In the custard apple flower, pollen shedding and ovary receptivity are often poorly synchronised and this minimises the chances of adequate and complete pollination. As a result, natural fruit set and shape are poor in some varieties. Hand pollination is a technique developed to overcome these problems.

Understanding custard apple pollination

The custard apple flower
The custard apple flower is hermaphroditic (male and female parts in the same flower) and exhibits protogynous dichogamy, which means that the stigmas or female receptors are receptive before the pollen is shed by the male anthers. This separation of the female and the male stages limits the level of natural self-pollination in most varieties to between 0 and 8% (Table 24).

Table 24. Self-pollination percentages of major varieties

Varieties with no self-pollination (0%)	Varieties with low self-pollination (1 to 3%)	Varieties with good self-pollination (more than 3%)
Pinks Mammoth	Hillary White	African Pride
	Martin	Gefner
	Most cherimoyas	Maroochy Gold
		Maroochy Red
		Fino de Jete

The degree of natural self-pollination depends on several factors, some of which can be manipulated to some extent by the grower (Table 25).

Table 25. Factors affecting natural self-pollination

Requirement for self-pollination	Factors affecting self-pollination	Impact
1. Stigmas receptive	Relative humidity	70 to 90% ideal (afternoon showers beneficial) · less than 60% or more than 90% detrimental (heavy or prolonged rain detrimental)
2. Viable pollen	Temperature Relative humidity	· higher than 35° C detrimental · 40 to 65% ideal · more than 95% detrimental · optimum level beneficial
3. Pollen transfer	Boron nutrition Pollinator insects (see below)	· thought to be responsible for 70% of natural self-pollination
4. Fertilisation	Flower self-incompatibility	· high in Pinks Mammoth and Hillary White · low in African Pride

Wind has very little impact on the level of natural self-pollination.

 Insect pollination
Studies in Australia and Israel have shown that the main insect pollinators of custard apple flowers are nitidulid beetles (Family— Nitidulae) (Figure 59). These beetles can transfer pollen from the anthers to the stigmas in the same flower or to other flowers.


Figure 59. Closeup of nitidulid beetle

The population of nitidulid beetles in orchards at flowering time varies considerably and is influenced by many factors (Table 26).

Table 26. Factors affecting population build-up of nitidulid beetles

Factor	How it impacts
Availability of rotting fruit	Rotting fruit acts as a food source and attracts nitidulid beetles into the orchard. However, as it may be a more preferred source of food and shelter than custard apple flowers, it needs to be managed carefully. The most favourable conditions for build-up of nitidulid beetles in rotting fruit are high temperatures (25 to 30° C) and high humidities (75 to 95% RH).
Soil temperature	High soil temperatures (over 28° C) seem to stimulate massed flights of beetles and increase their likelihood of entering custard apple flowers.
Wind	Nitidulid beetles are weak fliers, even in light winds. This reduces their movement into custard apple flowers.
Rainfall	Rainfall and high humidities appear to be detrimental to beetle activity by reducing their flight and by reducing the attractiveness of the scent of custard apple flowers

Maximising natural self-pollination
The best techniques for improving natural self-pollination are:
• maintaining high humidities in the orchard by late afternoon sprinkling;
• increasing nitidulid beetle populations in the orchard by placing rotting fruit attractants;
• using windbreaks to reduce wind movement and improve the movement of nitidulid beetles into the flowers;
• ensuring boron levels are maintained in the optimum range to maximise pollen viability.

An understanding of the process of natural self-pollination is important for improving productivity and for developing improved varieties.
As varieties with high levels of natural self-pollination appear to have more self-compatible flowers, as well as heavily scented flowers more attractive to nitidulid beetles, these characteristics can be screened for in new variety breeding programs.

Hand pollination
Varieties with very low or no natural self-pollination (for example Pinks Mammoth and Hillary White) will have poor fruit set, yield and shape if allowed to self-pollinate naturally. Hand pollination is necessary to achieve commercially acceptable yields and fruit quality. On the other hand, varieties such as African Pride, which are self- compatible, produce heavy yields and good fruit shape without hand pollination.

