South African Avocado Growers’ Association Avoinfo Newsletter, 168:17-19 (Oct 2009).
A BETTER UNDERSTANDING OF ALTERNATE BEARING THROUGH
REVISED GROWTH MODEL CHARTS
B. Nigel Wolstenholme1 & Andrew Sheard2
Alternate bearing remains one of the major horticultural challenges in the management of
avocado orchards. It also presents marketing and cash flow challenges. It is worst when
growing areas are not geographically widely spread, as in small countries like Israel and
New Zealand; or if environmental growing conditions are fairly similar, as in California or
Peru. In South Africa, with avocados produced under fairly variable climates and soils, the
national crop shows less of the alternation typical of the above countries. However, on a
regional basis, and especially on a particular farm, alternate bearing is certainly an issue.
The senior author was recently (July, 2009) invited to present two talks on alternate
bearing to the 4th Australian and New Zealand Avocado Growers’ Association Conference
in Cairns, Australia. Alternate bearing (AB) was the major theme of the horticultural
sessions, in a Conference dominated by marketing, handling, consuming, and avocado
disease issues. New Zealand in particular is plagued by AB, as well as by irregular
bearing (IB) in which the regular “on”/”off” crop alternation is regularly upset by cold spells
during the critical flowering and fruit set period. The causes of AB and IB are still hotly
debated in Australasia. Vested interests muddy the waters by not seeing the whole
picture, and by shifting the focus away from the scientifically established facts.
More recently, a presentation was made to the KZN Avocado Study Group, organized by
Subtrop Technical Advisor (KZN) Andrew Sheard. The authors developed “on” and “off”
crop year phenological growth model charts for this meeting, and believe that they are
helpful in understanding and managing avocado AB. We herewith offer them as a first
approximation for humid, subtropical growing areas in South Africa.
We firstly emphasize that the causes of AB are complex and many. There are two main
scientific theories. The “starch depletion” theory is based on the well-known correlation
between starch storage reserves and yield potential in tree crops. Young avocado
orchard yield typically rises in a fairly predictable way until a major crop is set, due to very
favourable environmental conditions during flowering and fruit set. Until this time, the
vegetative : reproductive balance is tilted heavily in favour of vegetative growth, as the
basic tree structural framework develops. Therefore there has been a surplus of well-lit
photosynthesizing leaves to meet the relatively small energy (carbohydrate) demands of
light fruiting. Surplus carbohydrates are stored as starch reserves – the tree’s buffer
against extreme environmental and pest events.
The first heavy crop heavily depletes the starch reserves (from the pre-flowering winter
peak), and reduces the recovery in starch reserves during summer and autumn. The
following season’s flowering is therefore less intense – mainly due to adverse effects on
1
Prof. (Emer.), University of KwaZulu-Natal; 30 Wavell Drive, Pietermaritzburg 3201, RSA, E-mail:
nigelw@telkomsa.net
2
Prof. (Emer.), University of KwaZulu-Natal; 30 Wavell Drive, Pietermaritzburg 3201, RSA, E-mail:
nigelw@telkomsa.net
shoo
ot and root flushes during an “on
n” season. The resultt is the “offf” crop. Th
his pattern
then becomes entrenched
e
, with “boom
m and bustt” cycling.
However, this explanation
e
is only partt of the storry. We now
w know thatt the heavyy “on” crop
n hormoness, mainly in
n growing
resullts in high concentrattions of antti-flowering gibberellin
seed
ds. Some gibberellins
g
moves to nearby sho
oot buds, and reducess flower bud
d initiation
(in la
ate summer and autum
mn) for the
e next (“off””) crop. Th
his is easilyy observed on heavy
fruitin
ng as compared with light fruitin
ng branche s. Undoub
btedly, we do not yet know the
ultim
mate causes
s of AB, butt seed gibbe
erellins und
doubtedly p
play a role.
The grower is more con
ncerned witth manage
ement of A
AB – how can the b
boom/bust
cropp
ping cycle be
b ameliora
ated? In sh
hort, fairly d
drastic interrvention is required – especially
in mo
ore stressfu
ul environm
ments where
e AB is mo
ore problem
matic. Grow
wers have to buy into
a full package of remedia
al measures
s, and by-a
and-large tthese are cconsistent w
with more
nsive “best practice” managemen
m
nt. Removval of excesss (small) ffruit early in
n the “on”
inten
seas
son is surely the most obvious, but
b understa
andably the
e least pop
pular option. Pruning
resto
ores vegeta
ative vigou
ur in “on” years whe
ere the balance has moved exxcessively
towa
ards fruiting
g. Girdling
g has a ro
ole to play (if properrly used), a
as does th
he growth
retarrdant unico
onazole (S
Sunny®). Obviously, other ma
anagement practices must be
optim
mized, and Phytophtho
ora root rot properly co
ontrolled.
Figure
F
1. The total growth cycle of cv
v Fuerte show
wing the relattionship betw
ween vegetative and
reproductive
r
growth and reserve
r
starc
ch in the trun
nks of trees (Whiley & W
Wolstenholme,, 1990)
Typical
T
of a humid, subtrop
pical climate; low ABI.
dard tool forr guiding avvocado orchard management – the Whiley
This brings us to the stand
enological growth
g
mod
del (Fig.1), well-known
n today thro
oughout the
e avocado
et al.. (1988) phe
grow
wing world – with adap
ptation to lo
ocal conditions. But itt is importa
ant to reme
ember that
this model was
s drawn up
p for very specific
s
con
nditions, viz. avocado
o orchards in a mild
(mes
sic) humid subtropica
al area, wh
here AB is not a big problem ((Whiley, A..W., pers.
comm
m., 2009). It does nott therefore look at the
e two-year ((“on”/”off” crops) cycle
e typical of
2
AB orchards.
o
The
T New Ze
ealanders were
w
the firsst to draw u
up a two-ye
ear chart, in
n the form
of a circle
c
with two
t
hemisp
pheres repre
esenting th
he on-off cyycle, and picctorial repre
esentation
of ph
henological events (flo
owering inttensity, fruitt drops, cro
op size, sh
hoot and ro
oot growth
flush
hes) (Hardy
y et al., 200
08). This is
s necessarry to accom
mmodate th
he fact that the Hass
fruit here takes more than
n one year to
t mature, and can be
e hung on the tree for up to 18
montths from flo
owering (inc
cidentally, th
hereby grea
atly aggravvating AB).
