Pine trees (Pinus spp.) are valued for their canopy of evergreen needlelike leaves that filter air pollution, buffer noises and provide shelter for birds. Sometimes pine needles turn yellow, which are outward symptoms of pest damage and disease. However, yellowing needles often indicate a natural end-of-life stage with newer and greener needles arising to take their place.
Natural and Environmental Influences
Although pine trees are evergreen and do not drop their needles all at once, they naturally shed their older foliage. When interior needles reach a certain age, which is different for each pine species, they turn yellow in autumn before turning brown and dropping from trees. If current-season needles are green, trees are healthy. The University of Florida reports that pines have wind-damage sensitivity. Trees may languish for six months to two years after windstorms before they die.
Of the 600 species of bark beetles in the United States, 200 of them attack pines in California. These insidious intruders are often invisible until irreparable damage is done. Female beetles lay eggs underneath bark, where hatching larvae create mines as they feed on phloem-cambium layers inside trees. They pupate inside galleries and emerge from trees as adult beetles. Yellowing needles result as tree health is compromised. Eriophyid mites, also called gall and blister mites, feed within needle sheaths and cause stunted, distorted and yellowed leaves.
Pitch canker is a fungal disease caused by the pathogen Fusarium circinatum. Prior to 1986, pitch canker was confined to the southeastern United States but spread rapidly to California. Although most California native pines are susceptible to this disease, Monterey pine (P. radiata) is most affected. The disease may be spread by wind that carries airborne spores or transmitted by bark beetles that vector this disease. Symptoms include yellowing needles and oozing resin. Repeated prunings of infected branches and insect management may slow the spread of disease, but the University of California notes no known prevention or cures for this disease.
Annie Esperanza, a park scientist at Sequoia National Park, has a 30-year history of studying air quality. She points to ozone damage as the culprit in weakening many ponderosa and Jeffrey pine trees. As a vital step in photosynthesis, plants must absorb sufficient carbon dioxide to make food that sustains healthy growth and green foliage. Pine trees that absorb ozone instead of carbon dioxide are unable to photosynthesize. As a result, their needles turn yellow, and, over time, trees weaken because they are nutrient-poor.
- University of Florida School of Forest Resources and Conservation: Pinaceae -- The Pine Family
- Colorado State University Cooperative Extension: Are Your Pine Trees Showing Yellow and Brown Needles?
- University of Florida IFAS Extension: What’s Bugging My Pine Tree?
- University of California IPM Online: Bark Beetles
- University of California IMP Online: Gall and Blister Mites
- University of California Division of Agriculture and Natural Resources: Frequently Asked Questions About Pitch Canker
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