Yellowing needles on evergreens such as pine trees cause concern and warrant immediate control to prevent spread and retain health. Because a number of factors cause the needles to turn yellow, you need to diagnose the cause of the discoloration before treating the problem and adopting strategies to prevent future outbreaks.
A number of reasons cause yellowing needles on a pine tree. An improper planting site causes the needles on a newly transplanted pine tree to turn yellow. Lack of irrigation or exposure to extreme drought damages causes the needles at the top of the tree to turn yellow and wilt, before spreading downward to cover the entire foliage. Excessive salt or herbicide damage causes the tips of needles to turn yellow first, before progressing and turning the entire surface purplish-red or brown. Blights and needlecast diseases cause needle discoloration and spotting, along with other symptoms such as stunted growth, cankers and premature needle drop. Insects such as aphids, spider mites and Zimmerman pine moth cause yellowing foliage on a pine tree.
Treatment varies according to the cause and scale of the discoloration. Transplant the pine tree to a site that meets its growing conditions, or replant the pine positioning its root collar at ground level. Avoid planting the tree in wet soil, since roots fail to absorb oxygen, causing the foliage to turn yellow. Spread the roots in different directions in the planting hole. Provide a steady stream of water around the tree to alleviate drought stress. Douse the soil with water to leech out accumulated fertilizer salts that cause yellowing needles. Snip off diseased parts of the pine tree with sharp, sterilized scissors and discard clippings immediately. Spray the tree with registered fungicides to achieve control and prevent spread. Douse small infestations of insects with a horticultural oil, or use a low-toxicity insecticide to treat large infestations.
Plant the pine in a loamy or sandy soil with pH between 5.5 and 6.5. Adopt good cultural practices to keep the needles of the evergreen from turning yellowing. Needle discoloration not only causes aesthetic damage, but increases stress that makes the tree susceptible to pest infestations and diseases. Irrigate the pine tree regularly, especially during prolonged periods of drought, to keep the soil from drying. Follow a regular fertilization schedule to prevent over-fertilizing the pine tree. Minimize chances of diseases by adopting good sanitation practices and minimizing plant stress.
Similar to other trees, pines undergo the natural process of needle loss. Older needles on the foliage turn yellow and fall off to make way for new growth. If you rule out other possible causes of needle discoloration, the only possible culprit is natural aging, which leaves you no choice regarding treatment. Sometimes, sudden exposure to drying winter winds or extreme temperature causes the needles to turn yellow before becoming brown. This discoloration is not cause for concern because the needles regain their original color in most cases.