Diseases and insects cause problems with flowering cherry trees that can lead to the eventual death of the tree. Early detection is vital if the tree has a chance to bounce back. When problems appear, contact your agricultural extension service to find out what fungicides or insecticides are permitted and available in your area.
Black Knot Disease
Black knot is a fungal disease that breeds in wet weather, spreads by spores and appears as black warts that can grow up to 1 foot long, making the branches look thick and black. This disease can affect trees in all of North America. Young twigs will succumb to the disease first; older, and therefore larger, branches will last longer. The tree will decline more and more each year, become stressed and eventually die. The weakened tree will also be susceptible to other types of infection.
In the first year, the disease will appear as small, light brown swellings on the new growth and the previous year's growth. During the second season, the swellings turn olive green before getting darker and becoming hard. The symptoms are easier to spot in the winter when they are not hidden by the leaves, but by this time the disease is already progressing to the next stage. Remove infected parts to prevent the spread of the disease. Generally, fungicide use is not recommended to treat black knot in the home garden.
Cherry Leaf Spot
Cherry leaf spot occurs in both the United States and Canada, most prevalently in humid areas. The fungus attacks not only the leaves, but also the leaf stems, fruit, and fruit stems. The disease appears as purple spots on the top surface of the leaves in the spring right after the petals fall off. Moisture will produce white, felt-like patches on the undersides of the leaves. These patches contain the fungal spores. Eventually, the spots will turn brown and drop out, leaving holes in the leaves. The infected leaves will turn yellow and drop off. Heavy infections can stop the cherry fruit from maturing. The disease is fast-moving, defoliating a tree by midsummer and weakening it to the point where the tree is susceptible to damage from the winter weather. Use fungicides such as captan or chlorothalonil to treat cherry leaf spot.
Rusty Plum Aphid
The rusty plum aphid attacks cherry and peach trees as well as plums. The insect is a rusty brown or deep purple in color and has distinguishing white bands on its legs. The female lays her eggs in the crevices on the twigs in late fall. The nymphs, which are all wingless females, hatch in the following spring just before the flower buds appear and produce young by themselves. Two or three generations later, winged insects are born and migrate to other plants. The aphids feed on the undersides of the leaves, disturb the growth of the stems and secrete a sticky substance called honeydew which produces a fungus known as sooty mold. Wash aphids from the leaves with a strong spray of water, or control aphids with insecticides before the leaves start to curl.