If your primary management goal is to earn income from a red pine, grow it at a distance at least twice the height of nearby mature red and jack pine. This precaution ensures that the vast majority of Diplodia (and Sirococcus) spores from residual pines will not rain down on younger pines.
When clearcutting, leave wildlife and seed-tree species other than red or jack pine. If there is only red or jack pine on the site, leave clumps of trees with as little shoot blight and branch death as possible. Encourage other overstory species such as white pine or hardwoods to grow next to young red pine.
Clearcutting scenarios for different levels of precautions for minimizing Diplodia (and Sirococcus) in young red pine:
- Growth loss from Diplodia is not acceptable: encourage young trees other than red pine within twice the height of remaining red or jack pine clumps. Also, consider this option if the site has a high number of dead lower branches and shoots on older pines or many understory pine saplings have dead shoots.
- Some growth loss from Diplodia is acceptable: encourage young trees other than red pine within one mature tree height distance from remaining older red and jack pines.
- Growth loss from Diplodia is acceptable: grow red pine seedlings and saplings next to older pines, if desired. Diplodia often kills some red pine saplings and slows the growth of others growing close to older red and jack pines. The level of impact varies enormously from site to site and year to year, ranging from no impact to more than 30 percent mortality (losses can be considerably higher for pines growing under a closed canopy or in gaps). Also, consider this option if many older pines on the site have healthy canopies and if there are abundant red pine saplings free of Diplodia.
It will be more challenging to grow young red pine in settings other than clear-cuts. You may have success with planted or natural red pine in the middle of harvested gaps. Consider creating gap sizes of 3/4 acre or larger, since red pine is not very shade-tolerant and there should be lower risks with Diplodia (and Sirococcus) in larger gaps. In situations where Diplodia is abundant or causing severe disease, or other species are competing aggressively for light (regardless of the presence of Diplodia), only a few red pines may survive in gaps regardless of size.
In any location where you plant bare-root red pine, expect a small percentage to die from latent (invisible) Diplodia infection. On any seedling type, another small percentage often dies from insects, other diseases, animals, and weather-related factors. When drought follows planting, large percentages of red pine seedlings can die from lack of water, latent infections, Diplodia infection from nearby pine slash, or other diseases such as Armillaria root disease.
A ponderosa pine planted in Minnesota with severe Diplodia shoot blight.
Shoot blight from Diplodia is common in the ornamental landscape but only becomes severe on stressed pines. Ponderosa and Austrian pines are so susceptible to severe shoot blight that we recommend not planting them in Minnesota unless an ornamental tree expert has determined the soil and other site characteristics are optimal for those trees.
One way to reduce shoot blight is to prune out blighted shoots. In most cases this would be an overwhelming task, so prune shoots soon after you notice blight. Prune out some healthy branch tissue back along the stem (closer to the trunk) to ensure you're removing the entire infection.
You can apply a fungicide registered to prevent Diplodia just before shoots elongate in the spring, typically between late April and mid-May. Consider hiring an arborist to do this. Fungicide applications do not get rid of the current disease in the canopy and multiple applications per year may be necessary.
Diplodia commonly hides in healthy-looking plant tissues until the tree is stressed, so preventing stress to trees is imperative. A great way to prevent serious disease from Diplodia is to ensure your trees have enough water during drought. Ensuring the soil pH is appropriate for the tree species and avoiding other sources of stress such as compacted soils and improper planting will also help prevent disease.