Colorado blue spruce, a conifer cherished for its bluish-green foliage, is falling prey to fungal diseases as a result of rainy spring weather for the last two years.
This year’s weather, with periods of drought and frequent rain, has been especially hard on spruce trees, as evidenced by the number of dying trees in local landscapes.
Colorado blue spruce (Picea pungens ‘Glauca’) comes in dwarf, weeping and prostrate forms, but the species has a symmetrical, pyramidal shape and grows 30 to 60 feet tall. It prefers moist but well-drained soil, full sun and good air circulation.
Cytospora canker, a common fungal disease, generally affects older blue spruces and sometimes Norway spruce trees. Cankers typically appear first on the trunk, lower branches or smaller twigs as localized, sunken, slightly discolored, brown-to-reddish lesions.
While the cankers themselves are inconspicuous, they cause resin to flow and drip down the trunk of the tree. Needles brown and drop, and eventually entire branches die back to the trunk. Wet weather spreads fungal spores to other branches and the disease spreads upward throughout the tree.
Fungicide sprays cannot be timed to effectively control Cytospora canker. Instead, remove dead branches during dry weather or in late winter. Disinfect pruning tools between cuts to avoid spreading the disease. In time, the trees will become unsightly and should be removed.
Rhizosphaera needle cast is another fungal disease affecting spruces. Symptoms may first appear in mid- to late summer as yellowing of the innermost needles on the middle and lower sections of the tree. The needles change from yellow to brown or purplish-brown and eventually fall.
This fungus is also spread during wet weather and affects specialty cultivars of blue spruce, Serbian spruce, mountain pine (Pinus mugo) and white pine (P. strobus).
Fungal black fruiting bodies may be seen on the underside of needles with a 10X hand lens. Control of this fungal disease is difficult because infection extends from spring through autumn, depending on rainfall. Fungicides containing copper or chlorothalonil may be applied in spring to protect new growth but will not cure infected needles.
Since infected branches can remain on trees and spread disease for years, it’s best to remove them during dry weather, making sure tools are disinfected between cuts. Severely infected trees should be replaced. For large trees, contact a professional arborist.
Because of Colorado blue spruce’s susceptibility to these diseases, consider planting less vulnerable conifers including concolor fir (Abies concolor), Oriental spruce (Picea orientalis), Serbian spruce (Picea omorika) and cultivars of Eastern red-cedar (Juniperus virginiana). All are hardy in our region.
Silloo Kapadia is a Penn State Master Gardener. This volunteer program supports the outreach mission of Penn State Extension and provides research-based information on best practices in sustainable horticulture and environmental stewardship. Information: alleghenymg@psu.edu or 412-482-3476.
First Published: October 25, 2019, 12:00 p.m.