Sirococcus shoot blight of spruce was diagnosed recently in the Plant Disease Clinic. The disease is caused by the fungus Sirococcus conigenus and predominantly affects the current year's growth on pine, spruce, and hemlock.
Damage is most severe on succulent, current year's shoots but may extend to one-year-old twigs. Symptoms appear similar to injury from winter dying or frost damage; however, shoots killed by Sirococcus are scattered rather than uniform. Young seedlings may be killed by the disease, but damage to large trees mainly is confined to lower branches and has slight effect on plant health.
The fungus infects needle bases in late spring and early summer. Small purple lesions are formed and a drop of resin is exuded at the point of infection. Infected needles become chlorotic, die, turn reddish-brown, and are shed. Infection may spread into the stem, causing a canker. If infection occurs in the region of shoot elongation, growth in the infected tissue is stopped, causing hook-shaped tip growth.
The fungus overwinters in killed shoots and cone scales. Small, black fruiting bodies form on killed tissue in late summer, early fall, or in the spring following infection. Spores are dispersed from infected tissue to susceptible hosts by rain splash. High humidity, mild temperatures, and low light favor disease development.
Infected shoots should be clipped and destroyed to prevent spore dispersal
Preventative fungicides such as chlorothalonil (Bravo) and Bayleton 25wp may be effective on nursery stock and young trees.
This article originally appeared in the August 26, 1994 issue, p. 132.