Plum trees are separated into three distinct groups: European, Damson and Japanese. The Santa Rosa plum tree is in the Japanese group. This group of plum tree cultivars produces plums that are large, heart-shaped and firm. They are extremely popular in the United States. Plum trees are susceptible to two fungal diseases, black knot and brown rot.
Santa Rosa Tree Characteristics
The Santa Rosa plum tree will reach a mature height of 15 to 25 feet. It requires a site that receives full sun, and it will tolerate most soil conditions. Santa Rosa blooms in late May and will begin producing fruit in three to five years---it is self pollinating. Harvest time begins in late July. The Santa Rosa plum tree is hardy in zones 5 through 9.
Care of the Plum Tree
According to the University of Rhode Island, plum trees require pruning/training, fertilization, mulching, and pest and disease control. The tree should be pruned and trained to have a vase shape, and it should be fertilized yearly (early spring). The suggested amount of fertilizer is 1/20 lb. actual nitrogen and 8 ozs. 10-10-10 fertilizer per year of the tree's life (for example, a 2-year-old tree would require 16 ounces). Mulch should be applied at the base of the tree to maintain moisture and to keep the weeds down.
Black knot is a fungal disease caused by Dibotryon morbosum. It is considered a serious disease of the Santa Rosa plum tree. Symptoms are hard black knots/swellings on the small branches of the tree. These elongated knots can be a foot long. In a severe case of black knot, the elongated knots will kill these small branches. Black knots can also appear on the trunk and larger branches of the plum tree. Infected branches should be pruned out and destroyed, and a fungicide should be applied. (North Dakota State University recommends using Orthorix or Bordeaux.) This should be done before bud-break.
Brown rot is also a fungal disease; it is caused by Monilinia fructicola. This fungus can attack flowers, spurs, shoots, and fruit of the plum tree. Infected blossoms turn brown, the fungus can also move further down into the fruiting spur. When brown rot attacks young fruit you will notice small, brown, round spots on the fruit. When this fungus attacks mature fruit it is quite serious. The entire plum (under certain conditions) can rot within a few hours. Some rotted fruit falls to the ground, and some infected fruit shrinks and remains on the tree. The shrunken fruit are called "mummies." The fungus over-winters in the infected fruit and twigs.
Controlling Brown Rot
Begin controlling brown rot in late summer or early fall. Any remaining fruit, mummies (both on the tree and on the ground), and infected twigs should be removed and destroyed before spring blossoming begins. Fungicide (North Dakota State University recommends using benomyl or captan) should be applied when the blossoms begin to show color. Any further fungicide applications will depend upon the weather conditions. If you are experiencing wet weather and evening temperatures above 55 degrees Fahrenheit, apply the fungicide when the flowers have completely opened and again when the petals begin to fall off. Once the plums have developed but have not begun to turn color, apply the fungicide again. If after 10 to 14 days you are experiencing high humidity and temperatures of between 60 to 80 Fahrenheit, apply the fungicide again.