- Diplodia kills young needles and actively growing shoots of Austrian, red and other 2 to 3 needle pines.
- If trees are stressed, resin soaked cankers form on branches and kill them.
- Use good cultural practices to minimize stress on pine trees to reduce the damage caused by Diplodia blight.
- Fungicides can be used to protect young needles and shoots from infection but cannot prevent branch cankers.
How to identify diplodia blight
- Needles of new shoots remain stunted, turn straw-colored and are glued in place from excess resin.
- Mature needles on branches killed by girdling cankers turn tan and die.
- Sticky, clear-to-white resin is found on killed twigs, the main stem or branches with cankers.
- Brown needles and dead shoots and branches are usually found in the lower part of the tree.
- Tiny, black, pimple-like, spore-producing structures (pycnidia) can be found at the base of dead needles, on cone scales, or on twig and/or branch bark.
Trees affected by diplodia blight in Minnesota
Diplodia blight is most common and severe in pines that have two to three needles per bundle. Disease is significantly worse in stressed trees than in vigorously growing, non-stressed trees. This includes:
- Austrian pine (P. nigra)
- Ponderosa pine (P. ponderosa)
- red pine (P. resinosa)
- Scots pine (P. sylvestris)
- Jack pine (P. banksiana)
Pines that have five needles per bundle are highly resistant to diplodia blight. In Minnesota, this includes:
- The only native five-needle pine, eastern white pine (P. strobus)
- The non-native bristlecone pine (Pinus aristita)
Other evergreen trees like spruce, fir, larch and arborvitae occasionally become infected. This is most common when trees are stressed and planted near severely infected pine trees. Even though the disease can be found in other evergreen trees, it rarely causes much damage.
How does diplodia blight survive and spread?
Diplodia blight is a fungal disease caused by two species of Diplodia. Diplodia pinea is more aggressive and can cause more severe damage than D. scrobiculata.
- These fungi overwinter in infected dead needles, twigs and cones either on the tree or on the ground.
- During wet weather, spores ooze out of the top of tiny, black, spore-producing structures.
- These spores are then spread by wind, splashing rain, animals, insects and pruning tools.
- Shoot blight begins when spores infect developing needles, buds or succulent shoots.
In a healthy, non-stressed tree:
The fungus infects and kills young growing needles and shoots but is unable to progress into older branches. In these cases, the damage to the tree is minimal.
Wounds are necessary for the fungus to directly invade mature needles and branches of a non-stressed tree.
Cones become susceptible to infection in their second year. Although infected cones don’t damage the tree, they do provide a source of spores that can lead to future infections in other parts of the tree.
In a stressed tree the infection may spread from infected shoots into older branches. This results in resinous cankers that kill mature needles and branches.
Fungi take advantage of stressed or weakened trees
Not all spores of Diplodia spp. start disease immediately after infection. Research has shown that the fungi that cause Diplodia blight can live within branches of a host tree for years without causing disease. These dormant fungi remain inactive until the plant becomes weakened or stressed.
The fungi that cause Diplodia blight are well known for their ability to take advantage of weakened or stressed plants. Stress caused by improper planting, compacted soils and drought, weaken the tree's natural defenses.
- When Diplodia infect stressed trees, the infection easily progresses into larger twigs, branches and even the trunk, causing cankers.
- This type of infection often results in a slow steady decline of the tree.
- Some stressors, like mechanical damage from insect attacks or hail storms cause dormant infections of Diplodia to rapidly flare up.
- This can cause severe cankers and branch death in a very short period of time.
How to manage diplodia blight
Plant pine trees native to the area and from local seed sources. These trees will be best adapted to Minnesota’s environmental conditions. Water during dry conditions and mulch to conserve water.
Protect trees from injury
Create an area of mulch surrounding the tree. This reduces the possibility of damaging the tree with a lawn mower or weed whipper.
Don’t drive, park or use heavy equipment (like cars or trailers) under the tree. This can compact the soil in the root zone and increase the risk of wounding the tree.
Do not fertilize infected trees
Do not fertilize trees and shrubs suffering from Diplodia blight, unless it is recommended by a soil test to correct a nutrient deficiency.
Never apply nitrogen to an infected tree. Research has shown that disease severity increases with increased levels of nitrogen in the soil and tree.
When and how to prune an infected tree
To improve the appearance of a diseased tree, prune to remove and destroy infected branches and twigs during dry periods.
Do not prune or shear pine trees during wet weather because wounds are easily infected under these conditions.
If pruning to remove infected branches, sterilize the pruners with 10 percent household bleach before pruning healthy pine trees.
Fungicides can be used to protect young shoots, needles and cones from becoming infected but they won’t prevent branch cankers on stressed trees.
To prevent shoot blight:
Apply a fungicide labeled for use on pine trees just before bud break.
Make two more applications at the interval listed on the label directions.
Applications should occur at bud break, when candles are half elongated and when needles emerge from the needle sheath covering young needles.
Hire a professional arborist to apply fungicides to large, mature trees. These professionals have the training and equipment necessary to safely and effectively apply the fungicide. Gardeners may be able to apply fungicides to shrubs and young, or small trees.
Fungicides used to protect new growth from infection include products with the following active ingredients:
Liquid copper, copper hydroxide or copper sulfate
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Reviewed in 2019