Guidelines for Growers on
the Purchase of Citrus Nursery Trees
By Kei th P yle
Backgr oun d
In the i nterests of a s ustainabl e and profit abl e commer ci al citr us industry, New Zeal and Citrus
Grow ers Inc (N ZCGI) w ishes to pr omote the adoption of best practices by nursery men that
w lil ens ur e that trees of hi gh quality and health ar e avail able to gr ow ers. In bri ef, trees shoul d
be pr opagated fr om citr us rootstocks and budw ood t hat are true to type genetically and
tested to be fr ee from potentially har mf ul virus es and other pathogens. N urseri es shoul d
obt ai n i ndependent certification that this is the case and t hat a r ange of best pr actice
gui deli nes have been adopted i n the propagation of t heir trees. NZCGI promotes the use of
certified trees from certified citrus nursery tree pr opagators.
This is vitally important as orchards establis hed fr om s ub-standard nurs ery stock w ill never be
as vigorous, pr oductive, or profitabl e as those est ablis hed fr om high quality dis ease f ree
material. Citr us have a high rate of natur al mut ation s o care must be tak en to ens ur e
budw ood is only taken from s uperior trees. As w ell as this, citrus trees may be inf ected w ith
diseases that show no symptoms i n young pl ants but w ill have serious i mpli cations over the lif e
of t he tree. This article w ill ex plai n these iss ues i n gr eat dept h and ends w ith a set of key
questions w hich all grow ers shoul d ask their nursery man.
Reasons for usin g certifi ed pr opagatin g materi al
Intro duct ion
The rational e for usi ng certified propagati ng material is the w ell know n princi ple that
genetically superior, disease and pest free propagating materi al is t he economic f oundation
of all successful horticult ural industries. It is an ess ential first step and pay s huge divi dends over
the lif e of t he tree as opposed to f ailur es w ith inferior materi al that may not reach y iel d
potenti al and may even have to be uprooted after only a f ew y ears. If mother trees used ar e
not genetically “true to ty pe” or contai n a har mful vir us, then so w ill the hundreds of trees
propagated each y ear from its buds.
Five differ ent but closely rel ated pr ogrammes ar e needed i n any citrus i ndustry :
1. A quar ant ine program m e to ensur e the safe i ntroduction of new
introduced i nto t he country (M AF controlled).
ger mpl asm
2. A clean stock pr ogr am m e to produce sour ces of pat hogen-free propagati ng stock of
new and ex isting commerci al vari eties.
3. A par ent tr ee pr ogr amm e inw hich par ent trees of each cultivar are vir us indexed and
cert ified. This certifies absence of pathogens i ncl uded i n the pres cri bed testing
progr amme w hich is based l ar gely on r ootstocks us ed i n t he i ndustry , their
susceptibility to t he vari ous dis eases and other risk assess ments.
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4. A pr ogramme of hort icult ural eval uat ion to ens ur e t hat parent trees r eflect the true
genetic attributes of a variety , w ithout any dis advantageous mut ations.
5. A qualit y ass ur ance pr ogr amm e looks at the above el ements and w hether a range
of best practices have been adopted duri ng the nursery pr opagation cy cle to give
grow ers the ass urance that the end product is of hi gh quality and is free of diseases
and pests.
Some of the mai n consequences on production if the above f actors ar e negl ected ar e
outlined below :
Genetic sele ctio n
Citr us trees have a fairly high rate of nat ur al mutation, and quite commonly tend to t hrow
'sports'. The most obvious of t hes e mutations ar e sector al chi mer as w hich can be s een on
trees as fruit w ith ridges, or variegated l eaves. Very occasionally, a sport is an improvement
on the origi nal parent, but unfort unately the majority of mutations res ult in a reduction i n
quality. If budw ood is cut from parent trees w hich are not r egul arly i ns pected f or fr uit quality
and abnor mal grow th habits, then there is a hi gh risk of propagating new trees w hich ar e
derived fr om inf erior sports.
Citr us budw ood is usually cut w hen t her e is no fr uit on the tree, so it is very diffi cult to see
w hether poor sports are pr esent. Records theref ore need to be kept from t hes e parent trees
at harvest, and i nferior br anches marked or trees eli mi nated fr om budw ood s election. M any
New Zealand orchards s how high v ariability , indicating t hat trees have been grow n from
inferior budw ood sel ections. Yiel ds and pack-out of fr uit could have been dr amatically
impr oved i n many or chard blocks si mply by ensuri ng that each tree w as grow n from a
superi or sel ected bud. This is best achieved by regul ar and strictly documented ev al uation of
budw ood sour ce trees to ens ur e they produce good y ields of good quality fruit, true to the
cultivar's characteristics.
