Heterobasidion root diseases remain some of the most damaging pests to conifer forests in California. They consist of two different root rotting fungi that attack different hosts:
Heterobasidion irregulare (on pines, incense cedar, juniper and occasionally some hardwoods) and
H. occidentale (on true fir, Douglas fir, giant sequoia, western
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Fruiting body of Heterobasidion occidentale. |
hemlock, western redcedar and spruce). The two fungi were jointly known as
H. annosum and previously as
Fomes annosus. Infestations occur on wounds and freshly cut stumps and spread through natural root contact.
Prevention is the key to management of
Heterobasidion root diseases. It is important to remember that treatment can significantly reduce the potential for tree losses in the future. This is particularly important now with increased cutting of conifers after the massive tree loss due to the recent bark beetle epidemic and to the state-wide push for thinning and greater forest management to reduce threats from both bark beetles and wildfires. Once established these fungi can survive and spread through a stand for years and even decades, weakening and gradually killing more trees.
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| True fir forest affected by Heterobasidion occidentale. |
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Although these fungi are difficult to detect, it is important to be aware of them because their presence is tightly related to bark beetle attack: trees that appear otherwise healthy but have infected and compromised root systems are more vulnerable to attack and less likely to survive drought years. Once a site is infested with one of these fungi it is very difficult to eradicate.
Preventive management for
Heterobasidion root disease consists of treating stumps at the time of cutting to avoid infection. Presently, the only pesticide registered for use in California is Cellu-Treat. This pesticide is a soluble borate powder that can be applied with a backpack sprayer on freshly cut stumps. It is a restricted use pesticide with a "Caution" label. In a high value/high hazard sites such as recreational areas or sites near homes or infrastructure, all conifer stumps should be treated. Generally, pine stumps 8 inches in diameter or greater should be treated in forested areas. Stumps of dead trees still bearing needles need to be treated, since stump infection is still possible. Dead trees that have lost their foliage do not require stump treatment. Treatment of all conifer stumps is a form of cheap preventive insurance to avoid future damaging root disease centers and related bark beetle attacks.
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