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Heterobasidion Root Disease (HRD)

Formerly annosum root rot

Heterobasidion root disease (HRD), formerly known as annosum root rot, is considered one of the most destructive diseases of conifers in the northern parts of the world. It was found in Wisconsin in 1993 where it is caused by the fungus Heterobasidion irregulare (H. irregulare). Prevention of HRD is key, as it is difficult to treat and control. Many tree species can be hosts, but it is most common in Wisconsin in red and white pine plantations.

Learn the biology and impact of Heterobasidion root disease (HRD), how to identify it, how to prevent its introduction and much more in this video [exit DNR].

Distribution

A map of Wisconsin, with counties where HRD has been confirmed highlighted green.
Wisconsin counties (shown in green) where Heterobasidion root disease has been confirmed (as of January 2020).

Since 1993, HRD has been confirmed in 28 Wisconsin counties (see map).

HRD is most damaging in plantation-grown conifers (especially pine and spruce) where stumps of trees that were cut down offer a place for infection to start. Once a stump is infected, HRD spreads to other living trees through root contact underground.

Site factors/history

Site factors are characteristics of the specific area that influence how likely it is for a disease to occur there. Site factors include the type of soil, temperature, slope of the land and more.

In the southeastern United States, the disease is more common on former agriculture land with a soil pH of less than 6. Sandy or sandy loam soils at least 12 inches deep, with good internal drainage and a low seasonal water table are also considered good sites for disease development.

Biology

The cause of Heterobasidion root disease

Infection most often happens when basidiospores produced by the fungus land and grow on the surface of a freshly cut stump. This infection process is why HRD can be so damaging in areas where trees are cut down.

A diagram showing the HRD infection process.

Trees that can get Heterobasidion root disease

Many tree species can be hosts to HRD but in Wisconsin, it is most common in pine and spruce plantations. On overstory trees, infection has been observed on red, white and jack pines and white and Norway spruce. Infection has been found in understory trees of red, white and jack pine, balsam fir, white and Norway spruce, eastern red cedar, red and white oaks, black cherry and buckthorn.

Not all species appear to be killed by HRD. Of all the species found with the disease, mortality has been observed or suspected on red, white and jack pines, balsam fir, white and Norway spruce, and eastern red cedar. Although fruit bodies have been found on deciduous seedlings, dieback symptoms have not yet been observed on them in Wisconsin.

Heterobasidion root disease can spread both above and below ground

Basidiospores are most often produced when the temperature is between 41 and 90 degrees F and can be carried by the wind over hundreds of miles, though most spores only move to within 300 feet.

The fungus starts living in the stump and moves into the root. Where roots connect, it moves from tree to tree at the rate of 3.2- 6.5 feet per year. Although infection through root and lower stem wounds can also occur, the major point of entry for the disease is through freshly cut stumps.

Impact, signs and symptoms

A close-up of a person pointing at an infected seedling's white fruit bodies.
Infected seedling with HRD fruit body at base.

HRD causes a decay of the roots and lower stem, attacks the cambium and kills infected trees. Individual tree impacts include thin foliage, reduced height, diameter and shoot growth and eventual mortality. Three to eight years after nearby trees are cut down, mortality of individual trees becomes noticeable. Understory seedlings and saplings that are near infected older trees may also become infected. The number of infection centers in a stand can vary widely. As these pockets of trees die, gaps are created in the forest canopy where brush and early successional trees can grow.

Both HRD and red pine pocket mortality can occur in the same stand and even within the same pocket.

A picture of HRD fruit bodies. They resemble popcorn, covered in pine needles.
Popcorn stage of fruit body, typically seen in summer.
A picture of an HRD fruit body. It's white and resembles a ruffled edge of cloth.
Fruit body in the fall.
The underside of a fruit body. It's white, with twigs and leaf litter clinging to it.
Underside of fruit body.
Dark yellow, stringy decay caused by HRD.
Stringy yellow decay caused by HRD.

Prevention

Once this disease is in a stand, it is very difficult to control. Prevention is the best approach.

If you are planning a thinning or harvest, consider treating freshly cut stumps with fungicides. Stumps must be treated as soon as possible after cutting (no later than the end of each cutting day). Fungicides will help prevent new infections but will not stop the growth of the fungus if a stump is already infected. 

  • Currently there are two products registered and available in Wisconsin to prevent HRD. Cellu-Treat is a Borate-based chemical and RotstopC is a biological fungicide that contains spores of a naturally occurring wood decay fungus. Both products can be mixed in water and applied either manually using a backpack sprayer or mechanically with an attachment on a processor.

The risk of infection by HRD is higher when a stand is close to infected stands. It is likely that unknown infection centers are present in Wisconsin. A risk-based guide [PDF] is available to help landowners/property managers determine whether a fungicide treatment should be considered to reduce the risk of HRD.

Management

Management information is available in the HRD treatment guidelines [PDF]. For recommendations when HRD is in a stand, view Appendix A.

The Wisconsin Department of Natural Resources maintains the locations of confirmed HRD-affected stands. If you suspect HRD in your stand, please contact forest health staff for confirmation and consultation.

Managing for Heterobasidion root disease when it is not yet in a forest or stand

Prevention is the best approach when HRD is not yet in a stand. Review the information under the Prevention heading for information about fungicide treatments applied on freshly cut stumps.

Revised Heterobasidion root disease guides

The HRD treatment guidelines [PDF] were developed to reduce the risk of introduction and spread of HRD in Wisconsin. These guidelines were reviewed, revised and implemented on Jan. 1, 2019.

The original guidelines were implemented in 2013. A review of those guidelines by internal and external stakeholders began in October 2017. Final draft guidelines were posted for public input in September 2018. The guidelines were finalized and approved by the DNR Forestry Division in December 2018.

The HRD guidelines are designed to help property managers and landowners determine whether the preventive pesticide treatment should be used to reduce the risk of introduction and spread of H. irregulare at the time of harvest in a pine and/or spruce stand. The guidelines should also be used by foresters and loggers to help communicate with property managers and landowners about the pesticide treatment option. These guidelines were developed to be scientifically sound, based on currently available scientific information and operationally practical in the field.

The stump treatment guidelines for the prevention of HRD are used for forest management activities on state lands managed by DNR and are recommended on county forests and private lands. Treatment of stumps from merchantable pine and/or spruce is recommended under certain conditions described in the guidelines.

For more information, please contact Kyoko Scanlon.