I have a windbreak of blue spruce trees across the back of my
property; it also provides privacy from our neighbor’s back yard.
They are about 15 feet tall. I noticed in the fall that low branches
on some of the trees seemed to be dying. The needles turned almost
lavender, then brown. Can you tell me what is causing this and what
I can do about it?
A. Colorado blue
spruce (Picea pungens ‘Glauca’) trees are susceptible to a number of
needle or foliar diseases. The lavender coloration of the needles
suggests that Rhizosphaera needle cast is causing the
problem. While different species of Rhizosphaera cause disease on a
wide range of conifers, it is more severe on conifers growing
outside their native range. The disease is not considered serious in
Species native to the
western United States such as Colorado blue spruce and Engelmann
spruce are highly susceptible, while Norway spruce is relatively
Needle diseases of
conifers are often more severe on the lower part of the tree
because it stays shaded longer, and therefore the needles stay
wet from rain, dew and overhead irrigation longer than the top
of the tree. Air circulation is generally better higher in the
Blue Spruce needles
Rhizosphaera needle cast
generally starts on the older, inner needles, but moves to new
needles as the disease progresses. Infected needles display
yellowish splotches mid- to late summer, and then turn brown or
purplish-brown in the case of Colorado blue spruce. On close
examination, the white stomatal lines (where the stomata or
pores are found) on underside of the needles are filled with
tiny black spots the size of a pinprick. These are the fruiting
bodies of the fungus. Although needles are infected in spring,
they do not show symptoms until fall or the following spring. In
severe cases, the only needles left on the tree are the current
season’s growth. When the new growth is killed annually for
three or four years, the branch dies. The fungus overwinters on
infected needles and is dispersed by spring rains.
Since moisture favors
disease development, cultural controls focus on improving
air circulation and sun penetration to reduce drying time after
rain or dew. Be sure to grow Colorado blue spruce in full sun.
Avoid overhead irrigation that wets the needles, especially in
the evening. Keep grass mowed or remove it from the base of the
tree to maximize air circulation. Rake up and destroy fallen
needles and prune out heavily diseased and dead branches when
the tree is dry.
Fungicides containing the
active ingredient clorothalonil are effective in controlling
Rhizosphaera needle cast. Applications should be made to protect
the new growth in spring, starting when shoots are
one-and-a-half inches long and again three weeks later.
Fungicides available to homeowners that contain chlorothalonil
include Bonide Fung-onil, Monterey Fruit Tree, Vegetable &
Ornamental Fungicide, and Ortho Garden Disease Control.
Species of Evergreens
If trees are damaged enough
to lose their ornamental appeal, consider replanting with more
resistant species such as Norway (Picea abies), white
spruce (Picea glauca), or Douglas-fir (Pseudotsuga