Borax Versus Killer Fungus
January 1994

Conifer forests are threatened all over the northern hemisphere by the tiny, ubiquitous spores of a naturally occurring fungus called Heterobasidion annosum. This disease, better known as Fomes, has reached epidemic proportions in Scandinavia, and is a growing menace in the managed forests of Canada, United States, Britain, and Russia. Fomes rots the roots and heartwood of growing trees. It could be called the acid rain of the fungus world.

Supporting the UK's Forestry Commission, Borax Group scientists Kieran Quill and Jeff Lloyd are fighting back against Fomes, and discovering how to do so with maximum effectiveness and economy. Their principal weapons are Tim-bor (disodium octaborate tetrahydrate) and the analytical capacity of the Borax Research laboratories.

Fomes cannot live freely in soil nor can it infect live trees except through root contact or wounds. Its spores however can colonize freshly cut stumps - both the "thinnings" which are essential as forests mature and the stumps left when the crop is finally felled.

The spores are produced by hoof-shaped fruiting bodies near ground level at a daily rate of about six million per square centimeter. Because these spores can be dispersed over distances of at least 300 miles, Fomes can be considered ubiquitous in most managed forests. Once established the fungus can remain viable in a stump for decades, posing a continuous threat to any conifer growing or planted near it. Fomes can survive both extreme cold and extreme heat.

But how are healthy trees infected? Fomes spores germinate on the stump surface, whence the fungus gradually colonizes the root system of the felled tree. From there it enters the root systems of living trees that are in contact with the stump's roots, causing both roots and heartwood to decay, eventually killing the tree.

The fungus is almost impossible to eradicate, except by the removal of all stumps soon after felling - an expensive and rarely practicable option. However, germination of spores on the surface of stumps can be stopped by chemical and biological agents. In the past, this has been carried out manually by the tree feller, but now with increasing mechanization, the requirements have changed. Today a material is needed that can be sprayed automatically onto the stump while the harvesting machine is actually severing the tree. The material must give value for money, be easy to obtain, have low mammalian toxicity, be non-corrosive and environmentally benign.

Among several fungicides tested, borates have consistently given good control. Tim-bor (known as Tim-Bor® in North America) and borax are the only chemicals to have EPA approval for the control of Fomes in the U.S. However materials that are effective over large areas of North America may behave differently in northern Europe where rainfall, climatic conditions and forest management techniques could result in a completely different set of disease and control characteristics. In the light of this, the UK Forestry Commission and the Borax Group have carried out trials in Scotland with the object of determining borate efficacy. What is the threshold at which Tim-bor becomes toxic to the fungus? How little will do the trick?

Undiseased Sitka spruce near Peebles, Scotland were felled and their stumps were treated with Tim-bor at four percent, two percent, one percent, 0.5 percent, or with water. Twenty-four hours later Fomes was applied dropwise by hypodermic syringe.

The stumps were left to mature for a year, during which time samples of wood were regularly extracted with a core borer for borate analysis. At the end of a year, the amount of stump colonized by Fomes was measured on a one inch thick disc cut from a standard depth. Each disc was incubated at 10ºC to 15ºC for ten days.

During incubation, fruiting structures of the fungus emerge from infected wood. These can be seen quite easily under a dissecting microscope, and allow any diseased zones of the stump to be mapped. A comparison of the measured diseased areas on the sample discs provides a means of judging the success of a particular treatment.

All analytical work for the project was carried out at the Borax Research laboratories in Chessington (UK).

The results from this experiment indicate that at a borate concentration of around four percent, the mean area of infected heartwood was reduced from 22 percent to less than 0.5 percent. This represented less than one square centimeter, an insignificant inoculum. However, at concentrations of two percent and below, no significant control occurred. In an earlier experiment it was found that a concentration of five percent totally prevented infection. So a working concentration of four to five percent of Tim-bor is indicated for full disease control.

As a result of this research, Tim-bor is being assessed for full commercial application by the UK Forestry Commission, and has aroused widespread interest across Europe.