Getting to the Root of Branch Dieback
Written by: Guest Author: Brian Hudelson, UW-Madison Plant Disease Diagnostics Clinic
I often have clients who contact me because branches on their trees are dying. Their assumption is that there is something going on directly in the branches that is causing the dieback. While that can be the case, there are also a myriad of other reasons why tree branches can die. Here is a laundry list of what I commonly see causing branch dieback in my clinic.
Canker diseases. Canker pathogens (most commonly fungi and bacteria) locallly infect areas on individual branches and eventually girdle branches leading to branch death. Often, the dead branches are scattered randomly in an affected tree. These types of diseases are most commonly managed by pruning out the diseased branches to keep the pathogens from moving into the main trunk, where they can cause more extensive damage. Classic canker diseases include Nectria canker (caused by a fungus that can infect a wide range of woody plants), golden canker (caused by a fungus that is relatively specific to pagoda dogwood) and fire blight (caused by a bacterium that infects a wide range of woody rosaceous hosts, but has its biggest impact on apple, crabapple and pear trees).
Vascular wilt diseases. For woody plant species, vascular wilt diseases are typically fungal in nature. The pathogens find their way to a plant’s xylem (i.e., water-conducting tissue) through direct infection (typically via roots) or via insect feeding or egg laying activities. Vascular pathogens eventually lead to blockages in the xylem that limit water movement to branches. This lack of water is what leads to branch dieback. Dieback typically starts with a single branch and progresses to encompass a cluster of branches. Often vascular wilt diseases eventually kill trees. The most common vascular wilt diseases are oak wilt, Dutch elm disease, and Verticillium wilt. Once infection occurs, these diseases are challenging to manage. Management typically involves using preventative fungicide injections (for oak wilt and Dutch elm disease) or avoiding planting susceptible tree species in areas where the pathogen is known to occur in the soil (for Verticillium wilt).
Root rots. Root rots tend to be more of an issue in sites with heavier soils and poor drainage, as well as in years with above average rainfalls. These wetter conditions are favorable for soil-borne fungi and water molds (in particular Phytophthora) to infect roots. Infections lead to deterioration of roots, which in turn leads to reduced ability of the affected plants to take up water, resulting in leaf thinning and branch dieback acoss a broad section of the upper tree canopy. Modifying soils to improve drain prior to planting and use of fungicide treatments (if root rots are detected early) can be useful management strategies. Fungicide use, however, requires proper diagnosis of the causal organism(s), as root rot fungicides are somewhat specific in terms of the organisms that they control.
Other root diseases. The primariy disease that I see in this category is Armillaria root disease. The fungus involved tends to infect through wounded roots (e.g., from construction damage or trenching to sever root grafts) and colonizes roots and the lower trunks of trees. This colonization inhibits water uptake and also disrupts water transport in the trunk. Trees with Armillaria root disease often survive for a period of time, with symptoms (i.e., branch dieback) intensifying when trees are stressed (e.g., during a droughty growing season). Eventually infected trees succumb to the disease. Spore-producng mushrooms and rhizomorphs (root-like fungal structures that grow through the soil) help the fungus spread from tree to tree. Once trees are infected, there is no cure. Removal of infected trees including stumps and roots (to remove the pathogen’s food source) is the typical management of choice.
Fungal wood rots. These diseases are less common causes of branch dieback, but can contribute on occasion. The fungi involved often enter through wounds, leading to rot of interior wood in tree branches and trunks. In the process, they can sometimes disrupt the xylem around the point of infection, killing branches fed water by this compromised vascular tissue. Look for fungal conks (i.e., shelf fungi) or mushrooms associated with these diseases. Unfortunately, once infection occurs, there is no treatment. So, management relies on preventing wounding that provides entry points for the pathogens.
Non-disease issues. Often, branch dieback is not caused by disease. Other potential causes can include:
- Girdling roots. These roots that encircle the trunk of a tree most commonly form when trees are planted improperly. Girdling roots eventually come into contact with the trunk of a tree and compress the vascular tissue under the bark. This limits water flow, and any branches fed by this compressed vasculature can decline and eventually die.
- Winter burn. This dieback most commonly occurs on evergreens (yews and boxwood can be particularly hard hit) and is really a problem with dehydration in affected tress and shrubs over the winter. Proper watering, particularly late into the fall, can help prevent winter burn from being a issue.
- Flooding of soils for an extended period purges oxygen from the spaces between soil particles. Without this oxygen, plant roots die, leading to reduced water uptake, subsequent branch dieback, and often tree death. I personally killed a dappled willow this way years ago. When I planted the shrub, I wanted to make sure it received suffient water. So, I set up a soaker hose and turned it on. I then neglected to turn the water off for 18 hours, leading to a small bog in my yard in the area around the shrub. The soil was totally saturated and went anaerobic, and the willow died shortly thereafter.
- Many insects can lead to branch dieback, but the most common one I see is two-lined chestnut borer. I often get requests to test dying oak branches for oak wilt. Often the branch dieback is not due to this disease but is due to two-lined chestnut borer. This insect and many others that can cause branch dieback are attracted to stressed trees, so reducing tree stress can prevent them from being a problem.
As you can see, diagnosing the cause of branch dieback in trees and shrubs can be a complex process, and there may be multiple causes of dieback in a given plant. But that’s why the UW Plant Disease Diagnostics Clinic (PDDC) and Insect Diagnostic Lab (helmed by PJ Liesch) are around. Feel free to tap into our expertise whenever you have plant disease or insect questions. You can reach me at (608) 262-2863 or email@example.com, and PJ at (608) 262-6510 or firstname.lastname@example.org. Also, for additional information on many of the diseases/disorders discussed in this article, check out the fact sheets and web articles on the PDDC website (https://pddc.wisc.edu/). For updates on new PDDC educational materials and training opportunities, you can follow the clinic on Facebook or Twitter (@UWPDDC) or subscribe to the clinic’s email listserv by sending a subscription request to email@example.com.