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Pine Wilt Continues to Kill Nebraska Trees
submitted by Sarah Browning, UNL Extension Educator
As winter progresses towards spring, homeowners should inspect their yards and windbreaks for trees showing signs of pine wilt. At this time of year, most affected trees are easy to spot, and it is important to remove them before the next generation of pine sawyer beetles emerge, carrying pinewood nematodes on to new trees.
Identifying Trees Affected by Pine Wilt
Trees infected from early to mid-summer last year stand out now like a sore thumb in windbreaks and landscapes. They are holding onto their needles, but have turned completely brown. Trees that were infected in the past couple years, but weren’t removed, have dropped their needles leaving them completely dead and bare.
However, pine wilt symptoms are harder to distinguish on trees that were infected late last year. Pine sawyer beetles, which move the pine wilt nematodes from tree to tree, can emerge as adults anytime during the summer, May through September. Beetle emergence in September results in late season pine wilt infections, and affected trees may not have had enough time to turn completely brown before cold winter temperatures stops the breakdown of green chlorophyll in their needles.
Instead trees go into winter with one or more completely brown branches, called “flags”, that stand out against the remaining foliage, which may still look relatively green and healthy. Flagging branches are usually located randomly in the upper portion of the tree’s canopy, not in the lowest branches. Inevitably, these trees continue losing their color as spring progresses, and are usually completely brown within a couple months.
Removing Dead Pines
Pine trees that are completely dead should be cut down, and burned, chipped or completely buried in the soil anytime from now until May 1st, to prevent the emergence of additional pine sawyer beetles. Do not keep any of the wood from affected trees for firewood, as it may also harbor immature pine sawyer beetles that could later emerge.
However, be slow to remove any pine tree that is not completely brown. Late season pine wilt infections could easily be mistaken for a very common fungal infection of pines, known as Diplodia tip blight. This disease can kill entire pine branches, usually the lowest branches in the tree canopy, but it will not kill the entire tree. Diplodia is relatively easy to control with two fungicide applications in spring, and it would be a shame to lose a good tree by mistakenly thinking it had pine wilt. Diplodia is very commonly found in the longer needled pines, particularly Austrian and Ponderosa pine, which have needles ranging from 4-10 inches in length. Pine wilt is found primarily in Scotch pine, which has shorter needles, 2-2 1/2 inches long.
Wait and watch pine trees that have some brown branches, and assess their condition as we move into spring. If they are infected with pine wilt, the browning of branches will pick up again as spring progresses, and trees will not develop any new growth. Once you're sure it's pine wilt, then the tree can be removed.
For more information on pine wilt and Diplodia tip blight, visit the Nebraska Forest Service, nfs.unl.edu, and click on “Tree Care”, or “Forest Health”.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office
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