Special Feature
Disease of the Month: Rhizosphaera Needle Cast on Spruce Disease of the Month: Cytospora on Spruce
by Heather Prince
Disease of the Month: Rhizosphaera Needle Cast on Spruce
One of the major needle cast diseases hitting spruces hard, especially mature specimens is rhizosphaera needle cast. Rhizosphaera infections occur during a roughly two-month period following bud-break in the spring. However, symptoms do not show up until fall or the following spring. The fungus attacks the oldest needles which develop a characteristic lavender-purple or brown color before dropping from the tree. This results in bare branches with only tufts of new needles on the Rhizosphaera infection ends. A slow-moving disease, rhizosphaera will generally start low in the tree and advance upward over many years as spores bounce up with spring rains. When scouting for rhizosphaera, use a hand lens or microscope and look for fungal fruiting bodies protruding from the needles’ pores. Fruiting bodies have smooth edges and develop in parallel rows on the needles. It does not cause straw-colored needles, banded needles, or twisted twigs.
Treatment:
Rhizosphaera can be managed with fungicide applications twice in spring, usually for multiple years. Effective fungicides include active ingredients chlorothalonil, chlorothalonil + thiophanate-methyl, copper, and copper hydroxide. Prompt treatment can save a specimen tree as infected and fallen needles will not regrow, so recovery can take many years if the infection is severe. Trees under stress are more likely to become infected, so keep trees mulched, fertilize lightly in spring, and water in dry periods, especially going into winter.
Additional resources:
University of Illinois Extension Service https://web.extension.illinois.edu/state/ horticulture/index.php 217-333-0519 54
Disease of the Month: Cytospora on Spruce
Caused by Leucostona kunzei fungus, cytospora canker on spruce has become a common disease of these valuable landscape trees. It appears as browning and drying of needles at the tips of twigs and dying of the lower branches of affected trees starting at the bottom and progressing upwards, although occasionally branches high in the tree are attacked. Its conspicuous cankers produce amber, purplish white or white patches of resin on the bark. If you carefully removal of a thin outer layer of bark in the diseased area you will find tiny, Cytospora on spruce black, pinhead-like fruiting bodies of the fungus. These fruiting bodies contain minute spores which can be spread by rain, wind, or pruning tools.
Treatment:
There are no fungicides that effectively control this disease. The best chance of tree survival is to prune diseased branches as soon as suspected fungus is confirmed. Cut back all diseased branches to the nearest living laterals or to the trunk. Avoid injuring bark and clean pruning tools between cuts to reduce fungus spread. Do not prune when affected trees are wet, as spores exude from cankers onto the surface of the bark. Trees under stress are more likely to become infected, so keep trees mulched, fertilize lightly in spring, and water in dry periods, especially going into winter.
The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/ 630-719-2424 The Landscape Contractor May 2022
Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972
