Treatment of a birch tree with a honey fungus (Armillaria) outbreak in the soil

About two years ago the first fruiting bodies were discovered in the meadow at a distance of about 5 m from the main stem of an 80 year old silver birch tree (Betula pendula). The owner had already removed the other trees as a result of honey fungus infection. Now his wish is to try to save the remaining birch using a Trichoderma treatment against honey fungus.

Armillaria species (honey fungus) attack and kill the roots of many trees in urban areas and is the most destructive garden disease across Europe.

Armillaria species (honey fungus) are considered the most harmful organisms on trees worldwide. In Central Europe there are eight different species which differ in distribution, host plants and aggressiveness. For trees in our latitudes the most frequent and most dangerous in built up areas are the light honey coloured Armillaria mellea and the dark honey fungus Armillaria ostoyae. Honey fungi can spread for years and decompose the wood without the tree showing immediate or visible signs. Honey fungus fruiting bodies are the most reliant indicators of its presence – capped fungi with stem ring- and usually appear in groups from September to November, on the lawn or at the base of the tree trunk. If a tree is infected, it usually dies between a single season to several years due to white rot caused by honey fungus. Armillaria sp. are highly pathogenic on urban and garden trees, where they destroy the inner bark and cambium causing a canker that can result in the rapid death of trees.

Preferred entry points for honey fungi are through root injuries in generally weakened trees (e.g. stressed due to drought or lack of nutrients). However, the fungus can also penetrate the healthy tree through root bark (without obvious damage) and infect it using fine black rhizomorphs. It then becomes a cambium killer. This means that the fungus kills the tissue between bark and wood that is responsible for cell division. The honey fungus can also spread to other woody plants via root contacts aided by the fast underground spread of rhizomorphs.

Monitoring honey fungus with the help of wood stakes

In order to measure the treatment success of Trichoderma in the soil, monitoring stakes (25 x 2 x 2 cm) with consecutive numbering made of beech wood were placed into the soil.

The stakes were placed where fruiting bodies have been observed and in zones where they would become critical for the tree if it became infected (the earlier treatment begins, the more successful the treatment).

The stakes are driven far into the soil.

To make the stakes easy to find, they are colour marked and a site plan with measurement of the stakes (e.g. every 2 m, aligned with prominent surrounding points) is recommended.

One year after the introduction, the stakes are taken for inspection and the degree of decomposition is measured from a table (0 = intact, no wood decomposition, 4 = very high wood decomposition). Depending on the result, the stake is placed back in the same location and checked ad hoc over a period of years.

During the early stages of a honey fungus infection reduced shoot growth and small leaves are evident. In conifers, an infection can be noticed through discoloration of the needles on the youngest growth. In deciduous and coniferous trees, premature leaf fall and yellowing of the leaves/needles occur. Depending on the vitality of the tree, the tree species and the degree of aggressiveness of the honey fungus, wood degradation can progress faster or slower. The white rot that results from honey fungi also represents a danger for the stability of the tree.

Bio Control potential of Trichoderma during the early stages of honey fungus infection

The small group of honey fungus fruiting bodies have been observed for over two years several metres from the base of the stem. None were observed in the immediate vicinity of the stem base. With the aid of beech wood monitoring stakes, the infection level in the soil can be measured regularly at various points (best to measure stakes and draw them into a plan on a yearly basis). The degree of decomposition (from none to high) indicates where and to what extent the fungus has spread.

Trichoderma atrobrunneum can make a significant contribution to containing and controlling the spread of honey fungus. Due to the excretion of enzymes, the beneficial fungus dissolves the cell walls of the hyphae of the harmful fungus and feeds on the cell contents. This process is called mycoparasitism. In addition, Trichoderma atrobrunneum grows faster in comparison to honey fungus, and besides the ability to parasitize the host, it also outcompetes for space and food. Over time, the infection level of honey fungus is contained and the infection pressure decreases. This allows the tree, in this case the birch, to recover.

Honey fungus outbreak during the early stages

Spreading of maize granules with Trichoderma spores 1×108 CFU/ml several times a year over several years

Application video Avengelus granulate and monitoring stakes