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Yellowing pine needles normal in autumn

Yellowing, dead needles on southwest Colorado’s ponderosa pines may not be related to bark beetles activity or tree disease, but instead simply a sign of changing seasons.

Bark beetle activity around the state has heightened landowners’ awareness of the trees around them. It also has increased the number of calls the Colorado State Forest Service Durango District receives from concerned citizens about their trees, including yellowing ponderosa pines.

Landowners tend to be more concerned about changes in their trees when there has been an ongoing insect or disease outbreak. Fortunately, the yellow, dying needles they see on their ponderosa pines in September and October is likely part of a normal cycle.

Every autumn, many Colorado evergreen tree species shed some of their older, interior needles as part of an annual growth cycle. Needles closest to the trunk are most commonly shed, but trees stressed due to drought or root damage may shed more needles to keep the tree in balance with its root system. Soon-to-be shed needles typically turn yellow first, then a reddish-brown color before dropping off; very small branches with few needles on them also may die.

By comparison, when a ponderosa pine is infested by bark beetles, needles throughout the entire tree often turn an off-shade of green at first, eventually becoming red by the following summer. Gartner says that in addition to changing needle color, bark beetle-infested trees also will show additional signs of attack, such as fine sawdust at the base of the tree.

This discoloration and loss of pine needles in the fall is often called “needle cast,””but the term”actually refers to a fungal disease of spruce-fir trees.

For more information about tree and forest health, contact the CSFS Durango District at247-5250 or visit csfs.colostate.edu.