A cherry tree reacts to injury or stress by exuding sap; that is, the sticky stuff you see on a branch. This process is known as gummosis. Over time, this sap becomes thick and sticky. Damage by insects, diseases or mechanical injury can trigger this response. Cherry trees, both sweet and tart, are part of the group of fruit trees known as stone fruit. Along with cherries, this group includes peaches, plums, apricots, nectarines and other fruits. Because trees belonging to this group are so closely related, they share the same insect and disease problems.
The peach tree borer is a destructive insect pest of cherry trees. Look for sap oozing from the trunk a few inches above the soil. Sap mixed with a sawdust-looking material -- known as frass -- normally indicates peach tree borer.
If oozing sap or frass is located higher up the trunk or on side branches, suspect the lesser peach tree borer, a different species. This insect requires an area on the tree already damaged by disease to gain access to the inner tree bark.
In certain parts of the country, perennial canker is the most common cause of gummosis. Look on the branches for oval lesions surrounded by a callus. The gum produced by this fungal disease is often clear but can change to a darker colour; it will not contain frass. Diseases such as brown rot, botryoshaeria and viruses will also cause gummosis.
Winter injury will stimulate gummosis. Temperatures approaching -25 degrees Fahrenheit will begin to kill tree parts. A more common form of winter injury happens during a cold spell when, on a clear day, the sun suddenly warms the tree trunk and raises the temperature enough to cause the cells in the inner bark to become active. During the night, temperatures fall enough to kill the active cells. This symptom is most commonly found on the southwest side of the tree. No frass is associated with this gummosis.
Mechanical injury caused by mowers, string trimmers, ladders, careless pruning or harvesting will cause gummosis. Herbicide damage, poor growing sites, hail, drought or damage from any cause can stimulate cherry trees to exude sap. Gummosis is a physiological response by a cherry tree to any form of stress.