Hand Pollination of Custard Apples
A DPI Technical Feature
The
custard apple grown in Australia is a hybrid of the sugar apple (Annona
squamosa) and the cherimoya (Annona cherimola). More recently,
hybrids have also been developed with Annona reticulata. In America it
is known as Atemoya.
The
custard apple flower is hermaphroditic (male and female parts in the
same flower) and exhibits protogynous dichogamy (stigmas = female parts
are receptive before the pollen is shed by the anthers).
The
separation of the female and the male stages appears to be the the main
factor limiting the level of self pollination. Fruit set can occur as a
result of self pollination, insect-assisted pollination or hand
pollination. The level of wind pollination is low. Studies in Australia
and in Israel have shown that the main insect pollen-carrying vectors
are nitidulid beetles. These beetles are capable of transferring pollen
from the stamens to the stigmas in the same flower or to other flowers
at the time of anther dehiscence. The population of these beetles in
orchards at flowering time may vary and consequently fruit set may be
low.
When do custard apples flower?
Custard
apples flower between October to February inclusive. Flowering is
associated with the vegetative flushing pattern. Flowering intensity is
dependent on tree vigour, tree age, lateral size and variety. Some
varieties such as Pink's Mammoth, exhibit strong apical dominance,
producing few flowers until 5-7 years of age. Other varieties, such as
African Pride, produce an abundance of flowers within 2-3 years after
planting.
Hand pollination
The advantages of hand pollination are:
1. Increased fruit set of poorer yielding varieties;
2. Manipulation of the harvest period;
3. Better quality (size and shape) fruit.
Currently
hand pollination is mainly used to increase fruit set of poorer
yielding varieties. Considerable opportunity also exists to manipulate
flowering and fruit set periods to produce high priced early or late
set fruit. Fruit quality is also improved as a result of hand
pollination, making fruit more suitable for export markets. A premium
price is often paid for hand pollinated fruit.
Pollen collection and storage
Flowers
from which pollen is to be collected should be picked from trees during
mid to late afternoon. The petals should be nearly fully opened and the
pollen sacs should have turned creamy-grey in colour and less tightly
held together.
Immature pollen sacs are white and tightly
packed. On hot, low humidity days, the flowers will release their
pollen sacs readily. These can be collected by shaking the flowers over
a piece of paper. The pollen sacs should be placed in a small container
and stored at room temperature (<20°C) overnight for use the
following morning. Alternatively if the pollen sacs do not readily
separate from the flowers, the flowers should be spread thinly over
shallow layered trays and left overnight. Pollen sacs should be then
separated on the following morning. Flowers should not be placed in a
closed container as a build up of ethylene and moisture will cause the
pollen sacs to turn brown and pollen germination will be severely
reduced. Preliminary studies suggest that custard apple pollen can only
be stored for very short periods of time (2-3 days).
Frequency of pollination
Trees
can be hand pollinated every week from December to February. Records on
the number of flowers pollinated on each tree and set should be kept so
as to prevent overcropping and consequently reduced fruit size.
In
South-East Queensland, the fruit development period for African Pride
is approximately 26 weeks, while the period for Hillary White is 19
weeks. It is possible to manipulate components of the fruit development
cycle by such things as defoliation, hand pollination and summer tip
pruning and consequently the time of fruit maturity.
Environmental factors affecting fruit set
The
success of hand pollination depends on environmental conditions during
fruit set. Conditions conducive to high fruit set are moderate
temperatures, high relative humidity, lack of tree water stress and no
wind or continuously heavy rain. If relative humidity falls below 70
percent, fruit set and fruit shape are adversely affected. High
relative humidities in the orchard can be maintained by establishing
windbreaks. Light applications of irrigation in the late afternoon
using wide-throw minisprinklers also will help opening flowers maintain
their stigma receptivity until the following morning.
The most
favourable period to hand pollinate flowers is in the early morning
prior to 11 am. Fruit set is reduced if flowers are pollinated during
the hottest period of the day. Hand pollination should not be attempted
on days when light rain is falling as fruit set on these days is
reduced.
Tree physiological factors affecting fruit set
Fruit
set is reduced during periods of strong vegetative flushing, due to
competition between the growing points and the flowers. Larger flowers
set more heavily than smaller ones. Flowers produced on the basal nodes
(node positions 1-5) set more heavily than those closer to the apices.
Trees should be maintained in a state of good health. Foliar
applications of boron during the flowering period will also improve
fruit set and fruit quality.
Economics
Flowers
can be hand pollinated using the brush technique at the rate of 150-200
flowers per hour with success rates varying between 80-100 per cent.
Using pollination guns this rate can be increased to 400 flowers per
hour. Using the brush technique and assuming a minimum success rate of
80 per cent, 160-250 fruit can be set for every hour of hand
pollinating. This is enough to fill about ten cartons. Using 1995
prices of $12 per carton, a gross return of $120 could be obtained for
every hour of hand pollinating. Even greater returns could be achieved
if a more effective hand pollination gun could be developed.
Pollen sources for hand pollination in custard apples
Hand
pollination is required to produce a commercial crop of Pinks Mammoth
and Hillary White custard apples. Pollen from these varieties is not
always available at the right time in large enough quantities to set a
viable commercial crop. Other varieties, such as African Pride (which
produces large quantities of flowers), can be a good source of pollen
in these times of pollen shortfall.
Trialling alternative pollen sources
Trials
were conducted to assess the use of alternative pollen sources. Flowers
from African Pride, Pink's Mammoth and Hillary White varieties were
harvested mid-afternoon and left overnight. The following morning, the
pollen sacs were extracted from the flowers of each of the varieties.