Low yields, poor tree grow th, and a lack of unifor mity visible in orchar ds can also stem from
poor rootstock performance. Inferi or quality rootstock materi al i n citrus can r es ult from usi ng
seed source trees that do not produce tr ue to ty pe s eeds, or, mor e us ually , becaus e of
insuffi cient roguei ng of zy gotic seedli ngs from seedli ng rootstock popul ations i n the nursery .
M ost citrus rootstocks i n use are poly embry onic and the per centage of zy gotic seedli ngs
produced vari es betw een rootstocks. Zy gotic seedli ngs can be ex pected to perform poorly
or out of character compared to true to ty pe nucell ar (or cl onal) seedli ngs.
Viruses, Viro ids and ot he r similar o rganisms
Caref ul selection of healt hy nursery stock is the singl e most important factor that affects the
longevity and fi nanci al s uccess of new planti ngs. Dis eas es caused by most fungi and
bacteri a in establis hed orchar ds can be managed w ith appropri ate chemical pr eventative
meas ures, but systemi c diseases caused by viruses, viroi ds, bacteri a and ot her r el ated
microscopi c or ganis ms ar e a different matter.
Graft trans missi ble dis eases may be caus ed by virus es, viroi ds or other pathogens and ar e
vegetatively pr opagated from an i nfected budli ne by buddi ng or gr afting. They can s eriously
affect fr uit production and quality as w ell as tree healt h and longevity . They may be spr ead
to neighbori ng or chards of healthy trees by insects in s ome cases, by us e of i nfected
PO Box 10 6 29 , Wellington, Ne w Zeala nd
Phone: (04 ) 9 17 7 163 Fax: (04) 473 6041 Em ail: info@ citrus.co.nz
budw ood, or by farmi ng equi pment. Infections by them can not alw ays be recogniz ed
merely by looki ng at the pl ant - it sometimes requires s ubtle changes i n the envir onment for
sy mptoms to be seen. Sy mptoms can be mask ed in certain stock/sci on combi nations but the
trees can act as carri ers to other sus ceptibl e combinations. These organisms can only be
controll ed by t he use of dis ease free planti ng mat erial.
Citr us e xoc ortis viro id (CEV)
If citrus trees on Poncir us tr ifoliat a rootstock, or its hybri ds, ar e infected by citrus exocortis viroid
(CEV) variable performance can be expected, manifested as variabl e tree size and shape
and appear ance of the bud uni on and stock. These symptoms r ange i n severity from a
reticulum of surf ace cr acks on the shoul der bel ow the bud union to har d, persistent bar k
scali ng. It is probable that this diversity of symptoms is due to vari ous combi nations of different
strains of the viroi d.
Citr us ex ocortis viroi d and ot her citrus viroi ds ( CV) like cachex ia are only spread through
infected budw ood and on i nfected cutting tools eg. secateurs, cli ppers, buddi ng k nives and
hedgi ng machi nes. Ther e is no evi dence t hat the vir oids ar e transmitted by ins ects or i n citrus
seed, but they can be spr ead from i nfected to neighbouri ng trees w ithi n an orchard by r oot
gr afting. Fort unately , becaus e of the li mited w ays in w hich they can be trans mitted, the
spread of CEV and CV are easily prev ented thr ough the us e of clean budw ood and si mpl e
hygi ene techniques. Ex ocortis is the mai n viroi d of concer n i n New Zealand because of the
susceptibility of trifoliate rootstock and its hy bri ds. Cachex ia is a viroi d t hat has not been
know n to occur in N ew Zealand fr om i ndexing done by MAF and HortResear ch and it has a
low inci dence i n A ustr alia. Overs eas it is mai nly a probl em on sw eet limes, mandari ns and
tangelos es peci ally w here they ar e used as r ootstocks. Ti mmer and D uncan (1999) list trifoliate
and its hy bri ds as bei ng tolerant.
Psorosis
The Psorosis virus is thought to be w idespr ead i n N ew Zeal and and is trans mitted by
mechani cal transmissi on, i n infected budw ood and i n s eed. Budw ood must be cl ean and it is
important to ensur e t hat a psor osis-fr ee source of seed is confir med bef ore propagati ng
rootstocks. The best w ay of doing this is to i mport rootstock s eed from a certified overs eas
seed supplier or index seed source trees establis hed in N ew Zealand. M ost citrus speci es ar e
sy mptomless carriers of this diseas e compl ex (New Zealand Citrus handbook), but the current
status and distribution of the dis ease is unk now n. Tests are now available f or this vir us and can
be i ncor porated into testing pr ocedures.
