Some ways to fight trees' needle disease
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Q. I have a windbreak of blue spruce trees across the back of my property; it also provides privacy from our neighbor's backyard. 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 natural forests. Species native to the western United States such as Colorado blue spruce and Engelmann spruce are highly susceptible, while Norway spruce is relatively resistant.
Needle diseases of conifers are often more severe on the lower part of the tree because it stays shaded longer, and 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 tree, too.
Rhizosphaera needle cast generally starts on the older, inner needles, but it 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 the 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.
Because 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 11/2 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.
If trees are damaged enough to lose their ornamental appeal, consider replanting with more resistant species such as Norway (Picea abies), white spruce (P.glauca) or Douglas fir (Pseudotsuga menziesii).
First Published February 9, 2013 12:00 am