For average
a
So
outh African
n conditions
s, we present an avoca
ado “on” cro
op growth ccycle (Fig.
2), and
a the con
nsequent “o
off” crop gro
owth cycle (Fig. 3). T
This theme was develloped in a
“work
kshop on th
he avocado
o growth cy
ycle” at the Cairns con
nference, a
and was em
mphasized
in th
he senior author’s
a
ta
alk on AB (without a
actually dra
awing the explanatorry figures)
(Wolstenholme,, 2009).
Figure
F
2. Phe
enological gro
owth cycle in an "on" crop season, with vegetative-re
eproductive b
balance
heavily
h
biased
d towards flo
owering and fruiting
f
(Wolsstenholme, 20
009, adapted from Whileyy et al.,
1988).
1
Typica
al of more stre
essful environments, follow
wing an "off" sseason; High ABI.
Figurre 2 (hea
avy crop, i.e. “on” year)
y
grap
phically represents tthe situatio
on where
repro
oductive ev
vents domin
nate. Thus
s flowering is very inttense: starch reserves start off
very high, and there
t
are plenty of fruiting sites ((peripheral shoots) fro
om the prevvious (light
crop)) season. Fruit
F
drop will
w be prom
minent due tto the shee
er volume o
of flowers and initially
set fruits.
f
The harvested crop will still
s be very
ry high beccause fruitin
ng starts u
under very
fruitin
ng-friendly conditions, and prefe
erentially a ppropriatess most of tthe tree’s rresources.
All of
o this, of course, has a trade-offf – vegetative growth
h loses outt big-time. Thus the
sprin
ng growth flush
f
is red
duced; the summer g
growth flussh even more. Thesse flushes
proviide the fru
uiting sites for next season’s
s
(“ off”) crop – simply p
put, not en
nough are
formed. Just as
s importantt, the two ro
oot flushes (late spring/early sum
mmer, and especially
the autumn
a
flus
sh) are red
duced. So the tree g
goes into a
autumn and
d winter witth a huge
crop, but rundo
own starch reserves, fe
ewer fruitin
ng sites, lesss flower bu
ud initiation
n because
g about in late summe
er; and leavves which h
have been
of alll the surplus gibberelliins sloshing
3
over--worked ph
hotosynthettically, are “tired”
“
and more pron
ne to photo-inhibition, and have
been
n inadequattely renewe
ed by flushin
ng. Talk ab
bout a recip
pe for disasster!
Fig.3
3 depicts the
t
conseq
quences of
o the “on”” crop on the next season’s “off” crop
phen
nological ev
vents. Here
e reproducttive events play secon
nd fiddle. T
There is lesss intense
flowe
ering, with equivalentt fruit drop
ps, and a llight crop. This resu
ults from lo
ow starch
reserrves, stressed leaves
s and rootts during frruit set, an
nd other p
physiologica
al factors.
Vege
etative flush
hing of both
h shoots an
nd roots ho
owever reco
overs dram
matically – ssetting the
scen
ne for next season’s heavy
h
crop, along with
h starch re
eserve build
dup due to
o relatively
low fruiting
f
dem
mand.
Figure
F
3. Phe
enological gro
owth cycle in an "off" crop season, with vegetative-re
eproductive b
balance
heavily
h
biased
d towards flo
owering and fruiting
f
(Wolsstenholme, 20
009, adapted from Whileyy et al.,
1988).
1
Typica
al of more stre
essful environments, follow
wing an "on" sseason; High ABI.
The challenge for
f the orch
hard manag
ger is to mo
ove the gro
owth modells in Figure
es 2 and 3
ards the “id
deal” shown in Fig. 1,
1 where th
he crop iss average, and vegettative and
towa
repro
oductive ev
vents are nicely balanc
ced. Starch
h reserves peak at abo
out the sam
me level in
succ
cessive seasons. Figu
ures 2 and 3 suggest what need
ds to be done. This iss the topic
for another occa
asion.
Ackn
nowledgement
The authors acknowledg
a
ge the pioneering wo
ork of Ton
eagues in
ny Whiley and colle
deve
eloping the avocado growth model concept.. Colin Partridge (Southern Prod
duce, NZ)
also contributed
d useful ideas.
erences
Refe
Hard
dy, J., Parrtridge, C., Barber, R., Pak, H. and D
Dixon, J. 2
2008. Sprring: Tree
manageme
m
ogy and flow
wering. N.Z
Z. Avo. Grs’’ Assoc., Pp
p 35.
nt, phenolo
While
ey, A.W., Saranah,
S
J.B., Cull, B.W.
B
& Peg
gg, K. G. 1988. Mana
age avocad
do growth
cycles
c
for productivity
p
gains. Qld
d Agric. J. 114: 29 – 36
6.
4
Whiley, A.W. & Wolstenholme, B.N. 1990. Carbohydrate management in avocado trees
for increased production. S. Afr. Avocado Growers’ Association Yearbook 13: 25-27.
Wolstenholme, B.N. 2009. Alternate bearing in avocado – an overview. 4th Aust. & N.Z.
Avocado Grs’ Conf. Proc. In press.
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