By: Emily Jack-Scott (Garfield County Colorado Master Gardener)
In late summer and early fall evergreen trees often have needles that dry out and shed. This can be alarming for gardeners, and it can be difficult to tell if early needle drop is cause for concern or just part of the evergreen tree’s annual needle shed. In this blog post we’ll review some signs and symptoms of when summer needle drop in evergreens could be indicative of underlying health problems.
It is typical for all evergreen trees to shed out old needles in the fall. Fall needle drop is usually preceded by yellowing of old (often interior) needles. Sometimes needle drop happens in early summer, especially if a tree has been recently transplanted. Trees can lose up to 90% of their roots when transplanted, which seriously hampers the tree’s ability to provide its needles with vital water and nutrients. Therefore it is common in the first few years after transplant for a new tree to have needles that turn yellow or brown weeks or even months before they typically do in fall.
Premature needle drop in newly transplanted evergreen trees is especially common in interior needles. Evergreen needles often turn a pale sickly shade of green during transplant shock. Once needles discolor, they never recover their original dark green or blue hues. Instead, the tree will retain the weakened discolored needles for several years until the tree has put on enough new healthy growth that it no longer requires the old weak needles for photosynthesis. At this point, the old interior needles will turn yellow or brown and soon after drop off. In spruces and pines, interior needle shed is common August – November. In these instances, summer needle shed is not usually cause for concern.
There are, however, some instances when summer needle shed and discoloration may be symptoms of more concerning health threats such as beetle activity, mites, fungal disease, root damage, or winter burn/injury.
When bark beetles such as mountain pine beetle or spruce beetle attack their respective host trees, they first chew through the bark and then tunnel around beneath the bark in the thin layer of the tree’s growing tissue known as the cambium. This can result in sudden red discoloration of needles (in pines), or slow and steady browning and dropping of needles (in spruces). To tell if needle discoloration is caused by beetle activity, look for other symptoms of bark beetles such as small entry holes in the tree’s trunk (with or without sap), fine sawdust (or frass) collecting in the bark or base of the trunk, and/or thin sap streamers dripping down the trunk of the tree.
Once trees have been attacked by bark beetles, there is little that can be done to prevent the decline and death of trees. But there are preventative sprays and pheromone packets available for trees that have not yet been attacked. These can be used in areas where bark beetle activity is high.
Another critter that can cause extensive discoloration of needles is the humble spider mite. Spider mites cause extensive damage and stippling to affected needles as they feed on their host’s sap. Colorado blue spruces are especially prone to spider mites. Webbing may even be visible on heavily infested branches. Improving the general health of affected trees is recommended. Miticide sprays are available to treat heavy infestations.
Spruce and pine trees can be prone to several fungal infections that can lead to needle discoloration and early drop. One of these fungal infections is known as cytospora canker, which primarily affects Colorado blue spruces. Needle discoloration from cytospora is typically purple or rust-colored, primarily on lower branches of mature spruce trees. Cytospora canker also causes resin-covered cankers on affected branches.
Another common fungus in Colorado that can cause needle discoloration and shedding is rhizosphaera needle cast. Rhizosphaera needle cast primarily affects Colorado blue spruce but can also affect lodgepole and ponderosa pines. Needle discoloration caused by rhizosphaera will typically start off yellow in mid-summer, turning to reddish-purple in late summer, before turning brown and shedding out. Spruces and pines affected by rhizosphaera needle cast will lose their interior older needles, leaving only the current year’s growth (which has not yet been infected). Infected needles are often covered in tiny black dots running lengthwise, which are the visible fruiting bodies (pycnidia) of the fungus.
There are preventative fungicide sprays available that can prevent spreading of fungal infection to new growth and unaffected trees in the vicinity. Timing of these sprays is critical, so be sure to carefully follow fungicide directions. Cultural treatments such as removing fallen needles and creating better airflow through tree canopies are also recommended.
If the discoloration and shedding of needles occurs in a spiral pattern up the tree, there is a good chance the premature needle drop is the result of root damage. Trees tend to spiral slightly as they grow (twisting slowly over time in response to prevailing winds). So when a root is damaged, it is unable to provide vital nutrients and water to the living tissue it is connected to running up the length of the tree in a slightly spiraled pattern. This results in the spiral pattern of dieback from damaged roots.
Root damage can be difficult to troubleshoot, since they are not visible below ground. But common causes of root damage include root girdling, root severing/damage from digging (e.g. utility repairs or sidewalk installations), or damage from absorption of salt, chemicals, or pesticides. Needle drop from root damage can be isolated, and the tree will recover in time. The exception to this is when roots have been girdled, in which case the tree will continue to lose needles and will likely need to be removed if the girdling root(s) cannot be isolated and removed.
Winters in Colorado can often be very cold, with heavy snowfall and strong winds. The thin wax coat on needles is all that protects evergreens from succumbing to harsh winter conditions. It’s no wonder that after particularly bitter winters, some evergreens show signs of winter injury by way of discolored needles. This can be especially true when affected trees endured stressful conditions in the summer growing season before (e.g. transplant, drought), limiting its ability to grow thicker waxy needle coatings. Winter injury can often present itself in a pattern, such as all the needles on the windward side of the tree showing discoloration. Discoloration from winter injury is often reddish-purple in hue.
This has been a review of some of the more common biotic or abiotic causes of premature discoloration and dropping of evergreen needles. There are other causes of needle drop, such as nutrient deficiencies and soil conditions that can also cause problems. If in doubt of the cause of your needle discoloration, contact your local Colorado Extension office or a certified arborist for more information or a site visit.
Cytospora canker https://extension.umn.edu/plant-diseases/cytospora-canker
General needle drop https://utahpests.usu.edu/ipm/notes_orn/list-treeshrubs/needle-drop
Mountain Pine Beetle https://csfs.colostate.edu/forest-management/common-forest-insects-diseases/mountain-pine-beetle/
Rhizosphaera needle cast https://www.ag.ndsu.edu/cpr/forestry/needle-cast-diseases-of-spruce-diagnosis-and-treatment
Spider mites: https://utahpests.usu.edu/ipm/ornamental-pest-guide/arthropods/mites/spruce-spider-mite
Spruce beetle https://csfs.colostate.edu/forest-management/common-forest-insects-diseases/spruce-bark-beetle/
Winter injury https://www.canr.msu.edu/news/sizing_up_winter_injury