to ensure constant supply ofbudwood‚Foundation & Increase Trees testedyearly for CTV by ELISA‚Foundation Trees completely re-indexedperiodically

Viruses/Viroids Endemic to LRGV

‚CTV - effects sweet orange and grapefruiton sour orange rootstock‚CTLV- most scions affected on trifoliateor trifoliate hybrid rootstocks‚citrus viroids (Exocortis, Cachexia) - mostscions affected on trifoliate and trifoliatehybrid rootstocks‚citrus psorosis virus - affects mostscions on sour orange and otherrootstocks

Mechanisms of Virus/Viroid Spread

‚CTLV, citrus viroids•mechanically or through infected budwood

‚Psorosis•through infected budwood

‚CTV•by aphid vector or through infected budwood

Effectiveness of VF BudwoodPrograms

‚Thus, spread of CTLV, Viroids, andPsorosis could be well prevented by thedistribution of VF budwood and educationabout mechanical transmission‚prevention of CTV spread nearlyimpossible with endemic CTV and efficientaphid vector (BrCA)

Other CTV Control Mechanisms‚Switch to CTV-tolerant rootstocks‚Pulled out all trees infected with CTV‚cross-protection‚frequent ELISA testing, removing all treesthat react only with antibody indicating thepossibility of severe CTV strains present‚improvement through biotechnology•isolating CTV resistance gene in onerootstock & inserting into Sour

Timeline to a Virus-Free Texas

‚Barring a severe freeze wiping out all oldertrees, it could be a long time before all citrustrees in the LRGV are only those distributedfrom the VF budwood program‚it may never happen after arrival of theBrCA‚but we can test for and eventually replacetrees infected with viruses/viroids that arenon-insect transmitted

Importance of Testing Nurseries

‚The CTLV spread to other source trees ina nursery again highlights the need toreplace these trees with virus-free sources‚problem - VF program concentrating onincreasing commercial budwood supply‚homeowner trees ie. lemons, limes,mandarins, tangelos) are a concern-symptomless reservoir for disease

Homeowner Trees

‚Many VF varieties imported fromCalifornia•TX doesn't have resources to increase thesupply (large # varieties, collectors, etc.)•TX considering letting large nurseriesincrease supply

In the MeantimeYY.

‚Study of other endemic pathogens suchas CTLV‚Kahlke research•survey commercial nurseries<try to detect CTLV in commercial grapefruitand orange sources$by Biol. Indexing & ELISA•molecular tests on known TX CTLV strains -determine if a difference between bud-unioncrease and non bud-union creasing strains

Kahlke Research (contd.)

‚Same molecular testing if CTLV detectedin commercial sources•examine the possibility of cross-protection‚replacement of all source trees withCTLV-free sources