Sirococcus blight of conifers
North Central Research Station USDA FS, Bugwood.org
Fig. 1. Brown, wilted needles on red pine caused by Sirococcus blight.
Sirococcus blight of conifers is most prevalent on small landscape trees or on understory trees below infected mature-sized trees. Damage to large trees is confined to lower branches whereas infection can kill seedling aged trees. In Minnesota, Sirococcus blight only causes noticeable problems in years with consistently cool, wet weather.
Pathogen and susceptible plants
Sirococcus blight of conifers is caused by the fungus Sirrococcus conigenus. The disease affects several evergreen species but is most common on red pine (Pinus resinosa) and blue spruce (Picea pungens) trees in Minnesota. Other species that can be affected include ponderosa pine (Pinus ponderosa), white fir (Abies concolor), tamarack (Larix laricina), Norway spruce (Picea abies), white spruce (P. glauca), black spruce (P. mariana), red spruce (P. rubens), jack pine (P. banksiana), mugo pine (P. mugo), scots pine (P. sylvestris), and Douglas fir (Pseudotsuga menziesii).
- In summer, infected needles, shoot tips and one year old twigs wither, brown and fall off.
- Infections on expanding shoots often cause the shoot to curl downward in a shepherd's crook.
- Infected red pine needles often droop conspicuously.
- On mature trees, infection is often limited to the lower branches.
- Tiny, brown to black (pycnidia) fungal spore producing structures appear as raised bumps on the surface of newly infected shoots and needles.
- On spruce, pycnidia are most common on the young branches whereas on red pine they are most abundant under needle sheaths.
The Sirococcus blight fungus overwinters in infected needles and shoots. During wet periods in spring and summer, when temperatures are 60-68° F, the spores spread through splashing or dripping water to start infections on young growing plant parts. Years with longer than normal wet periods have the most severe infections.
Infections occur on or next to the base of new needles and spread to shoots. Lesions eventually expand, killing the shoots. The infection may move into and kill one year old twigs. In a mature tree, infection is typically limited to the lower branches and does not significantly impact the health of the tree. Young trees can become severely disfigured by infection or killed.
- Prune to remove infected shoots and branches during dry weather.
- At planting time, space trees according to their mature size in order to provide adequate exposure to sunlight, encourage air movement and promote drying of branches and foliage.
- If Siroccocus shoot blight has been a regular problem in an area, plant resistant or immune species including:
- Balsam fir (A. balsamea)
- Austrian pine (P. nigra)
- Eastern white pine (P. strobus)
- Have your soil tested to check magnesium levels. If soil is lacking in magnesium, fertilize your tree with the recommended rate according to test results. Soil can be tested at the University of Minnesota Soil Testing Laboratory: http://soiltest.cfans.umn.edu/
- If trees have a history of infection with Sirococcus blight and weather forecasts predict long periods of cool, wet weather, a fungicide can be applied to protect new growth. Fungicide with an active ingredient of chlorothalonil should be applied in spring while new shoot tips and needles are elongating. Follow label instructions for timing of additional applications.