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The duality of Diplodia

Diplodia is a commonly encountered disease of hard pines.  And, it's full of apparent contradictions.

  • Diplodia can occur on exotic pines or on native pines.
  • It can occur on shade trees, in plantations and in natural stands.
  • It can occur on environmentally stressed trees or on otherwise healthy trees.
  • Infective spores can enter through the epidermis of expanding shoots or through a wound anywhere on the tree.
  • Diplodia can cause a localized tip blight or become systemic and cause large cankers and even tree death.
  • The shoot blight phase or the canker phase predominates depending on the weather.
The reality of the picture is not a grim as it sounds. Let's take a look at each of these statements.

Diplodia can occur on exotic pines or on native pines.
Exotic pines, especially Austrian, mugo and Scots, are particularly vulnerable to Diplodia infection and damage.  Ponderosa pine planted in the Midwest is also vulnerable.  Native species, red and jack pines, only become vulnerable to significant damage when under environmental stress, planted outside of their natural range or when wounded.

Diplodia can occur on shade trees, in plantations and in natural stands.
Natural stands are much less likely to suffer significant damage than shade trees or plantations because they are much less likely to become stressed.  Examples of things that stress pines are water shortage, compacted soil, frost pockets, improper planting, other pest problems, and excess shade or heat. Shade trees and pine plantations are much more likely to be planted in marginal or poor locations and so they?re more vulnerable to Diplodia damage.

Diplodia can occur on environmentally stressed trees or on otherwise healthy trees.
In trees that are relatively free from stress, Diplodia only kills current season shoots and remains a localized infection.  ( Repeated annual infection and destruction of many shoots may cause a gradual decline and eventual death of these trees.)  However, when pines are under stress, quiescent local infections flare up and move into older twigs, branches and stems.  They become cankered and become increasing diseased over time and eventually die.

Infective spores can enter through the epidermis of expanding shoots or through a wound anywhere on the tree.
Germinating spores can infect expanding pine shoots during a two to three week period in the spring, causing shoot blight.  Cankers are produced after spores germinate on a fresh wound.  Wounds are brought about by hail, snow damage, insect feeding, shearing or vehicle damage.

Diplodia can cause a localized tip blight or become systemic and cause large cankers and even tree death. The shoot blight phase or the canker phase predominates depending on the weather.
When the weather is wet during late spring, rain-splashed spores infect the elongating pine shoots.  Often, the disease only kills the current year's buds and shoots.  When there is a water shortage, new and existing infections grow and expand, causing cankers.  High incidence of Diplodia canker is often recorded after successive years of drought, but subsides once the drought ends.

So, it all boils down to this. During wet weather, Diplodia infects jack and red pines but usually only kills current shoots.  If the trees are stressed by such conditions as drought, improper planting, soil compaction and frost pockets, or wounded, then the fungus grows, causing cankers and much more damage.  Exotic pines are very susceptible to this disease and, if growing under stressful conditions or wounded, they are likely to suffer significant damage.

What can be done?  Here are a few suggestions (that mostly involve planning ahead and prevention).
1.  Plant disease-free stock.
2.  Avoid planting exotic and non-native pines or pines out of their natural range.
3.  If possible, plant seedlings 200 to 250 feet away from the nearest tall red or jack pine.  Since the spores are spread in rain drops, this distance prevents most of the spores from reaching the young trees.
4. If Diplodia was already a problem in a location, either avoid planting pines there again or eliminate the source of the spores.  Tall pine tree removal is most applicable in nurseries and seed orchards with infected pine windbreaks.
5.  Choose the ?right site for the right tree?.  Avoid planting pine seedlings on poorly drained, droughty sites or compacted soils, in frost pockets, or where spittlebugs are a problem.
6.  Plant the seedlings properly.  Avoid J-rooting the trees because this leads to water shortages, among other things.
7.  If the pines are destined to be Christmas trees, avoid shearing them while the foliage is wet to prevent transferring infective spores to healthy trees on the shearing tools.
8.  Prevent all types of wounding from May through August.
9.  For nursery seedlings, Christmas trees and high value shade trees, use a preventative fungicide against Diplodia infection.  Several fungicides are registered and should be applied at bud break and at two week intervals until shoot elongation and needle growth is complete.  Read and follow all label directions.

Diplodia tip blight and canker are caused by Sphaeropsis sapinea, just in case you haven?t been following the name changes.