Why are my pine trees dying?
By Tracey Payton
OSU Horticulture Extension Educator
If you have a pine tree that is turning brown, you are not alone. Many homeowners have experienced problems with their pines. In fact, if I had a nickel for every time I got this question, I could go into early retirement. Pines have definitely filled a niche, they are flexible with evergreen appeal and a delicate appearance, but they can also be used for windrows or privacy. However, most pines can be stressed by the climate in Oklahoma, especially non-native pines such as Austrian, Scots, mugho, and Japanese black pines. Overall, stressed and weakened plants can be more susceptible to disease and pests than healthy specimens. These problems can be due to environmental problems or disease induced. Recently, I've seen three major diseases effecting pines in our area of the state.
The disease that most are worried about in Oklahoma is Pine Wilt. Pine Wilt is relatively new to Oklahoma, as it was just confirmed here in the late 1990s. Pine Wilt is caused by the Pinewood nematode that feeds in the resin canals of the tree, blocking the flow of water and nutrients. Because of the blockage, the needles will turn brown, sometimes one branch at a time. There also tends to be no sap leakage, since the resinous cells are blocked. But, you ask, how do these nematodes get into the tree? Pine sawyer beetles have a very close relationship with the nematodes. Female pine sawyer beetles feed on and also lay eggs in dead or dying pines. The feeding wound allows entry by the nematodes that are already present in the beetle. Now the tree is infected. But, then the eggs hatch and before the adult beetle emerges from the tree, the nematode basically hitches a ride in the breathing tubes of the beetle. And the cycle begins again. Symptoms of pine wilt are brown, droopy needles that remain on the tree. The death of the tree is also very quick, typically less than one month. Unfortunately, there is no cure for pine wilt. There are some treatments, but they are ineffective and expensive. If you have a tree infected by pine wilt, it is recommended that you chip, bury, or burn the wood promptly and also grind the stump. Pinewood nematodes also feed on blue stain fungi. So, if you have a tree that you have recently taken out and think it might be pine wilt, the blue stain will be visible in the cut stump. If you are not sure that you have pine wilt, you can wait until April or early May to take the tree out, as the beetles do not emerge until this time.
Next we will focus on fungal diseases. Fungi are mostly spread by wind and rain, and thrive in moist conditions. So, it is not unreasonable to see an increase in fungal pathogens due to our recent wet weather.
Dothistroma needle blight is a fungal disease of pines. Needle blight is not as fast acting as pine wilt and may take several years to kill the tree. This fungus begins with a banding in the middle of the needle. The band will typically begin white and progress to brown, with yellow tissue on either side. The browning will progress to the end of the needle, so that the needles will be half brown, half green. With this fungus, the overall middle growth of the tree branches will be brown. However, over time, the entire canopy may be affected. You can apply copper fungicides to needles if the tree is not too large. Two applications of fungicides are necessary to protect the needles, the first in mid-May, and the second in mid-June to July.
The other fungal pathogen is Diplodia tip blight. I have seen this disease, the least of the three mentioned here. The tip blight fungus affects the growing point or tip of the branch. The tip of the branch will be very stunted compared to the rest of the growth. The branch tip will also be leaking sap or resin, and eventually the needles will turn brown. In addition, pine cones affected by tip blight will have a tiny black, "pepper"-like appearance all over the surface. Tip blight can also be controlled by applying copper fungicides. Fungicides should be applied in early spring at bud break, and also two more times weekly until new needles emerge. (Note: if the terminal bud, or candle, is removed, it will not be hollow. Hollow terminal buds are a sign of Nantucket pine tip moth damage.)
What can be done to prevent disease besides applying fungicides? To be proactive against any disease, try to minimize stress to the trees and practice sanitation. To relieve stress, make sure the tree has adequate water and nutrients. We recommend you do a soil test every 3 to 5 years to measure the amount of nutrients present in the soil. Without a soil test, apply 9 pounds of 34-0-0 to the drip line of trees in February and November and add water to incorporate. Sanitation in pines would be done by collecting fallen needles and disposing of them. Affected needles can harbor the disease and be a source of reinfection. Pruning is also a good method of sanitation. In pines, prune out completely dead limbs back to the main trunk and dispose of the wood. Be sure to sanitize shears or loppers after every cut to minimize the spread of the disease.