The two most common blue spruce diseases in the United States are Cytospora canker and Rhizospharea needle cast, both of which may kill the tree if they are left unchecked. Blue spruce trees are popular in many areas of the United States for their blue color and their ability to serve as screens or wind breaks. While usually quite hardy, many of the trees suffer from common blue spruce diseases.
Cytospora canker is a fungus that attacks many species of hardwood trees, conifers, and even shrubs. Blue spruce trees may easily catch Cytospora canker if they are older or are stressed by excessive heat, drought, or freeze injury, or if they already have another disease. Trees infected with Cytospora canker have branches that die off, usually starting near the bottom of the tree. On close inspection, these branches are often covered by a sticky, opaque white sap, which the tree emits as its way of fighting back against the disease. As the disease progresses, more branches die, until eventually the entire tree is dead.
The second most common blue spruce disease is Rhizospharea needle cast, which is a fungal disease that causes blue spruce needles to turn purple or brown and fall off the tree. The infected needles have rows of small black dots on them when viewed under a microscope. If the tree's needles are allowed to die from Rhizospharea needle cast for several seasons, the branches of the tree may also begin to die.
Trees with Cytospora canker or Rhizospharea needle cast should have dead and dying branches pruned during dry weather. Pruning during wet weather may simply spread the disease further. Most fungicides will not effectively treat either Cytospora canker or Rhizospharea needle cast. It is best to prevent blue spruce trees from becoming infected if possible by planting them in well-drained soil with enough room to grow freely. Mulching the trees can also help protect them from fungal diseases.