Plum trees produce a variety of stone fruits, which typically ripen from late July to September, depending on the variety. However, sometimes a plum tree may drop fruits during the growing season. This process can be uncontrollable and significantly reduce the yield. So, why is the plum tree dropping unripe green fruits? If plums fall off the tree before ripening, it could be due to poor pollination, pests, diseases, an excess of fruits on the tree, or a combination of these factors.
Poor Pollination
Like other stone fruit trees, plums are pollinated by bees. Fruits that are not properly pollinated may stay on the tree for a few weeks before eventually falling off.
Poor pollination can occur due to rainy, humid weather, which prevents bees from pollinating the plum tree’s flowers.
The lack of a compatible pollination partner tree can also be problematic for plums. While some plum varieties can set fruit using their own pollen, many hybrid varieties need to be planted near another compatible plum variety to achieve good pollination results. Even “self-pollinating” plum trees will produce a better yield if planted next to another variety.
Plum Tree Pests
Plum trees can be infected by insects that feed on the fruits, leading to premature fruit drop.
Plum Weevil (Conotrachelus nenuphar)
The most well-known pest is the plum weevil, a type of beetle that lays eggs inside plums and other stone fruits. Both the weevil larvae and adults feed on the flesh of the plums. Affected fruits may fall from the tree in May and June.
Experts recommend spraying plum trees with insecticides immediately after the flowers drop their petals, as this is when the weevil typically attacks the trees.
It is important to know which insecticides you are spraying and alternate specific types, as they have different uses and effectiveness. Also, it’s crucial not to spray the trees before the petals have fallen, as insecticides that kill the weevil can also kill the bees that pollinate the trees.
Additionally, you should collect and destroy all fallen plums (as they may contain larvae) to prevent adult weevils from emerging.
Codling Moth (Cydia pomonella/Grapholita funebrana)
Infestation by the codling moth can also lead to premature fruit drop on plum trees. The moth larvae burrow into the fruit, causing it to rot and fall from the tree.
To combat codling moth larvae, prune the trunks and branches of the tree to remove old bark, collect plant debris, steam treat fruit containers and supports, dig up the soil to destroy overwintering caterpillars, and burn unsuitable packaging materials in gardens, platforms, and storage rooms.
Insecticides such as carbaryl or permethrin should be applied when the first adult moths are seen. Use the insecticide two or three times, with a gap of three to five days between each spray.
Fungal Disease in Plums
Another reason for the premature dropping of unripe green fruits is fungal infection, specifically fruit rot caused by the fungus Monilinia fructicola. This fungus thrives in warm, humid conditions and affects many parts of the plum tree, including the fruit, causing it to rot.
Rotten plums “mummify” and often remain attached to the tree, sometimes even into the next growing season, but in some cases, they fall off. Removing infected fruits from the tree and the ground is a key step in fighting this disease.
Fruit rot can be treated using fungicides designed for edible fruit trees, such as captan, sulfur, chlorothalonil, and thiophanate-methyl. It’s important to use the correct product at the right time to avoid harming the tree.
Chlorothalonil can be used until the fruits fall from the tree but not after. Captan and thiophanate-methyl are often used together. Sulfur is washed away in rainy weather and may require more frequent applications.
Tip: When using fungicides on plum trees, it’s important to read the product instructions to determine the harvest interval, i.e., the time that must pass between the last spraying and harvesting.
Too Many Fruits Overloading the Branches
Plum trees may drop fruits prematurely in the summer simply because the tree has produced more fruit than it can bear. Too many fruits can also overload the branches, causing them to break. One way to prevent this is thinning. Thin the fruits on the plum tree around June, leaving enough space between the fruits on each branch.