Aphids

• Aphids are tiny insects that suck sap out of soft plant tissue like leaves. Cherry trees are among hundreds of species susceptible to them. As they feed, they exude a sticky substance called honeydew, which can cover any or all parts of the tree as it drips, including leaves, twigs, branches and even trunks. Weeping cherry trees droop, so branches tend to be closer together and closer to the trunk, which can cause more branches to be affected by the dripping. The honeydew is exceedingly sticky and difficult to clean.
  
  

Bacterial Canker

• Bacterial canker is caused by the pathogen Pseudomonas syringae, which lives on plant surfaces and is carried to new sources by splashing rain. Cherry trees between the ages of two and eight are the most susceptible to this disease, while older, thriving individuals are the most resistant. Bacterial cankers produce rough cankers--sunken, dead spots on the bark that ooze amber-colored, sticky gum. Canker disease can also split the bark, causing the tree's inner sap to ooze out. Other symptoms include sour smelling inner bark, leaf spot, dieback and blast of young flowers and shoots.
  
  

Borer Insects

• Insect borers that only affect cherry tree branches will likely leave telltale signs--primarily spiral-shaped sap that oozes out of holes in the wood. This is the result of the insects' boring activity. The peach tree borer, however, a common parasite of cherry trees, can also cause damage to branches higher in the tree. Peach tree borers live in the bottom foot of the trunk, but their activity can cut off the flow of nutrients and water to areas higher up. When this occurs, sap oozes out of the top section of dying bark, reports New Mexico State University.
  
  

Mechanical Damage

• Cherry trees, including weeping cherry, have very thin bark all over and are highly susceptible to mechanical damage. Common sources of this damage include lawnmowers, weed-eaters, pruners, hail, sun (as in sun scald) and wire wrapped around the trunk. Cherry trees produce sap as a protective mechanism against injury. Damaged bark can split, providing an opening for the otherwise healthy sap to ooze out.