Plum trees are popular garden performers with fragrant, white or pink spring flowers and attractive green or purple foliage. Their green, red or purple fruit is sweet-and-smoky flavored. It is edible right off the tree or can be used in preserves, cooking and wine. Dried plums become fiber- and antioxidant-rich prunes. These commercially important fruit trees are susceptible to two serious--and sometimes fatal--parasites.
Black Knot and Brown Rot
Black knot (Dibotryon morbosum) affects the trees' branches and twigs. Left untreated, the parasite progresses each year until the infested parts of the trees die. Black knot can destroy entire commercial plantings of plum trees, according to the Cornell University New York State Integrated Pest Management website. A second parasite, brown rot (Monilinia fructicola), attacks plum tree flowers, fruit and the twigs on which they grow. In the right weather conditions, notes Cornell, the parasite can be severe enough to destroy a tree's entire crop.
Black knot appears as soft, green, tumor-like 1/4- to 1/2-inch growths. Becoming brown and eventually black as they age and harden, the growths can reach more than 12 inches and completely encircle the branches. Heavy infections may spread from small to large branches and even to tree trunks.
Brown rot symptoms include withered, brown flowers that form a gummy substance on the twigs. If the flowers stay on the tree, the fungus may spread to twigs. Leaves on diseased twigs wither and brown, but stay on the trees. Infected fruit develops expanding, mushy brown areas covered in tan spores. Rotting rapidly, the fruit may also remain on the trees.
Black knot overwinters in branch growths, producing and releasing spores in rainy spring weather. The wind-borne spores attack green twigs of surrounding trees, advises Cornell. Spores infect new twigs that remain wet long enough for germination. Brown rot fungus overwinters in growths and dried fruit. Its rain- and wind-spread spores spread to other trees to germinate on blossoms wet for five hours or more. It also infests injured fruit.
Controlling Black Rot
Pruning and destroying infected twigs before buds open is the recommended treatment for black knot in established orchards, notes Cornell. Burying, burning or stripping the bark from the pruned twigs prevents further spread. Pruning back to 2 to 4 inches below each growth removes tissue infected beyond the canker's margins. Cutting growths down to bare wood--and an area 1/2 inch beyond them--treats infested trunks and large branches.
Controlling Brown Rot
Removing and destroying growths and dried fruit from dormant trees, and dropped and rotted fruit from the ground, minimizes brown rot spore production. Spraying blooming trees with systemic and protective fungicides in early spring will prevent fungal spread to flowers and fruit, while minimizing spore production on already-infected tissue.