Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES
Rhizosphaera Needlecast on Spruce
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Spruce,
in particular, Colorado blue spruce, can be infected with a needlecast disease caused by the fungus Rhizosphaera kalkhoffii. Trees
planted in nurseries, Christmas tree plantations, and landscapes can be
infected. Trees are not usually killed by this disease; however, premature needlecast results in trees that are not marketable, or
which are not acceptable in the landscape. Symptoms and Disease Cycle A
healthy spruce will retain its needles 5 to 7 years. A spruce severely
infected with Rhizosphaera needlecast
may hold only the current year's needles. Rhizosphaera
needlecast infects needles on the lower branches
first and gradually progresses up the tree (Figure 1). This pattern holds
true for most needle diseases on conifers and is the result of more favorable
conditions for disease development near the ground. Under epidemic
conditions, lower branches may be killed by this fungus. |
Figure 1. Lower needles of blue spruce showing symptoms
of needlecast. |
Although
needles on new growth become infected in May and June, symptoms are not visible
until late fall or the following spring, when infected needles turn purple to
brown and begin to drop (Figures 2, 3, and 4). Tiny fruiting bodies of the Rhizosphaera fungus protrude through the stomata of the
infected needles. Under a hand lens, these stomata appear as fuzzy black spots
instead of their usual healthy white color (Figure 5). During wet weather in
late spring, spores are released from these fruiting bodies and are rain
splashed onto newly developing needles where infection occurs and the disease
cycle is repeated.
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Figure 2. Purple needles in the interior of a spruce
infected with Rhizosphaera. |
Figure 3. Purple needles of spruce infected with Rhizosphaera. |
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Cultural
Control Very
little is known about cultural control of Rhizosphaera
needlecast. The following guidelines will help
prevent serious losses. |
|
Use
Healthy Stock
At
planting time, the foliage of blue spruce should be examined for fruiting
bodies of Rhizosphaera protruding through needle
stomata. If these bodies are present, the tree should not be planted.
|
Figure 4. Defoliation of spruce caused by Rhizosphaera. |
Figure 5. Fruiting bodies of Rhizosphaera
erupting from infected needle. |
Maintain
Tree Vigor
Although
detailed studies are lacking, it has been observed that trees suffering from
environmental stresses are often more seriously attacked by Rhizosphaera.
Spruces in Ohio are particularly sensitive to heavy, compacted soils which
become quite dry in late summer. Vertical mulching such soils to improve
aeration and water penetration may help lessen the severity of the disease.
Root irrigation during dry weather should also be carried out whenever possible
Prevent
Spread by Shearing Tools
Shearing
when the foliage is wet may result in spread of the spores on shearing tools.
To avoid this possibility, do not shear infected trees when the foliage is wet
(such as when dew is on the foliage in the morning). Shear healthy trees first
to avoid carrying the spores from a diseased tree to a healthy one. If this is
not possible, tools should be sterilized after shearing a diseased planting.
Denatured alcohol, available at most paint stores, will kill the spores and
also remove pitch from tools. A three- to five-minute dip will do the job.
Chemical
Control
If the
decision is made to use chemical treatment, applications should be made in the
spring because Rhizosphaera infects newly emerging
spruce needles. Begin treatment when needles are half elongated.
Information obtained through the Ohio State Extension Factsheet HYG-3059-96
Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES