Root diseases — Management

Root diseases are the most damaging group of tree diseases.  Forest land may be rendered non-productive when disease centers regenerate on susceptible tree species that are subsequently killed before they reach merchantability.  Furthermore, if mortality is not an outcome, disease impact can be great enough to reduce net productivity.

Managing for root disease has been an effort among forest pathologists in British Columbia for decades.  Understanding the biology, epidemiology and edaphic and other environmental conditions of the different root diseases has led to the research and development, and the eventual application of root disease management techniques, protocols and recommendations.

The following information on root disease management draws on a 2018 publication entitled “Managing root disease in British Columbia” (View the complete document), which brings together the best available science from throughout western North America on root disease management.  The document recommends actions for detecting and evaluating root disease incidence, and for using this knowledge to develop viable plans and treatment prescriptions.

Since there are currently no practical or cost-effective methods for controlling root disease in forested stands, strategies for controlling future root disease risk should begin in the planning and pre-harvest stages of forest management.  Once a stand is harvested, the treatment options for forest management can be implemented.

Stand Risk Assessment

Assessment of future risk to a stand is an essential component to be considered when developing a root disease management plan.  Assessing treatments to be conducted within the framework of a forest steward ship plan will help avoid future problems created by considering only short term gain.  Future risk for root disease is based on:

  • the incidence level of root disease,
  • the existing and future host-tree population,
  • the biogeoclimatic environment,
  • the recommended disease management strategies,
  • stand management practices (past, present, and future) that have affected or may affect root disease incidence levels

These latter four points require the evaluation of risk while considering the potential root disease management strategies.

Evaluating Root Disease Hazard

The potential threat posed by root disease varies across British Columbia (. The decision to apply a root disease treatment is based on an assessment of the future risk from root disease and the overall management objectives for the site.

  • Reviewing which pathogens may be present is the first step.
  • The second step is to acquire detailed site-level information from a walkthrough or more detailed ground survey. Detailed, site-specific information is required to estimate risk,
  • The third step is the decision on whether to treat for root disease or not to treat, and if treatment is required, choosing treatment(s) are most suitable.

FIRST STEP – Root Disease Hazard Assessment

Provincial maps for root disease distribution (View Appendix 2 in the Management of Root Disease in BC Guide) and regional hazard tables arranged by Biogeoclimatic Ecosystem Classification (BEC) variants (View Appendix 3 in the Management of Root Disease in BC Guide) must be consulted when determining the site-specific root disease hazard and the potential risk to timber management.

SECOND STEP - Root Disease Surveys

Surveying for root disease requires skilled surveyors that pay close attention to host tree species and are proficient at recognizing the signs and symptoms of the pathogens. The survey must sufficiently cover the site and use systematic and quantifiable methods of recording the intensity and location of the damage to make well-informed decisions about treatment type and intensity.

There are four recommended methods for surveying root disease.  Details on the survey methods listed below are available in the “Root Disease management in British Columbia” document (View the complete document).  Also, contact the regional forest pathologist for additional information.

  1. Preliminary walkthrough

The preliminary walkthrough provides an early opportunity to confirm disease occurrences, assess the risk posed to management objectives, stratify the areas by root disease, and estimate the presence of other damaging agents.  However, further surveys may be needed to accurately define treatment areas.

A walkthrough survey may not provide enough information to properly delineate root disease treatments.  The following situations indicate that you must use one of the other three survey methods outlined below to collect adequate information:

1) The distribution could not be accurately determined by the preliminary walkthrough;

2) Small, dispersed root disease centers are found in the preliminary walkthrough; or

3) Multiple species of root disease are found and the treatment boundaries need to be refined.

  1. Pre-harvest sketch mapping survey

An area-based sketch map survey can delineate the approximate areas where root disease exists to stratify areas for treatment.  This method is ideal in areas with well-defined root disease centers. Sketch mapping is very difficult or impossible in situations where there is scattered mortality due to root disease interspersed with asymptomatic trees.

Following the pre-harvest sketch mapping, treatment buffers can be added around each infection center.  These buffers delineate the extra area likely occupied by the infected root systems.

