Eva Grimme, Montana State University, and Brian Hudelson, University of Wisconsin-Madison
Spruce and pine trees are staples in many landscapes. Diseases and abiotic disorders can adversely affect these trees, resulting in needle discoloration and loss, as well as potential dieback. The following are tips to help identify common pine and spruce needle problems using a 10X hand lens (or dissecting microscope) and compound microscope. Incubation of samples for 24 to 48 hours (or longer) in a moist chamber may help with confirmation.
Spruce (Colorado blue and other spruces)
- Reddish to brown discoloration of the needles at the tip of branches:abiotic issues such as winter injury/burn or drought stress (Figure 1).
- Brown, bronze, or dark-purple discoloration of inner needles: possible needle cast diseases.
- Spherical fruiting bodies (pycnidia) emerge from stomata with small, oval, hyaline, aseptate conidia: Rhizosphaera needle cast (Figure 2).
- Clusters of conidiophores emerge from stomata (possibly easier to detect on green than brown needles) with light brown, elongate, five- to eight-septate conidia: Stigmina needle cast disease.
- Spherical, spiny fruiting bodies (ascocarps) form on branches or pulvini (needle pegs) (Figure 3) with pale to medium brown, septate ascospores (asci may or may not be readily visible): Sudden needle drop (SNEED).
Figure 2. Conidia of Rhizosphaera sp. Photo by Eva Grimme, Montana State University.
Figure 3. Fruiting bodies of Sudden needle drop on a spruce tree branch. Photo by Eva Grimme, Montana State University.
Pine (primarily Austrian, ponderosa, mugo)
- Tan or brown needle tips with discoloration fading into green tissue at the base of needles (on trees near roads where salt is used for winter snow management):salt spray damage.
- Tan or brown needle tips (often with a distinct interface between dead and live tissue) that start as dark-green (water-soaked), tan, or reddish-brown spots, eventually forming a band (possibly with small resin drops) (Figure 4) around needles; dark fruiting bodies (stromata) with hyaline, elongate, and one- to five-septate conidia: Dothistroma needle blight.
Figure 4. Reddish-brown band with resin droplet on pine tree needle caused by Dothistroma sp. Photo by Eva Grimme, Montana State University.
Pine (primarily Scots pine)
- Tan or brown needle tips with discoloration fading into green tissue at the base of needles (on trees near roads where salt is used for winter snow management):salt spray damage.
- Tan or brown needle tips (often with a distinct interface between dead and live tissue) that start as yellowish or reddish-brown spots, eventually forming a distinct band around needles; dark fruiting bodies (acervuli) with olivaceous, elongate, curved, one- to five-septate conidia: Brown spot needle blight.
Resources
- Compendium of Conifer Diseases, Second Edition. 2018. Edited by Everett M. Hansen, Kathy J. Lewis, and Gary A. Chastagner. APS Press. ISBN: 978-0-89054-597-3.
- Insects and Diseases of Woody Plants in Colorado. 2014. W. Cranshaw, D. Leatherman, B. Jacobi, and N. Tisserat. Colorado State University Extension. ISBN: 978-1-889143-25-5.
- https://pddc.wisc.edu/2015/07/14/conifer-disease-quick-reference/
- https://extension.umn.edu/plant-diseases/rhizosphaera-needle-cast
- https://hortnews.extension.iastate.edu/stigmina-needle-cast
- https://extension.usu.edu/pests/ipm/notes_orn/list-treeshrubs/dothistroma-needle-blight
- https://www.fs.fed.us/rm/pubs/rmrs_gtr335.pdf
