Tree Facts: Shot hole disease of trees and shrubs in the plum family

ff_admin
Farm Forum

Do your chokecherry or plum tree leaves look like they have been blasted with a shotgun? If the leaves are peppered with holes it is probably Shot Hole Disease. Common hosts include trees and shrubs in the plum family such as Russian almond, Mongolian cherry, Nanking cherry, chokecherry, American plum, western sand cherry, Manchurian apricot, Amur chokecherry, pin cherry, peach, nectarine and cherry. There are two possible causes of Shot Hole disease the most common is bacteria but there is also a fungus.

Symptoms

Shot Hole disease bacteria first appears as small, water-soaked, grayish areas on the undersides of leaves. Shot Hole disease fungi appear as small, dark purple spots. Both the bacteria and fungus caused spots become angular and are most numerous at the tip ends and along the midribs of leaves. The infected areas drop out, giving the infected leaves a shot-holed, tattered appearance. On plum, the shot-hole effect is more pronounced than on other stone fruits. Tattered leaves are still functional and normally do not cause significant harm to the tree. Severely infected leaves may eventually turn yellow and drop. Fruit is often reduced in size, sunburned, and cracked. Tree vigor and winter hardiness may also be reduced.

In severe cases, the bacteria will penetrate into branches, causing cankers. Cankers are often found along the trunk and major branch crotches. Cankers typically are dark brown and sunken in the branch; the inner tissues will turn orange-red. These pockets of bacteria will release a pus-like ooze and can plug the flow of water and nutrients, causing leaves to wilt and branch tips to die.

Disease cycle

Shot Hole disease bacteria overwinter in the twigs, buds, and other plant tissue. In the spring the bacteria are spread by rain to leaves, shoots, and fruit. Spring infections can occur after the leaves begin to unfold. Temperatures above 65°F and warm rains are needed for the bacteria to multiply, become exposed, and be spread. After these first infections, which are rarely noticed but do initiate the disease each year, the severity of the secondary infections depends on the weather. A moderately warm season with light, frequent rains accompanied by heavy winds or recent injury to the leaves or fruit, such as wind-blown soil particles and hail, may result in severe outbreaks.

Secondary spread of Shot Hole disease can occur from oozing summer cankers and leaf and fruit lesions during warm, wet weather. The systemic movement of the bacteria from leaves and shoots contributes to the formation of cankers. These cankers can be spread during budding to healthy nursery trees.

Control

No sprays are available to control Shot Hole disease bacteria. Rake leaves and twigs from under the tree to remove the bacteria from the area. Trim out cankers going at least 6 inches into healthy wood. Remove the infected branches and burn them. Fungicide sprays are usually not needed, especially for established trees. For young trees, a spray with products containing chlorothalonil can be used to prevent the spread of the Shot Hole disease fungus. This fungicide will not control bacteria.

Planting resistant varieties is the most effective control measure. There a quite a few varieties of peaches highly tolerant of Shot Hole disease. Resistance in plums, nectarines, and apricots is not as common. Nurserymen are well aware of the degree of susceptibility of the varieties they sell and they can provide good information for specific areas. Since trees in poor vigor are more susceptible, orchard management programs should be designed to maintain good vigor.

My sources for this news release were North Dakota State University and Penn State University Extension. If you would like more information about “Shot Hole Disease of Trees & Shrubs in the Plum Family,” contact Bob Drown at 605-244-5222 Extension 109 or by e-mail at robert.drown@sd.nacdnet.net.