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| Occasional Symptoms | Probable Causes |
| Needles Turn Yellow | Soil Too Alkaline - Sometimes soils lose their acidity over time which means the tree is then unable to take up iron from the soil. This deprivation is called chlorosis and is signaled by yellowing of the foliage. Restore acidity to the soil by adding sulfur to it or fertilizing with a product that has iron chelates. For quick treatment, dilute this fertilizer in water and spray it directly on the foliage. To maintain soil acidity mulch with acid materials such as peat moss, pine needles and oak leaves. |
| Twigs And Roots Damaged | Weevil - Weevils are sometimes called snout beetles because their heads are elongated into slender downward-curved snouts terminating in the insect's mouthparts. Most adult weevils are either brown or black, with tear-shaped, hard-shelled bodies averaging 1/10 to 1/4 inch long. Their larvae are grubs that lurk in the soil feeding on roots. Strawberry root weevil and Douglas fir twig weevil are pests of Douglas fir. The latter attacks smaller branches on trees in the Pacific Northwest, sometimes killing them. |
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| Leaves Chewed; Tree Defoliated | Caterpillars - Needles chewed at their edges or devoured signals caterpillar visitors. Several kinds of moths are known to be attracted to Douglas firs to lay their eggs. When their hungry larvae, caterpillars, emerge to feed on tree foliage, they are capable of serious damage to the tree. Hand-picking makes a sizable dent on most caterpillar attacks if you catch the infestation right away. |
| Needle Tips Yellowed | Needle Cast - Various fungi sometimes cause Douglas fir needles to develop yellow spots on their tips near fall. In spring the spots turn reddish-brown, spreading over the healthy green part of the needle so that it appears scorched. Moist conditions promote its spread. |
| Orange; Brown or Yellow Powdery Masses on Needles | Rust - Evergreens like Douglas fir sometimes fall victim to one of a variety of rusts caused by fungi. Some cake the needles with a powdery coating, others form galls or swellings on the trees which release fungal spores to harm nearby apple trees. |
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