Citr us tristeza vir us (CT V)
Citr us tristeza virus (CTV) is endemi c in New Zeal and and is trans mitted by several species of
aphi d, but mai nly by the brow n citrus aphid w hich is very common. M any CTV strai ns i n the
field ar e mil d ones w hich do little damage to the plant, but sever e ones or new virulent strains
can arise at any time and be trans mitted. The sy mptoms of tristeza vary accor di ng to the virus
strain and the scion-rootstock combi nation. Sour or ange rootstocks are very susceptible and
for this r eas on have not been s uccessf ul i n New Zealand ex cept if lemons ar e used as a scion
and aphi ds ar e ex cluded from the rootstock seedli ng stage. Sw eet orange s cions are most
under threat; mandari ns and l emons are not aff ected to the same degree but w ill
nevertheless carry the virus. Trifoli ate rootstock induces some resistance to tristeza but a
resistance br eaking strai n has been i dentified, and a severe stem pitting strai n has affected
some N aveli na and New hall trees on trifoliate at Kerikeri Research Station. Sy mptoms of CTV
infection can i nclude flat topped tree shapes, leaf cuppi ng, vei n cl eari ng, stem pitting, s mal l
fruit si ze, chl orosis, stunting, or total tree coll apse.
PO Box 10 6 29 , Wellington, Ne w Zeala nd
Phone: (04 ) 9 17 7 163 Fax: (04) 473 6041 Em ail: info@ citrus.co.nz
Pre-i mmuniz ation, w hereby plants ar e i nocul ated w ith mil d strains of CTV to pr otect them
from severe strai ns, hol ds the best long ter m strategy for tristeza contr ol. This es pecially applies
for new introductions w hich are tristeza fr ee after s hoot tip grafting and w hich may be very
vul nerable to chall enge fr om sever e stem pitting strai ns.
Nat ur al mil d strai n pr otection can occur i n established orchar ds and until a mild strai n
inoculation procedure is developed, a cl ean budw ood progr amme needs to identify superior
trees in ex isting or char ds that coul d have natural mil d strai n pr otection. I n our case t his
appli es especi ally to sw eet oranges (Navels and V alenci a). Sound horticultur al ev al uation
and dis ease inspection looking f or stem pitting of parent trees, dw arfing, flat tops and s mal l
fruit size w ill help to pick up under perf or mi ng trees that may have a s evere form of the
disease. Thes e s houl d be eli mi nated from use as budw ood s ources. Changes i n rootstock use
coul d have an i mpact on t he expression of diff erent tristeza strai ns. A mil d strai n under one set
of circumstances may not be consistently mild i n another environment or w ith different
stock/sci on combi nations.
Citr us vein e nation virus (CVEV)
Citr us vein enation virus (CVEV) is another vir us trans mitted by the brow n citrus aphid that is
w idespread in N ew Zeal and. It w ill not be a probl em unless there is a change from usi ng
trifoliate and its hybri ds as r ootstocks.
New t hre ats
Certification pr ogr ammes w ill need to be dy namic to cater for new diseases that may
become establis hed i n New Zeal and i n f ut ure. Some ex otic dis eases that are particul arly risky
if introduced to New Zeal and ar e tatterleaf, Citrus vari egated chlorosis, stubbor n, infectious
variegation, gr eeni ng, and new stem pitting strai ns of tristeza. Strict quaranti ne and biosecurity w ill hopefully keep t hes e diseases out.
W ith the range of diseases at pres ent, parent tree blocks i n N ew Zealand can be located
outdoors under strict sanitary control and re-i ndexed for nomi nated pathogens on a regular
basis as prescri bed i n t he best practices, toget her w ithsound horticultur al eval uation.
Further details on some viruses and viroi ds and their i mpact on production can be f ound i n
the book “Gr ow ing Citrus i n New Zeal and- a pr actical gui de” avail able thr ough NZCGI.
Disease an d pest fr ee nurser y trees
W hile viruses and viroi ds ar e covered above there are ot her pests and diseases that can
invade citr us nurseries at any stage. N urs ery trees should be free of i nj uri ous root pat hogens
and nematodes, all of w hich can have an advers e eff ect on tree s urvival and early tree
grow th, and shoul d not be s pread t hrough the i ndustry on pl anting material. Gr ow ers and
nurs erymen may be unaw are of the presence of these pests, because fr equently there ar e
no obvious acute sy mptoms. Control is achi eved by strict sanitation and usi ng pathogen f ree
w ater, and disease and nemat ode free soil or pr opagating medi ums.
