Plum trees (Prunus spp.) produce lovely spring flowers. There are many varieties, producing numerous colors of fruits, each with a different taste. Plums can be grown as shrubs and hedges or as trees. One of the dangers for plum trees is a serious disease caused by the bacterium Erwinia amylovora. Fire blight can kill a plum in just a few short years.
When a plum tree is infected by fire blight, its blossoms initially will look water-logged, then they will wilt and brown. This phase is sometimes called blossom blight. Once the petals fall, the leaves will turn brown and curl downward. Leaves may stay on the tree in this condition for several months. Fruit will also look waterlogged before turning brown. The fruit will shrivel and take on a mummified appearance, also clinging to the tree for an extended time. Infected twigs, sprouts and shoots may droop, giving the appearance of a "shepherd's crook," according to the Alabama Cooperative Extension.
Fire blight is a bacterium that lives in the tissue surrounding a canker. During warm weather, the canker oozes an amber-colored discharge that is spread by the splashing of rain or by insects. In some cases the ooze dries and forms a string, which can carry the bacterium on the wind to blossoms, leaves and twigs.
The disease affects young trees more frequently than older ones. Trees experiencing rapid sprouting caused by excessive nitrogen fertilizer, severe pruning or unbalanced fertilization regimens are more likely to be attacked. Warm, moist conditions when flowers are blooming increase the risk. Frost, high winds, hail or mechanical injuries can create wounds where the bacterium can enter.
The antibiotic streptomycin is effective in treating fire blight, particularly when the disease is extensive. It should be applied just as blooms begin to emerge. Agrimycin, applied every four to five days from the time the center bloom in a cluster opens through petal fall, gives added protection. Copper fungicides may be effective when the case is a mild one.
Planting disease-resistant varieties and engaging in sound horticultural practices are the best methods for reducing the risk of fire blight in plum trees. Excessive fertilization and pruning can lead to weak plants. Infected tissue should be removed and destroyed, and infected branches cut 12 to 18 inches below the cankers. An alcohol or bleach solution applied to pruning tools between cuts helps stop the spread of the disease. Insecticides can control flies, ants, leafhoppers and aphids, which can spread the disease. Insecticides should not be used when the trees are blooming, in order to protect pollinators like honeybees.