We call pine trees evergreens, but that description is misleading. No tree keeps its needles---or leaves---indefinitely. Just as the leaves on some deciduous trees become yellow, dropping in autumn, pine tree needles become yellow before dropping off the tree every two to three years. That yellowing, however, is distinct from the yellowing of needles on diseased trees, says Dennis Patton, horticulture agent for Kansas State University Extension of Johnson County.
When trees experience normal needle drop, the needles at the ends of their branches remain healthy and green. Only the older, inner needles are yellow, says Patton. The trees may have a threadbare appearance after the old needles fall. They bounce back with lush new growth the following spring.
Yellow Needles from Disease
Two diseases, each a type of needle cast, produce yellow pine needles. Naemacyclus needle cast attacks Scots pines, usually striking between April and June or from July to November if weather is mild and rainy. This fungus infects new and old needles, with new ones remaining asymptomatic, say the University of Kentucky Extension's plant pathologist John R. Hartman, Ph.D and professor Deborah B. Hill, Ph.D. Lophodermium needle cast affects Scots, Virginia, Austrian and red pines. Short-needled Scots pines grown as Christmas trees are the most vulnerable, say Hartman and Hill.
Symptoms of naemacyclus surface in the late summer or autumn of the year after the infestation. Second- and third-year needles develop pale green spots before becoming completely yellow. Brown bands appear as the needles deteriorate. The needles eventually turn solid brown, dropping from the trees between fall and spring.
In late fall of the year of infection, trees with lophodermium develop yellow-encircled brown spots on their newest needles. They remain in that state until spring. They then become completely yellow, then brown, and drop from the tree all summer. The tree's newly emerging, green needles also harbor the infection but don't exhibit symptoms.
Small, narrow, tan dwarf fruits litter needles killed from Naemacyclus. Lophodermium-affected needles contain black, football-shaped spore capsules. Both structures release wind-borne spores that can infect other trees.
Several control measures help prevent needle cast from spreading, advise professors Hill and Hartman. Begin with the removal of heavily infected trees and destruction of infected branches on tree stumps. Avoid transmitting spores from disease to healthy trees by pruning the healthy ones first. Prune the infected trees only when weather is dry. Keep the area around the pine trees free of weeds to enhance air circulation.
Scots pines growing in moist areas, especially those in shady or northern exposure areas, may benefit from fungicide application. During wet, mild weather, when spores are most likely to spread, spray the trees every two to three weeks with fungicide registered for needle cast diseases.