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When an evergreen like a Pine Tree starts to turn brown it’s an indication of problems that you need to take note of.
The two most likely causes of a pine tree turning brown from the top down are pine wilt and drought stress, which are serious concerns. Secondary factors such as pest or fungus infection can worsen the issue.
Let’s look at each of these causes and what you can do if you are seeing issues with your pine tree.
Pine wilt disease is one of the most common pine tree diseases. In fact, pine wilt is endemic in the U.S. but has also been found in pine forests throughout Europe and Asia.
Austrian, mugo, and Scots pines are the most susceptible to pine wilt, but other species can be infected, as well, especially in areas where summer temperatures are high.
Pine wilt is caused by a nematode known simply as the pine wilt nematode (PWN, Bursaphelenchus xylophilus). PWN is a tiny parasite visible only under a microscope. It’s carried from tree to tree by way of sawyer beetles.
On their own, sawyer beetles aren’t considered to be a significant problem for pine trees, unless the tree is already stressed. However, an infected sawyer beetle can transmit PWN by nibbling the trees’ new shoots or by laying eggs on the tree. This usually happens in the early summer.
PWN feeds on the tissues responsible for carrying water throughout the tree. They can reproduce very quickly, and as the population grows and continues to feed, the tree’s water transport essentially stops altogether (source).
High heat in the summer promotes the nematode’s development. If a tree becomes infected in early summer, and temperatures are high enough, PWN can advance quickly, killing the tree within a matter of weeks or months (source).
Officially, pine wilt can only be diagnosed by placing a sample of the wood under a microscope to see if PWN is present. However, the following are all signs that your tree may be infected:
- Wilting needles, often beginning near the top of the tree
- Browning needles (red needles do not indicate pine wilt); occasionally, the needles may appear brown with a greenish cast.
- Dieback of whole limbs and branches
Infected trees often decline very quickly until they ultimately die. Austrian and Scots pines, the most susceptible pine species, often die within three weeks of becoming infected. Some research shows that ponderosa pines have some resistance to pine wilt, but if PWN is robust enough, they will die, too.
What You Can Do
Unfortunately, if your tree is suffering from pine wilt, there is nothing you can do to save it. However, because PWN is so easily transmissible, there are some steps you should take to help control the spread. These are especially important steps if you live in or near a heavily forested area, where dead pine trees are a huge wildfire hazard!
- Contact your local Extension office to get an official pine wilt diagnosis. An official diagnosis is necessary for researchers to continue tracking the spread of pine wilt. The Extension agent will give you instructions for collecting a wood sample or will send a representative to collect one.
- After your tree dies, destroy it before sawyer beetles return the following June. The wood may be chipped and used as mulch, but don’t save any larger pieces for firewood or other purposes. Leave no stumps or logs to attract future populations of sawyer beetles.
- Do not replant a pine tree in the same location.
If you currently have a healthy pine that shows no symptoms of pine wilt, there are two available nematicidal injections you can give your tree. These work similarly to vaccinations in that the chemicals target PWN specifically, protecting your tree from large-scale damage.
Neither injection will heal a tree that is already infected. These are preventative only and worth considering if you live near a forested area.
While you may be able to find nematicidal products from a retailer, the injections work best when they are injected at a high pressure, between 30 and 50 psi. For this reason, it will be best for you to contact a local arborist or other expert to have it administered professionally.
For best results, inject your trees in the fall. You can repeat the injections every year or every other year, depending on your budget.
You should avoid using a broad-spectrum insecticide to kill the sawyer beetles who transmit PWN. Insecticides aren’t effective against the nematodes themselves, so even if the sawyer beetles were killed, the nematodes could still infect their feeding sites. Furthermore, broad-spectrum insecticides often harm populations of beneficial insects, which is never desirable (source).
Pine trees are fairly tolerant of drought, but like all plants, there is a limit to how much drought they can take. If you live in a dry region, or if your area is experiencing an unusually dry period, your tree may be suffering from drought stress.
Early symptoms of drought stress include the following:
- Thinning needles
- Yellowing canopy
- Roots growing near or emerging from the ground
As drought stress progresses, the thin, yellow canopy turns brown. If your tree is already at the browning stage, it may already be dying (source).
Drought stress weakens the tree’s overall health, including its natural defense mechanisms against pests and other diseases that it would be able to ward off under optimal conditions. These secondary problems accelerate a stressed tree’s decline.
Bark beetles are often found in drought-stressed pines. This is because stressed pine trees exude a chemical compound that bark beetles find irresistible (source).
Bark beetles bore holes through the tree’s outer and inner bark and into the interior tissue. A healthy tree “pitches” the beetles out by filling these holes with sap.
To produce sap, however, trees need water. A drought-stressed tree will not be able to produce enough sap to withstand a large population of bark beetles.
If you notice red sap around fresh holes in the trunk, pink or brown pitch tubes (masses of resin) on the trunk, or reddish dust around the base of your tree, your pine has a significant bark beetle infestation.
Once a large population of bark beetles settles in, you should remove the tree altogether. Bark beetles damage the structure of the tree itself, putting it at risk of collapsing, which in turn poses a hazard to you and your property.
While there are some insecticides that can help control the bark beetle population, it is important to remember that they are most attracted to trees that are already in distress. Maintaining your tree’s water levels throughout periods of drought will strengthen your tree’s natural pest defenses (source).
Cytospora canker is a fungal disease that is more commonly experienced by spruce trees, but pine trees undergoing drought stress can also host the fungus.
The Cytospora fungus causes cankers to emerge on branches that exude a gummy substance. The cankers are most likely to appear near the tree’s trunk. Cankers can cut off the already limited flow of water and nutrients.
Because of the tree’s weakened condition, the fungus can overtake it fairly quickly. Unfortunately, Cytospora is resistant to fungicide.
What You Can Do
Unless your local government has imposed drought-related water restrictions, drought stress is entirely preventable.
During dry periods, give your pine tree about one inch of water per week. If your soil is sandy, your tree may need up to two inches of water per week (source).
It may also be helpful to apply mulch around the base of your tree. This will help maintain the soil’s moisture level and protect the roots from drying out during hot summer days.
If your pine tree’s needles are turning brown, pay attention! Browning needles are a huge warning sign of a serious problem like pine wilt or drought stress and its related issues.
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