Why Do Plum Trees Drop Plums & Never Develop?
A bounty of spring flowers promises a full crop of summer plums, but a lot of things can happen between flowers and fruit. It can be frustrating to watch plum fruit drop without developing or see nearly ripe fruit succumb to pests or disease. Understanding the problem is the first step to intervention and eventually larger crops.
It is normal for unpollinated fruit to drop. Small plums form at each flower but drop without further growth. Most varieties of plum require cross-pollination by a plum tree of a different variety. Both trees must flower at the same time and must be planted close enough together for cross-pollination. Since bees are important to pollination, insecticides must be avoided before and during flowering. Some growers improve pollination by spraying their trees with a 20-percent sugar solution to attract bees.
Called June drop, this natural thinning occurs in April or May in warmer southern states. At this time, the fruit are about the size of marbles. If the tree has set too many fruit, competition for nutrients causes the tree to shed some fruit. This natural fruit thinning benefits the remaining fruit, which form larger and better quality plums. Further hand thinning may be warranted if the remaining crop is too heavy for the tree branches.
Weather affects fruit development in many ways. If the weather is too cold when blooms appear, bees may not be available for pollination. Flower buds may suffer freeze damage and fail to develop. Rain may keep bees away during critical bloom periods. Pollinated flowers may form fruit that fails to develop when temperatures are too cold or if sufficient sunlight is not available when blooms appear. While the weather cannot be controlled, blooms can be protected from freezing weather by covering them with tarps or blankets when temperatures drop much below freezing.
Fruit-damaging insects are a serious problem for gardeners. Fruit flies, apple maggots, caterpillars and plum curculio damage the plums, which eventually drop from the tree. Plums damaged by insects are usually scarred or rotted and the causes are easily traced to insects present.
Plum trees are susceptible to fungi, bacteria, viruses and nematodes. Damage to the tree may cause a decline in fruit quality or viability and fruit may drop. In addition to fruit drop, indications of damage on the tree, including discolorations, swelling on the tree, tumor like growths, gumming, yellowing leaves and defoliation may be present.