Many of the evergreens in North Dakota, especially pines, are not very green this spring. Some have turned completely brown, while others are showing needles with brown tips and green bases.

“There are a several potential causes, but the correct cause may be difficult to determine,” says Joe Zeleznik, North Dakota State University Extension Service forester. “Insects, diseases, last summer’s flooding and the extremely mild temperatures in March may all play a part in the current situation. Applying treatments to fix the trees may not work, especially if the exact cause is not determined. Much of what we’re seeing falls under the heading of winter injury.”

Symptoms cover a wide range. In some cases, almost the whole tree is brown or it's brown just on one side of the tree. On spruce trees, it might only be the needles that are underneath the branches that are affected, but not those on top of the branches. Sometimes just the needle tip is damaged but the rest of the needle is mostly green.

The reason that the symptoms are so varied is that the potential causes are so diverse:

• A traditional example of winter injury is when needles above the snowpack are brown, while those below the snowpack are green.
• Another cause is when sunny, warm days in late winter or early spring coincide with frozen soil. During this time, the trees lose water through their needles but can’t replace it because the ground is frozen. Tissues then dry and die.
• In other instances, the warm weather may cause trees to deharden too early. Then, when cold weather returns, some trees can’t reharden quickly enough, so the needles die.
• Tree health during the previous summer also affects a tree’s response to winter extremes. For example, aphid or mite damage might not be noticed during the summer but shows up the following spring.
• Lack of insulating snow cover can cause damage to the root system of a tree.

“Nothing can be done directly to fix trees that are suffering from winter injury,” Zeleznik says. “The only thing to do is minimize the stresses that the trees face during the upcoming growing season. Also, just because needles were killed doesn’t necessarily mean that the tree is dead because the buds still may be alive. Unfortunately, conifers that survived last summer’s flooding may not have survived the winter weather.”

While environmental damage, such as winter injury and last summer’s flooding, are likely causes, insect and disease issues also may be playing a part. Pines may be affected by pine needle scale, pine moths, diplodia tip blight or some other pest. Diseases of spruce trees include cytospora canker or one of the needlecasts. More information on pest problems is available in the “Insect and Disease Management Guide for Woody Plants in North Dakota” at http://www.ag.ndsu.edu/pubs/plantsci/trees/f1192w.htm.

“Several items are important to remember when treating trees,” Zeleznik says. “Make sure that the correct insect or disease is being treated. If you don’t, the treatment is a waste of time and money. Timing is critical because applying a treatment at the wrong time will not work and is a waste. Always follow all label directions when applying pesticides.”

For more information, contact Zeleznik at (701) 231-8143 or joseph.zeleznik@ndsu.edu.

NDSU Agriculture Communication – April 23, 2012