Gummosis

Gummosis is the oozing of sap from wounds or cankers on fruit trees. Gummosis can result from disease, insect infestation, or some type of mechanical injury.  Gummosis is not a disease, but it can be associated with disease or insect damage to the tree. Gummosis is most common on stone fruit trees such as plums, peaches, nectarines, and cherries.

Gummosis is often associated with mechanical injuries (such as lawnmowers or pruning), insects, winter damage, sunscald, herbicide injury, and various fungal or bacterial infections. In response to these stresses or injuries, a sticky amber ooze or gum is exuded from these lesions (see pictures). With time, cankers may become more obvious, as branches swell or form corky growths on the margins. Severe damage or infections may cause leaf wilting and the eventual death of fruit-bearing wood.

Insects such as peach tree borers feed under the bark, creating wounds and tunnels in the inner bark. As a result, branches exude gum through wounds. Fungal pathogens from the genus Botryosphaeria may also infect cherry trees and cause cankers between the trunk and scaffold limbs. These fungi are usually opportunistic and colonize plants when their defenses are low. On the other hand, bacterial cankers caused by Pseudomonas syringae can sometimes become a serious disease in commercial orchards. Bacteria colonize leaf surfaces and enter the tree via wounds, creating oozing cankers and girdled limbs. Sometimes entire limbs may wilt, and leaves and fruit may show spots. 

In summary, gummosis is the plant's reaction to stress. Pathogens or insects may be involved, but the best way to prevent gummosis is by taking an integrated management approach. Avoid unnecessary mechanical damage to your tree and prune during dry weather. Provide a good growing site for your tree and a balanced fertilization program to promote vigorous growth. Also, practice good sanitation by pruning and destroying cankered limbs.