The pine genus (Pinus spp.) is a group of conifers valued for distinctive cones and needles, which are usually evergreen and may display shades of green, blue or silver. A pine with yellow needles may suffer from a disease, improper cultural conditions or a pest infestation. Pine trees, though, naturally shed their old needles in fall. If yellow needles are primarily in a pine tree's center, then natural shedding may be the cause of the yellowing and no treatment is necessary.
Fungal diseases such as needle cast and pine needle rust cause yellow spots on needles, sometimes leading to premature shedding or even defoliation. Both diseases typically occur in spring. Chemical treatment is generally not warranted for either disease, according to Clemson Cooperative Extension. Help prevent fungal diseases by moving shade-producing structures and plants if possible to increase light and air circulation. Keep the area around the pine trees free from weeds and debris, including plant debris. Plant only pine trees that are hardy in your location's U.S. Department of Agriculture plant hardiness zone. Eastern white pine (Pinus strobus), native to the eastern United States and southeastern Canada, is hardy in USDA zones 3 through 8 while ponderosa pine (Pinus ponderosa), which hails from western North America, is hardy in USDA zones 3 through 7.
Root rot is a soil-borne fungal disease that causes a host of symptoms, including yellowing needles. Infected trees appear stunted, eventually declining and dying. At or near the soil line, cankers oozing resin may appear, and the wood around the cankers may be dark brown or black. Penn State Extension recommends removing infected trees. Pine trees that develop root rot are generally growing on sites ill-suited for pines. Do not plant another pine in the same spot.
Mealybugs and scale insects form dense, lumpy colonies on twigs and needles, resulting in damage such as yellowing needles, needle drop and twig die-back. Mealybugs resemble bits of white cotton while scale insects may be soft or hard, light-colored or dark and shiny, depending on species. Both insects can be combated by spraying the pine trees with horticultural oil. Protect yourself when mixing and applying horticultural oil by wearing a long-sleeved shirt, long pants, socks, closed-toe shoes, a face mask, safety glasses or goggles and chemical-resistant gloves. Thoroughly mix 2 1/2 to 5 tablespoons of horticultural oil per 1 gallon of water. The average mature tree requires 3 to 4 gallons of the spray solution, according to one horticultural oil company. On a day with no wind and no rain in the forecast for at least 24 hours, spray each pine tree's needles evenly with the solution, using a pump-up or trigger sprayer. People and pets shouldn't be in the treated area until the spray dries.
Inadequate nutrients in soil can yellow pine needles and disrupt growth. In spring, young trees can benefit from 2 to 4 pounds of a 10-10-10 fertilizer per 100 square feet of soil surface. Feed each larger tree's soil with 2 pounds of a 10-10-10 fertilizer for every 1 inch of the tree's trunk diameter. Scatter dry fertilizer on the soil under the tree canopies, and then water the site deeply to disperse the fertilizer into the soil. Proper growing conditions go a long way toward preventing pine problems. Generally, pine trees grow best in well-draining, fertile soil, full sunlight and regular irrigation. A 2- to 3-inch-thick layer of organic mulch placed on the soil surface will retain soil moisture and suppress weeds; don't allow the mulch to touch the trees.