Nurs eries must adopt practices w hich mi ni mise the risk of s udden death believed to be
associated w ith infestations of a complex of Fus arium species and ot her root pathogens on
trifoliate and its hy brid r ootstocks. There is evidence that stresses on the root system at any
stage can exacer bate later s udden deat h i n orchar ds w hich only manifests itself once the
trees come i nto beari ng w ith heavy crops. K eepi ng r ootstocks f or an ex cessively long time i n
beds pri or to buddi ng w ill mean t hat t he r oot sy stems w ill have to be sever ed for
trans pl anting. This can cause damage to the root systems, increase t he risk of inf ection w ith
PO Box 10 6 29 , Wellington, Ne w Zeala nd
Phone: (04 ) 9 17 7 163 Fax: (04) 473 6041 Em ail: info@ citrus.co.nz
species of Phyt opht hor a or Fus ar ium , and r es ult i n probl ems w ith grow th and l ater on s udden
deat h. Getting trees off to a good start in the orchar d is best achi eved by usi ng actively
grow ing trees that have been pr opagated w ithout any grow th checks thr oughout thei r
grow ing cy cle from s eed to the ready to pl ant tree.
Foli ar diseas es such as scab and alter nari a, shoul d be absent from nursery tree consignments.
Their pr esence could s peed up t he establis hment of t hese diseases in an or chard even
though they ar e endemic i n the ar ea i n w hich t he trees ar e to be grow n. Preventive
fungi cide progr ammes in the nursery shoul d not include sy stemi c fungici des as this coul d
select genes f or disease r esistance i n the fungal popul ation w hich can be spread to
commer ci ally i mport ant areas.
Insect pests should be absent from nursery consignments. Bud mite is a major pest in young
trees that can adversely af fect grow th. Scal e ins ects can be debilitating to the trees,
especi ally the ar mour ed s cal es that inject toxins into t he tree. Aphids ar e vectors of tristeza
and the Australi an citrus w hitefly should not be spread from i nfected to uni nfected ar eas
(e.g. from Kerikeri to Gis bor ne).
Key questi ons to ask your nurser yman
1. Are trees unif or m, dis ease and pest free, healthy l ooki ng w ith straight stems ?
2. Are bud uni ons at a mi ni mum of 10 cm above s oil level to mi ni mise the risk of
phy topht hor a i nf ection of t he sci on?
3. Have they been deriv ed fr om pathogen free seedli ng popul ations gr ow n in a
pathogen free soil or gr ow ing medi a?
4. Have seedli ngs undergone a ri gorous sel ection pr ocess to eli mi nate “ off- ty pes. ”
5. Are the tree root sy stems fr ee fr om any potentially har mf ul pathogens?
6. Are trees bei ng sol d w ithin a max imum of 18 months of buddi ng and 36 months from
seed?
7. Has the budw ood f or each cultivar come from parent trees that have been i ndex ed
for nomi nated viroi ds and viruses and w henw as the last i ndexing carried out ?
8. Has a process of horticultur al eval uation been carri ed out on budw ood parent trees
to ensur e they ar e true to ty pe?
9. Does the nurs ery have a Quality M anagement sy stem that incl udes traceability
procedur es ?
10. Has the nursery , its pr actices, and its sour ce materi al been audited and certified to
comply w ith the best pr actice protocols?
PO Box 10 6 29 , Wellington, Ne w Zeala nd
Phone: (04 ) 9 17 7 163 Fax: (04) 473 6041 Em ail: info@ citrus.co.nz
Levels of Cer tifi cati on
The ti me requir ed for horticultural ev al uation means that the pathw ay to full certification of
nurs ery trees may tak e ti me f or nurs erymen to achi eve, so an initial three level scheme of
certification is proposed to enabl e gr ow ers to judge how w ell trees comply w ith the best
pr actices r ecommended. Grow ers are advised to ai m f or planti ng Level 3 trees. Level 2 trees
w lil not carry any assur ance of their being tr ue to ty pe, w hile Level 1 trees coul d have many
disadvantages and tree healt h concer ns. The pr opos ed l evels of tree certification are:
Level 3 – Full certification. All best practice protocols have been complied w ith and
certification standards met.
Level 2 – Virus i ndexed parent trees and all ot her best pr actice protocols have been
compli ed w ith and certified, but horticultur al eval uation still i ncomplete.
Level 1 – Only virus i ndexed budw ood has been used and certified. Other best pr actices
have not been met.
All other trees w ould be consi dered uncertified.
Discl aimer: The view s ex pressed i n this articl e are ess entially those of the contribut ors and do
not necess arily reflect the view s of NZ Citr us Grow ers Inc. Reasonable car e and diligence is
taken i n the publication of mat erial but no li ability is accepted for any l osses arisi ng fr om the
use or misuse of any informati on cont ai ned i n this publication.
PO Box 10 6 29 , Wellington, Ne w Zeala nd
Phone: (04 ) 9 17 7 163 Fax: (04) 473 6041 Em ail: info@ citrus.co.nz