However, besides improving fruit set, yield and shape, there are other reasons for hand pollination. These are to:
• manipulate flowering and fruit set and hence the harvest period to take advantage of high-priced, early or late season markets;
• manipulate the length of the harvest season;
• improve fruit size;
• overcome low pollen viability in early season flowers where an early crop is being sought (particularly in African Pride);
• overcome low natural self-pollination in drier areas where low humidities reduce stigma receptivity and pollen viability.

Sources of pollen for hand pollination
The best source of pollen is flowers of the African Pride variety. This variety produces large numbers of flowers and has a high degree of self- compatibility. By contrast, Pinks Mammoth produces few flowers and has a low level of self-compatibility. Growers of Pinks Mammoth and Hillary White should have some African Pride trees in their orchard to provide a source of pollen for hand pollination.

Collection and storage of pollen
Pick flowers from which pollen is to be collected during mid to late afternoon. Suitable flowers have petals nearly fully opened (Figure 60). The pollen sacs are creamy-grey and less tightly packed together; immature pollen sacs are white and tightly packed (Figure 61).


Figure 60. Flower suitable for collection


Figure 61. Left: flowers ready for collection, showing the pollen sacs separating (this flower is in the male stage). Right: immature pollen sacs are white and tightly packed (this flower is still in the female stage)

On hot days with low humidity, the flowers will release their pollen sacs readily. Collect these by shaking the flowers over a piece of paper (Figure 62). Place the pollen sacs in a small open container and store at room temperature (less than 20 ° C) overnight for use the next morning. Discard the petals and flower stems.


Figure 62. Left: pollen sacs separated from the flower. Right: close-up of pollen sacs, showing released pollen grains

If the pollen sacs do not readily separate from the flowers, spread the flowers in a shallow layer in a tray and leave overnight. Pollen sacs can then be separated the next morning by again shaking the flowers over a piece of paper. Discard the petals and flower stems.

Do not store flowers in a closed container while waiting for pollen sacs to separate. A build-up of ethylene and moisture causes the pollen sacs to turn brown and pollen germination is significantly reduced.

Use the pollen within 24 hours of collection. About 20 to 30 collected flowers should provide enough pollen to pollinate about 50 to 60 flowers.

Methods of pollen application
There are three main methods for applying pollen to the flowers:
• camel hair brush
• pollination gun
• puffer.

All three can be used with the pollen undiluted or diluted with lycopodium dust or PVC dust. Dilution enables the pollen to go further as well as making it flow better through guns and puffers. However, the three methods are effective only when pollen is not strongly diluted.

Camel hair brush
This is a standard soft hair brush with the bristles trimmed (Figure 63).


Figure 63. Camel hair brush and pollen container for use in the field

The camel hair brush method is the most effective because it applies the greatest amount of pollen to the stigmas of the flowers. As a result, the best fruit set and symmetry is produced. Fruit set and quality is only affected when pollen is diluted more than 30%.

Pollination gun
Pollination guns have the advantage that flowers can be pollinated much more quickly (from two to five times the pollination rate) than with the camel hair brush.

However, most guns do not apply the pollen as accurately and as uniformly as the brush. Guns also do not work well at upward angles as there is often not enough pressure to push the pollen upwards into the flower. Pressurised guns would overcome this problem but this is not a feature of most currently available guns.

In Queensland, however, the Japanese persimmon pollination gun (Figure 64) has been redesigned and early field-testing has shown it to be highly effective. At low dilution rates, the pollination gun produced fruit symmetry comparable with that of the brush application tech- nique and fruit set only slightly lower. This slight reduction in fruit set would be more than compensated by the increased pollination rate of the gun.


Figure 64. Prototype of the Japanese persimmon pollination gun

Puffer
Puffers consist of an air bladder, pollen container and fine discharge tube (Figure 65). They have similar advantages and disadvantages to the pollination gun but they need to be held upright to discharge properly.


Figure 65. Puffer

Pollen dilution
Pollen can be used undiluted or diluted with a carrier such as lycopodium dust or PVC dust. Dilution enables the pollen to go further as well as making it flow better through guns and puffers.

Dilution, however, reduces the efficacy of pollination, the degree of reduction depending on the dilution rate and the carrier used. Research has shown that at low levels of dilution (1 g of carrier per pollen sacs from 100 flowers), there is only a 10 to 15% reduction in fruit set compared to undiluted pollen. Both carriers give similar results. Fruit symmetry for the diluted pollen is similar to that for undiluted pollen.

