When Evergreens Aren't: Part I Rhizosphaera Needle Cast

If you are familiar with the blue spruce, then you most likely have seen them in various stages of decline. These trees have been overplanted to a fault and often are sited in inhospitable areas. Stressed trees are more prone to insects and disease and one common fungal disease to which they are subject is Rhizosphaera Needle Cast. This disease caused by Rhizosphaera kalkhofii causes browning and early loss of needles starting on the lower branches.

Needles become infected in the spring, turn yellow in July, then purplish or lavender, then brown, and finally drop during the summer and fall.

If you use a handlense, you will see the small, black fruiting bodies of the fungus in the stomata of the needles. This disease is spread mainly by rainwater.

Infected trees should be treated with a fungicide or bordeaux mix when the new growth reaches 1/2 to 2 inches long. They should then be sprayed again 4 to 6 weeks later.

To combat Rhizosphaera Needle Cast, reduce stress by mulching and watering the tree. Also, promote good air circulation by keeping trees properly spaced and free of weeds around the base. Never prune or shear trees when the foliage is wet. Also, clean up and discard all fallen needles. If a spruce. gets so bad that it needs replaced, plant a tree that is not susceptible to the disease.

Any age tree is susceptible to this disease and conditions favoring disease development include: warm, wet weather, and poor air circulation around the tree.

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