The plum tree is an adaptable, vigorously growing tree that thrives in an array of soil environments. It produces fragrant white blooms in the early spring and develops sweet, rich-colored fruit in the fall. This hardy and tolerant tree produces its fruit on its previous year's wood. Although hardy, the plum tree is susceptible to several diseases and conditions that can cause it to drop its fruit.
A plum tree with a poor framework is unable to carry the weight of its fruit. A strong framework starts at planting. As it grows, the plum tree should be shaped into an open-vase structure. The pruning process should include removing any damaged and diseased twigs and branches from the tree. Interior branches should be thinned out to improve the air circulation and light penetration throughout the tree. These practices will help to keep the plum tree's branches strong and resilient against the weight of its fruit.
Thinning and pruning of the plum tree also helps to reduce the potential of plum diseases. This tree is susceptible to diseases such as bacterial spot, brown rot, plum pocket and plum pox virus. Although immediate symptoms of these diseases do not include fruit drop, plum drop can be a secondary symptom. Plum tree diseases cause the decay and spotting of its fruit, foliage spotting and wilting, growth stunt and dieback of its branches and stems. The subsequent stress and weakening of the tree can cause the fruit to drop.
As plums mature, they take on a full, rich color that ranges from rich reds to deep blues, and even golden yellows and greens. The branches will begin to hang from the weight of the mature fruit. If the plum tree remains without a harvest, some of the tree's plums will begin to drop. It is important that you harvest your tree daily until all of the fruit has been removed from the tree. Plums left on the tree throughout the dormancy period can increase the tree's susceptibility to disease. Fungal and bacterial spores often take shelter in decomposing fruit that remains on the tree throughout winter months.
Regular maintenance is an essential force in reducing fruit drop from your plum tree. Along with timely pruning, your plum tree should be irrigated and fertilized regularly throughout the growing season. Your plum tree should be irrigated at least once each week during the hot, dry summer months and at least once every two to three weeks during the remaining periods of the growing season. The Texas A&M AgriLife Extension also recommends an annual fertilization with a well-balanced, slow release fertilizer, such as a 10-10-10 or 20-20-20 combination. The fertilizer should be applied in the spring at the onset of the growing season.
A few dropped pieces of fruit throughout the growing season is not an uncommon thing for the plum tree. Even with detailed pruning and regular maintenance, it can be nearly impossible to prevent all fruit drop. If you find that general maintenance and pruning is not improving the plum tree's health, speak with a horticultural or nursery specialist for assistance.