Causes

• Unusual, reddish growths on the trunk of a weeping willow tree can be caused by an insect called the poplar borer, which drill into the wood of the tree. Another cause may be a bacteria called Agrobacterium tumefaciens, which lives in the soil and causes galls to form on trees and other woody plants. Or, the growth may be caused by a fungi from the Cytospora and Cryptosphaeria families, both of which commonly cause cankers to appear on willow trees.
  
  

Symptoms

• The poplar borer is a one-inch-long insect that bores into the tree, causing a swollen wound in the bark that oozes red sap. The sap may drip down the tree. Another tale-tell symptom of poplar borers is the presence of tiny sawdust piles, which the bug leaves when it exits the trunk of the tree. Galls are lumpy growths that usually form on the trunk near the base of the tree. They grow larger and become darker with age. Cankers are lesions of dead, cracked wood. They may also ooze with a reddish-orange substance that is composed of the fruiting bodies of the fungi.
  
  

Prevention

• Galls, cankers and insect pests are most damaging to trees that are stressed or weak. Keep your weeping willow healthy by giving it plenty of room to grow -- the trees can reach heights and widths of 70 feet -- and by locating the tree where it will receive at least partial sunlight. Don't plant a weeping willow in soil that has previously harbored other diseased trees, and remove any severely infected plants close to the willow.
  
  

Treatment

• Branches affected by boring insects, galls or cankers can be easily pruned off, but when the growth is on the trunk of the tree, treatment is much more difficult. There is no way to chemically treat galls, according to the Ohio State University Extension's website. As for cankers, fungicide will kill the fungi that caused the canker, but the canker itself is dead wood. Trying to dig a canker or a gall out of a trunk will only further wound the tree. Boring insects can be killed with insecticide, but not when they have burrowed into the trunk and are protected by the bark of the willow. Insecticide must be applied when the insects are laying eggs in the spring, or right when the eggs are hatching. Repeated applications are the most effective. Apply the first dose of insecticide at the beginning of June, and then again monthly until September.