  1. Variable-width transect survey

The tree-based, variable-width transect survey is suitable for estimating disease incidence in almost all stand types and for all root diseases. Depending upon the transect interval, this method also provides an accurate estimate of disease location. This survey method is ideal for situations where mortality and symptomatic trees are scattered throughout the stand or where there is a mixture of tree species that vary in their susceptibility to root disease.

  1. Post-harvest stump top survey

Stump top surveys are used to accurately confirm disease incidence after harvest and to delineate treatment strata based on observable signs and symptoms when clearly visible on freshly cut stumps. This method works best in situations where indicators of decay like stained, delaminated and pitted wood can be readily assessed from freshly cut stump surfaces.

It is important to recognize both the potential and limitations of technology in regard to locating, identifying and mapping stands infected by root disease.  Aerial photos, LiDAR or helicopter flights can help identify and delineate root disease centers from stand/landscape features however, technology is not a substitute for ground surveys by skilled surveyors.

THIRD STEP – Root Disease Treatment Options

Recommended root disease treatments for the major root disease found in British Columbia can be reviewed at this link (View Table 3 from the Management of Root Diseases in B.C. Guide). The recommended treatments are implemented during or immediately following harvest.

Regenerating with less susceptible species

Tree species vary in their susceptibility to different root diseases.  Losses to host-specific root diseases,  can largely be avoided by planting non-host species at the time of reforestation. Species conversion to less susceptible trees naturally occurs over time in both managed and unmanaged stands, but it can be facilitated through species selection prior to planting. Host susceptibility, future site productivity, climate change impacts, and other management considerations must be balanced when selecting species for reforestation.

Stump removal

Stump removal (also known as stumping) has repeatedly been shown to reduce root disease, in the majority of cases where it has been applied.  When used appropriately, stump removal is justified and compensated by reducing mortality to reforestation seedlings due to root disease and improving tree growth.

If tree size prevents the removal of all stumps in a stratum a treatment other than stumping must be selected.  Uprooted stumps should be flipped and placed back in their original holes to allow the roots to dry.  Large roots that break off during stump removal and hardwood stumps should be removed from the soil. Silvicultural systems that reduce the effectiveness of stumping (e.g., dispersed retention) should be avoided.

It is highly recommended that a post-stumping survey be completed to ensure that the stumping treatment was adequate. It is more economical and efficient to conduct the post-stumping survey while the machines are still onsite. If the original treatment did not remove a sufficient amount of stumps, roots and debris, a second removal may be required. Leaving behind stumps, stubs or residual trees of any species must be avoided to reduce the probability of retaining potential sources of inoculum, thereby reducing stumping efficacy.

While stump removal can significantly reduce root disease, a number of negative site impacts may occur. Excessive site disturbance can increase the risk of introduction and spread of invasive plants. The disturbance of soil may result in erosion, puddling, compaction, inversion of horizons, and nutrient loss.  Stumping operations should be monitored frequently to avoid excessive site or soil disturbance and must be postponed after periods of heavy rain or snow. An assessment of site disturbance can be completed using the soil conservation survey.

This quantitative decision support system exists to assist a forester in making a risk assessment on Armillaria root disease (View Figure 1 from the Management of Root Diseases in B.C. Guide).

Pushover harvesting

Pushover harvesting combines harvesting and stump removal in one step and is suitable for tree sizes up to 78 cm DBH.  Provided that the amount of root removal is comparable, pushover harvesting can be as effective as post-harvest stump removal in reducing root disease. There is also less risk of leaving tree roots in the ground because of missed stumps.

Facilitating hardwood regeneration

Hardwoods appear to limit the spread of root disease between conifers. Although the underlying mechanisms are poorly understood, hardwoods are less susceptible to infection and more tolerant to disease.  As a comparatively resistant species, the roots of birch can form a barrier to underground disease movement.  Highly susceptible hosts, such as Douglas-fir, benefit by having such impediments to disease spread.

Biological control

Biological controls are introduced agents that can displace or prevent colonization by pathogenic fungi and prevent the spread of disease. Biological agents are considered pesticides and, as such, must go through the same regulatory approval process and be registered for use by the federal government.

Stump avoidance

This strategy involves avoiding stumps when planting and is recommended for sites infected by tomentosus root disease only.

Stand management objectives should be adjusted where root disease incidence, site conditions, risks, or constraints dictate. This is preferable to prescribing an inappropriate root disease management treatment that places resource management objectives and future forest health at risk.