Evergreen needles do not stay green forever! Older, inner needles turn brown and drop off after a few years. Depending on the type of tree you have, this can happen gradually or all at once. White pines (needles 2-5” long in clusters of 5) are the worst of the bunch. This tree typically bears 3 years’ worth of needles at any given time. Third-year white pine needles turn yellow throughout the tree. The tree will appear particularly unhealthy if the yellowed needles outnumber the green ones of the current season. Scotch pine usually retains its needles for three years. Red pine ordinarily drops its needles in the fourth year. Thus, three or four years’ green growth will outnumber yellowed needles, even at peak periods, and neither of these species will appear as unhealthy as the white pine periodically does.
Winter injury is another very common cause of browning pine needles. Winter sun, wind, and cold temperatures can bleach and desiccate evergreen foliage. Foliar damage normally occurs on the south, southwest, and windward sides of the plant, but in severe cases the whole plant may be affected. Yew, arborvitae, and hemlock are most susceptible, but winter browning can affect all evergreens. New transplants or plants with succulent, late season growth are particularly sensitive. If an evergreen has suffered winter injury, wait until mid-spring before pruning out injured foliage. Brown foliage is most likely dead and will not green up, but the buds, which are more cold hardy than foliage, will often grow and fill in areas where brown foliage was removed.
If you have ruled out seasonal needle drop and winter injury as the cause of your pine’s problems, there are several fungal diseases, blights and insects that can also cause evergreen damage. To help diagnose your specific tree problem, visit the University of Minnesota Extension “What’s Wrong with my Plant!” website. (www.extension.umn.edu/gardeninfo/diagnostics/index.html)This interactive web page is designed to help gardeners in Minnesota diagnose problems in the yard and garden caused by insects, diseases, and nonliving factors. This tool will help you narrow down the possible causes of tree damage. Photographs are included to help you better identify your disease vector.
If you are still stumped, bring a sample (at least 6” of branch with needles attached), and pictures of your damaged trees to the Extension office on Tuesdays, Fridays, and Wednesday afternoons for further diagnosis. (This service is available free of charge and is offered for all sorts of plant diagnostics, and plant and insect identification.)