One reason why the soursop fruits may not be holding on the tree is because of insect bite. Or more specifically, fruit fly attacking. They may come most in the summer because it's usually their season.
Fruit flies may come and take a bite on the young fruits when they're about 3-4 weeks old and are about the size of a small grape. This could cause the falling of the fruits.
Usually, a soursop tree may bear fruits some 3-4 years after it's grown from seed. For grafted plants, it's usually sooner. And the fruits may develop about 3-4 weeks after a successful pollination.
Although soursop or most trees in the Annona family overall are not very yummy treats to insects, especially the leaves part because of its bitter taste, the soursop fruits may attract unwanted visitors like mealybugs.
To Protect Soursop from Insects
To protect soursop from insects, you could try bagging the fruits – even better when they're at a young stage. You can use some sort of plastic bag. Cut or tear a little opening hole about the size of the fruit in the middle of the closed end of the bag.
Then, like a T-shirt, fit the bag with the hole up onto the fruit, leaving the open end facing downward. Use plastic ties to tie the bag onto the branch. This can keep some insects away from your soursop.
The soursop fruit dropping could also be because of the weak bearing branch. Sometimes, it could also be the pollen quality or the pollinated flower.
For some trees in the Annona family, in the first year that they are grown from seed, they may be weak and not able to get fertilized or bear fruit. The fruits thus may not be holding on the tree strong and long.
For this, some growers advice that we hand pollinate the flowers with pollen from other Annona varieties in the first year. This way, in the next year of growing, the tree will have a better chance of growing strong, bearing fruits and holding onto them better.
Other times, the soursop dropping may be because it is too ripe. In other cases, if the soursop tree capacity is about 100 fruits per tree but we do the hand pollination for more fruits than that amount, the tree might decide to drop off some fruits to keep things in balance.
Also, water could be a problem. Too much water or not very good drainage might cause the stem to turn darkened or even the whole fruit might turn dark. Eventually, they drop off. This could happen in the scenario when the tree is grown in a container and it's getting much more water than needed. Or it could be long rainy days causing an overwater and thus fruit dropping.
Other environmental reasons could be little birds or squirrels in the garden. They might jump on the branch, sometimes creating a vibration by the movement of jumping up and down. Some little fruits then might not be strong enough to hold on and thus drop down. Strong winds or storm might also be a problem.
Also, be careful though of a transplant shock. In the winter, some growers move the trees in the containers into the ground in the greenhouse. For some fruit-bearing trees, they might experience a shock. And thus some fruits might fall off.
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