At higher rates of dilution (2 g of carrier per pollen sacs from 100 flowers), there is a 30% reduction in fruit set using PVC dust and an 85% reduction in fruit set using lycopodium dust compared to undi- luted pollen. It appears that the pollen grains adhere much more easily to PVC dust than they do to lycopodium dust. In addition, fruit symmetry for the lycopodium carrier was adversely affected, whereas it was relatively unaffected by the PVC carrier.

These results indicate that lycopodium carrier should never be used at the high rate. PVC dust is only suggested at the high rate in mid season, when pollen viability is high (Table 27). PVC dust is also cheaper than lycopodium.

Pollen carriers should be used in the knowledge that fruit set will always be slightly reduced. The benefit, however, is the increased number of flowers that can be pollinated per gram of pollen harvested.

Table 27. Guide to dilution rates for hand pollination

Carrier type	Dilution rate (grams of carrier per pollen sacs collected from 100 flowers)
	Early season	Mid season	Late season
Lycopodium	1.0	1.0	1.0
PVC	1.0	2.0	1.0

Pollinating flowers
Hand pollinate in the morning, preferably before about 7.00 a.m. but no later than about 11.00 a.m. Select flowers in the female stage (Figure 66).


Figure 66. Flower ready for pollination

If using the camel hair brush, carry the pollen in a suitable container (Figure 63) which can be conveniently placed in a chest pocket or hung around the neck.

Gently separate the petals and transfer pollen from the container of pollen sacs to the female parts of the flower. Gently twist the brush to ensure even pollination of all stigmas (Figure 67). The number of stigmas pollinated will determine the potential size and shape of the fruit.


Figure 67. Hand pollinating flowers with the camel hair brush

Frequency of hand pollination
Trees can be hand pollinated every week during the flowering period. For each tree, to prevent overcropping and reduced fruit size, record the number of flowers pollinated and those that set fruit. Manipulating the time of harvest

In south-east Queensland, the fruit development period for African Pride is about 26 weeks, and for Hillary White and Pinks Mammoth about 19 weeks. Depending on location, an earlier or later crop can be produced by intensifying the amount of hand pollination at the beginning of the season for African Pride and at the end of the season for Hillary White and Pinks Mammoth.

Economics of hand pollination
Using the brush technique, about 150 to 200 flowers can be hand pollinated per hour, with a success rate of between 50 and 100%. Using pollination guns and puffers, the rate can be increased to 400 flowers per hour.

With the brush technique, and assuming a minimum success rate of 75%, about 110 to 150 fruit could be set for every hour of hand pollination. This is equivalent to about 8 to 12 trays of fruit. Using an average price of $10 per tray, a gross return of $80 to $120 could be obtained for every hour of hand pollination. Gross returns could be increased significantly when hand pollination is used to target early season fruit for the high quality export market.

Factors affecting fruit set
The success of hand pollination depends largely on environmental conditions during fruit set. The physiological status of the tree plays a less important role.

Environment
Environmental conditions conducive to high fruit set are:
• high relative humidity
• moderate temperatures
• no heavy rainfall
• adequate moisture supply to the tree
• still conditions with little wind.

Of these, relative humidity, temperature and rainfall play the most important role.

Relative humidity. If relative humidity falls below about 70%, fruit set and shape is adversely affected. Good windbreaks help to maintain high relative humidity in the orchard. Light applications of irrigation using wide throw minisprinklers in the late afternoon also helps flowers to maintain their stigma receptivity until the next morning.

Temperature. Fruit set is reduced if flowers are pollinated during the hottest part of the day. The most favourable time to hand pollinate flowers is in the early morning before 11.00 a.m.

Rainfall. As fruit set is reduced by free water on the flowers, avoid hand pollination on days of light rain.

Trees
Fruit set is reduced during periods of strong vegetative flushing because of the competition between the growing shoots and the flowers.

Larger flowers (40 to 50 mm long) on the basal nodes of shoots set better fruit than smaller ones closer to the tips.

Trees in a good state of health set better crops. Pay close attention to nutrition and irrigation; boron nutrition is particularly important. Foliar applications of boron during the flowering period will generally enhance fruit set and quality.