Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 4 Western Australia, Series 4
Volume 7
Number 1 1966 Article 3
1-1-1966
Why does fruit drop?
Why does fruit drop?
K T. Whitely
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Recommended Citation Recommended Citation
Whitely, K T. (1966) "Why does fruit drop?," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 7: No. 1, Article 3.
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H O R T I C U L T U R E
WHY DOES FRUIT DROP?
By K. T. Whitely, B.Sc. (Agric), Adviser, Horticulture Division
THERE is always a dramatic difference between the number of blossoms that form on a f r u i t tree in spring and the number of f r u i t which ultimately mature. This is very obvious t o the grower in the " o f f " or light crop year when he feels a personal loss with every young f r u i t l e t or near-mature f r u i t that falls from the tree.
The fact is that even in such years only about 1 to 2 per cent, of blossom need reach maturity for a satisfactory crop to be harvested. But just the same there are seasons when the excessive drop of fruit is a big factor in deciding the ultimate crop. What is the reason for this shedding of fruit?
Most types of fruits in most conditions do not suffer a continuous shedding after blossom but undergo a series of two or three waves of shedding.
The dropping of the young fruitlets is due to changes in the nature of the con- necting walls between layers of cells in the stem of the fruit, called the abscission zone. These changes are controlled by hormones which in turn are influenced by other factors.
The first wave of shedding generally takes place soon after blossoming and is due to lack of fertilisation in the would-be fruits. This drop usually continues for a few weeks and depending on variety, may have several minor waves of dropping before it is finished.
The grower is able to reduce this first drop by providing the right cross-pollinat- ing varieties and increasing the density of bees in the orchard.
The second main wave of shedding takes place when the young fruits are about marble size, that is, about £ in. or more in diameter. This is generally in December.
Usually, the more flowers in proportion to the size of tree and the smaller the percentage of young fruitlets to fall in the earlier waves, then the larger the percent- age of fruit to fall in the second wave.
This shedding seems to result largely from competition between fruits for limited food and moisture supplies. In this drop, apple and pear fruits with the fewest and the weakest seeds are usually shed first. In fruits such as citrus and Bartlett pears, which are able to mature the crop without pollination, these unfer- tilised fruits are most apt to fall at this stage.
The grower can reduce the loss during the second main shedding of fruit by
• Ensuring soil moisture is always adequate. This is particularly important in the metropolitan sands where a few days of hot weather and easterly winds can quickly sap the available mois- ture.
• Preventing root damage by imple- ments or by waterlogging or root rot fungus.
• Insuring that the tree does not suffer from a lack of fertiliser, especially nitrogen. However, other elements such as copper can be important, and the mainten- ance of healthy foliage and satis- factory vigour is a good rule of thumb principle in this regard.
• Excessive application of fertilisers can be detrimental.
Over the main period of the growth of the fruit further shedding can occur, depending to a large degree on weather conditions. However the next serious shedding occurs on those types which are subject to pre-harvest drop.
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Journal of Agriculture, Vol 7 No 1 1966
In these fruits the abscission layer has a tendency to form before the fruit is properly mature, and the fruit falls.
There are certain known predisposing factors:
• Hot, dry summer conditions are apt to bring on pre-harvest drop of such apple varieties as Jona- than and Gravenstein.
• On the other hand persistent wet weather in the early winter is generally conducive to pre-harvest drop of Washington navel oranges.
This third shedding can be controlled by artificial hormones. The formation of the abscission layer is due to the critical balance of plant hormones in the tree and the fruit and this balance can be affected by these hormone substances.
The type of hormone which gives best results varies with the different types of fruit. The right hormone at the correct strength and timing can achieve outstand- ing results.
In Brief . . .
Although the grower is very much at the mercy of nature in the matter of fruit shedding, the good fruitgrower knows just what control he can expect when the fall is excessive.
By good management, the judicious use of fertilisers and irrigation, and later, the timely application of pre-harvest hormone sprays, he will prevent excessive fruit drop and so maintain his crop at a reasonable level.
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Journal of Agriculture, Vol 7 No 1 1966
