Diplodia blight causes shoot blight and branch cankers of pine trees and occasionally other conifers. Cankers can lead to branch death. Repeated infections disfigure trees and may eventually lead to death of the entire tree.
Pathogen and susceptible plants
Diplodia blight is a fungal disease caused by two species of Diplodia: D. pinea and D. scrobiculata. Diplodia pinea is more aggressive and capable of causing more severe damage than D. scrobiculata, but both fungi are capable of causing disease.
Diplodia blight is most common and severe in pines that have 2 to 3 needles per bundle (Table 1). Disease is significantly more severe in stressed trees than in vigorously growing non-stressed trees.
Pines that have five needles per bundle are highly resistant to diplodia blight, which in Minnesota includes the only native five needle pine, eastern white pine (P. strobus) as well as the non-native bristlecone pine (Pinus aristita).
Other conifers 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 conifers, it rarely causes much damage.
Table 1. Conifer species and their susceptibility to Diplodia blight in the Midwest.
|Most susceptible species||Moderately affected species||Occasionally affected|
|Austrian pine (Pinus nigra)||Ponderosa pine (P. ponderosa)||Spruce species (Picea spp.)|
|Red pine (P. resinosa)||True fir species (Abies spp.)|
|Scots pine (P. sylvestris)||Douglas fir (Pseudotsuga menziesii)|
|Jack Pine (P. banksiana)||Arborvitae (Thuja)|
- Needles of new shoots remain stunted, turn straw colored and are glued into place from excess resin.
- Mature needles on branches killed by girdling cankers turn tan and die.
- Sticky clear to white resin or pitch found on killed twigs, the main stem or branches with cankers.
- Infected young, actively growing shoots and dead branches occur most commonly on lower branches but can occur randomly throughout the tree.
- Tiny, black, pimple-like spore producing structures (pycnidia) found at the base of dead needles, on cone scales, or on twig and/or branch bark.
The fungi that cause Diplodia blight overwinter in infected dead needles, twigs, and cones either on the tree or on the ground. Spores ooze out of the top of tiny black spore producing structures called pycnidia during wet weather and are spread by wind, splashing rain, animals, insects, and pruning tools.
M. Grabowski, UMN Extension
Figure 3. Raised black dots on the pine cone are spore producing structures of Diplodia.
Shoot blight begins when spores infect developing needles, buds or succulent shoots. When young growing tissue of a healthy, non-stressed tree is infected, the fungus is unable to progress into older branches and the infection stops. In these cases, the damage to the tree is minimal. If the tree is stressed however, the infection may spread from infected shoots into older branches. This results in resinous cankers that kill mature needles and branches. 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 and although this infection does not directly impact the health of the tree, it provides a ready source of spores, often leading to future epidemics on young needles, shoots, and buds during wet periods.
Not all spores of Diplodia spp. infect and immediately start disease. 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 trees 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 a hail storms cause dormant infections of Diplodia to rapidly flare up resulting in severe cankers and branch death in a very short period of time.
- Plant pine trees native to the area and from local seed sources to insure that trees are adapted to Minnesota's environmental extremes.
- Water during dry conditions and mulch to conserve water.
- Create an area of mulch surrounding the tree that will not need to be mowed or weed whipped thereby reducing the possibilities of breaking or wounding branches or the trunk.
- Do not drive, park or use heavy equipment like cars or trailers under the tree as this will compact soil in the root zone and increase risk of wounding the tree.
- 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.
- In order 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 as wounds are readily infected under these conditions. If pruning to remove infected branches, sterilize pruners with 10% household bleach or 70%rubbing alcohol before pruning healthy pine trees.
Fungicides can be used to protect young shoots, needles and cones from becoming infected but are ineffective in preventing branch cankers on stressed trees. To prevent shoot blight, apply a fungicide labeled for use on pine trees just before bud break and 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.
Although gardeners may be able to apply fungicides to shrubs and young or small trees, applications to large mature trees should be made by a professional arborist who has the training and equipment necessary to safely and effectively apply the fungicide.
Fungicides used to protect new growth from infection include products with the following active ingredients:
- Liquid copper, copper hydroxide or copper sulfate
- Thiophanate methyl