Forest Health Notes:
A Series for the Non-Industrial Private Forest Landowner

Hosts: 2- and 3-needle pines, especially ponderosa pine

Quick ID:

• Stunted, discolored needles
  
• Needles attached to branch through winter, usually in clusters
  
• Shoots bent or curled
  
• Cankers on branches or stems
  
• Olive-green streaking on tissue underneath bark Small black fruiting bodies

Field Identification

Tree: The first obvious sign is stunted, discolored needles on the current year's growth. When these shoots are infected early in the season, they show characteristic curled or bent growth.
The twigs may be resin-soaked, and olive-green streaking may be found on the tissue below the bark. The disease spreads up the branch, killing older needles up to the main trunk. Cankers,
which are sunken dead areas, may form on the branches or the trunk. If a canker girdles a trunk the area above it will be killed. In contrast to needles that drop normally, needles that are killed by
Diplodia will stay on the branch all winter.

Fungus: Tiny black fruiting bodies (pycnidia) are formed on the twigs, recently killed foliage, and the face (umbo) of cone scales. These are spherical, smooth, and slightly nippled, and are imbedded in the tissue.

May be confused with: Normal needle browning, drought or frost injury, spittlebug damage, bark beetle damage.

Disease cycle: Pycnidia release spores during moist weather from spring to fall. The spores are expelled from the pycnidia in masses and accumulate close by until washed away. The fungus enters the stomata of needles or elongating shoots, or into small wounds, such as those caused by hail or pine spittlebug feeding. The disease then grows into the twig and causes a dieback or possibly a canker. Once the disease is present it moves along the branch killing other twigs and needles, and may kill the leader. Wood in the trunk may become sapstained. Diplodia alone does not usually kill trees. Although massive infection may kill a tree, more commonly the weakened tree is killed by bark beetle attack.

Predisposing agents: Wounding of the tree permits spores to enter. The most susceptible trees are those with reduced vigor, e.g. those planted off-site or stressed by drought, snow damage, or insect activity. As noted above, this disease is itself a predisposing factor for bark beetle attack.

Impact: This disease is found internationally, and causes a great deal of damage in exotic pine plantations. In this country, it hits plantings of Scotch pine, Austrian pine, and mugo pine more seriously than ponderosa pine. No figures for its impact on ponderosa pine in Washington are available, nor would it be easy to quantify as death is often associated with bark beetles.

Management: Trees under stress are most susceptible, so managing stress should reduce the likelihood of infection. Trees should not be planted on sites for which they are not adapted, nor should they be planted in unfavorable locations, on poor soils, or in densely shaded areas. Stocking levels can influence tree stress as well, so stands should be thinned to prevent overstocking. Mechanical injury should be avoided. Pruning high-value trees (e.g. those near houses, campgrounds, etc.) will not decrease the spread of Diplodia. As this disease is not in and of itself a tree killer, do not take hasty action but watch for signs of more serious problems such as Armillaria or successful bark beetle attack.

Adapted from Diplodia Tip Blight on Ponderosa Pine, University of Idaho Cooperative Extension bulletin by Chris Schnepf.