Premature shedding of citrus
This problem is difficult to prevent as normally about 98% of the fruitlets that originally set on orange trees shed when the fruit is about pea sized. The remaining 1% to 2% is sufficient to produce a commercial crop of fruit.
A second fall in midsummer or December occurs when the fruit is about 2 to 2.5cm in size and usually determines the ultimate fruit production. Trees growing in Perth’s sandy soils often suffer a very severe midsummer drop. At this time of the year rapidly rising temperatures and desiccating easterly winds intensify the fall.
Citrus fruit splitting
Citrus fruit splitting may start as early as mid summer, but most of it occurs in autumn. Navel oranges are most susceptible, followed by tangelos, some tight-skinned types of mandarins, and other oranges. The split usually starts at the blossom end of the fruit, which is the weakest point in the rind. The split may be short and shallow or it may be deep and wide, exposing the segments of the juice vesicles. Young trees are more prone to fruit splitting than older trees. As well as being wasteful, the splitting creates a good breeding ground for fruit fly, so the split fruit should be removed and treated.
Splits probably occur when water and sugars are transported from the roots of the tree to the ripening fruit, and the rind is unable to expand quickly enough to accommodate the added volume. The rind bursts open under the pressure.
Splitting appears to be most closely related to extreme fluctuations in temperature, humidity, soil moisture and fertiliser levels. It is thought that the trouble is caused by a combination of these factors rather than by a single cause. Splitting is usually observed when growing conditions become erratic such as under water stress, uneven fertiliser supply, temperature fluctuations like hot and cold nights, and sudden rainfalls.
Some varieties are more susceptible to splitting than others.
Reducing the loss
Irrigation and fertiliser management
There is no specific remedy to completely overcome this premature shedding or fruit splitting, but heavy losses can be reduced by extra attention before and during the critical periods. Trees should always have access to sufficient water and nutrients. In a garden situation with sandy soil and water restrictions, a number of steps can be taken:
Improve soil with compost and add clay, which is available from garden centres, to sandy soils. Citrus need a continuous supply of soil moisture and grow best when supplied with daily water in spring, summer and autumn. A hand held hose should be used on non watering days.
In summer a 1m by 1m tree will need about 7 litres of water a day, a medium 2m by 2m tree about 28 litres a day and a mature tree of 4m by 4m will need up to 111 litres per day.
In autumn and late spring a small tree will need about 5 litres a day, a medium tree about 20 litres and a mature tree up to 82 litres.
Rainfall should supply enough moisure in winter.
Use compost and slow release fertilisers to feed the tree. This will stop sudden spurts of nutrients. Citrus trees use comparatively large quantities of nitrogen. During active spring growth, shortages of this important element can retard the tree and accentuate fruit fall. Make sure soil supplies are replenished and available from August to December. Also make sure the tree has enough trace elements. If the leaves yellow and show nutrient deficiencies, apply a foliage spray. The most receptive time is following a flush of new growth.
Use mulch to retain soil moisture by reducing evaporation.
Insects
Parasitic insects, particularly the black citrus aphid which is active while the fruit is still small and vulnerable, directly contribute to excessive shedding of fruit. Observe trees carefully, and control aphids if necessary.
Location
When planting new trees avoid windy and shaded situations and be aware of possible competition from adjacent trees and plants. Lawn, particularly, should be kept well away from fruiting trees and if allowed to compete with the tree extra water should be applied. Digging around citrus trees should be avoided as trees have shallow roots.