Causal Agent

Rhizosphaera kalkhoffii 

 

Hosts

Spruce (Picea spp.) including Colorado Blue Spruce. 

 

Symptoms

Healthy spruce trees retain needles for 5-7 years. When the trees have Rhizosphaera needlecast, they may only hold the current year's needles. Symptoms begin on the lower branches where relative humidity is highest. In severe cases, the lower branches may be killed. 

 

The needles are infected in May-June, but symptoms are not generally apparent until fall. Needles will turn purplish and drop. With a hand lens, it may be possible to see black fruiting bodies protruding through the stomata of the needles. In late spring, spores are released and splashed onto new needles where they can initiate new infections. 

 

Control

Only plant healthy trees and avoid trees with evidence of infection. Promote health and vigor of trees by planting in proper sites and minimizing stresses. Trees suffering from environmental stresses (compacted soils, drought) are more likely to suffer from Rhizosphaera needle cast. Trees should be watered during periods of drought. Avoid pruning trees when the foliage is wet because the pathogen can be spread on pruning tools. Disinfest tools by dipping in 10% bleach solution. Rake up and discard fallen needles (do not compost). Thinning or spacing trees to increase air circulation may reduce disease severity. Fungicides may be needed to control the disease. Treatments should be made in the spring when spruce needles are emerging until half elongated. Consult your local county extension office for specific recommendations and always follow label directions.