Douglas-fir (Pseudotsuga menziesii) is affected by several needlecast and blight diseases. The following summarizes key diseases, symptoms, pathogens, and management.
| Disease | Symptoms | Pathogen/Cause | Management |
|---|---|---|---|
| Rhabdocline Needlecast | March through May: yellow spots on previous year's needles enlarge to reddish-brown patches on otherwise green needles; needles then fall. | Rhabdocline pseudotsugae and R. weirii | Plant in locations with good air circulation; maintain weed control. Apply fungicide when ~10% of buds have broken; repeat at 1 week and 3 weeks after bud break. If spring is cold, apply a fourth spray 2-3 weeks after third. |
| Swiss Needlecast | March through June: previous year's needles gradually brown and fall. Rows of tiny black fungal fruiting structures line midrib on needle undersides (magnification needed). | Phaeocryptopus | Ensure good air circulation and weed control. Apply fungicide when new shoots are 1.5 inches long; repeat 3 weeks later. |
| Twig Blight | Twig tips curl downward, turn dark brown, and die. Small black fungal fruiting structures dot dead tissue. | Diplodia | Prune and destroy blighted tips. |
Rhabdocline needlecast symptoms and signs
Rhabdocline fruiting structures
Swiss needlecast fruiting structures
Diplodia twig blight. Note black fungal structures on curved tip.
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