Flowers from both Pink's Mammoth and Hillary White were hand pollinated
with pollen from Pink's Mammoth, Hillary White and African Pride.
African Pride was only pollinated with its own collected pollen, to
confirm its pollen viability. African Pride was not pollinated with the
other two pollen sources as it usually does not require hand
pollination.
All pollinated flowers were tagged and the pollen
source recorded. Fruit, resulting from the pollination, was assessed at
harvest.
African Pride pollen is best
African Pride shows a high level of self-compatibility when using its own pollen for hand pollination.
In
contrast, varieties such as Pink's Mammoth show a high level of
self-incompatibility which results in reduced fruit set and poorer
fruit shape.
African Pride is therefore the preferred pollen source for most commercial varieties of custard apples.
Methods of pollen application in custard apples
A
variety of methods are available for applying pollen to flowers when
hand pollinating custard apples. These methods include pollination
guns, puffers and the use of a camel hair brush. All of these
techniques are effective when pollen is not strongly diluted with a
carrier such as lycopodium (a tan powder made from the spores of
clubmosses. It is highly flammable) or PVC dust.
Which application technique is best?
Flowers
from African Pride were harvested mid-afternoon and left overnight. The
following morning. pollen sacs were extracted and diluted with various
amounts of PVC dust. The pollen mixtures were used to pollinate Hillary
White flowers. Two methods of application were used - a camel hair
brush and a pollination gun (based on a prototype of the persimmon
pollination gun). All flowers were tagged and pollen dilution and
application methods were recorded. Fruit was assessed at harvest.
Discussion
The
brush application technique is a more effective method of increasing
fruit set and symmetry. The quality of the fruit from this method is
only affected when the dilution of pollen falls below 30 per cent.
The brush technique produces more reliable results due to a greater amount of pollen applied to the stigmas.
Pollination guns tested need to be modified so as to apply a more accurate and uniform dose of pollen to the flower.
The pollen storage and delivery system of these guns need to be pressurised so that the gun can be used at any angle.
Flower size affects fruit size in custard apples
Choosing
the right flowers to hand pollinate can improve custard apple fruit
quality and size. Hand pollination can be time consuming so it is
important to ensure that the right flowers are pollinated to give the
best results.
Trialing the effect of flower size
A trial was conducted to see if flowers size affected fruit quality and size.
African
Pride flowers were harvested mid-afternoon and left overnight. The
following morning, pollen sacs were extracted and used to pollinate
Pink's Mammoth flowers of three specific size grades.
These flowers were tagged and the subsequent fruit assessed at harvest.
Larger flowers produce better fruit.
It is evident that the size of the flower affects the size of the fruit produced and the level of fruit set.
Flowers that were 40-50mm long had a 98 per cent level of fruit set with excellent fruit symmetry.
This
is in contrast to flowers that were 20-30mm in length which produced a
low level of fruit set (48 percent) with reduced, but acceptable, fruit
symmetry.
This finding is similar to findings in other temperate
fruit tree crops which have shown that tree size control and canopy and
nitrogen management can all influence flower size.
It is recommended
that leaf nitrogen levels be maintained in the optimum range and for
greater productivity, larger flowers should be targeted in hand
pollinating operations.
Provided these problems could be overcome,
the pollination gun would be a much faster method of application
(nearly twice as fast).
In
California, the Japanese persimmon pollination gun has proven to be
highly effective in pollinating cherimoya, unfortunately these guns are
no longer manufactured.
At low dilution rates, the pollination gun
produced comparable fruit set and symmetry results to that of the brush
application technique.
The slight reduction in fruit set results
would be compensated by the increased pollination rate that is
achievable (x5 the brush application technique) with this device.
This would make the use of a hand pollination gun a much more profitable practice.
Pollen dilution for hand pollination in custard apples
Custard apple growers use hand pollination to increase and control fruit set.
Collecting sufficient pollen to pollinate an economical number of flowers is time consuming.
Pollen can be extended by using dilutants such as PVC dust, lycopodium and talc.
Pollen sources and dilution rates
Trials
were conducted to investigate the use of dilutants to extend pollen.
African Pride flowers were harvested mid-afternoon and left overnight.
The following morning, the pollen sacs were extracted and some was
diluted with either PVC dust or lycopodium. The dilution rates were 50
per cent (Pollen): 50 per cent (Dilutant), 33.3 per cent (Pollen): 66.6
per cent (Dilutant) and 100 per cent (Pollen). The prepared dilutions
were then used to pollinate Pink's Mammoth flowers. These flowers were
tagged and the subsequent fruit assessed at harvest.
What's recommended
At
low levels of dilution (less than one third - 33 per cent), there
appears to be less than a 15 per cent drop in the efficacy of
lycopodium or PVC dust when compared to undiluted pollen. At higher
rates of dilution, PVC dust performed better as a carrier than
lycopodium. There was approximately a 30 per cent reduction in the
level of fruit set using PVC dust, whereas, an 85 per cent reduction
occurred using lycopodium.
• Fruit symmetry appeared to be
affected minimally by the use of PVC dust, whilst the use of lycopodium
produced a drastic reduction in fruit symmetry.
• The results achieved through the use of dilutants are still lower than those achieved from undiluted pollen.
• The
benefit of using PVC dust as a dilutant is the increased number of
flowers that can be pollinated per gram of pollen harvested.
• It appears that the pollen grains adhere much more easily to the PVC dust than they do to lycopodium.
• PVC dust is also less expensive than lycopodium.
Back To
Sugar Apple Page
Custard Apple Page
Atemoya Page
|