Tress And Their Diseases

With the start of the New Year we are starting a new series of blogs on tree knowledge. This will mainly feature on causes of ill health in trees and will help you identify when further action is required.

Giant polypore, botanical name Meripilus giganteus, is a large bracket fungus, which usually affects beech trees. Its impressive fruiting bodies appear around the base of the tree as this fungus attacks the roots. It is parasitic, meaning it attacks live trees, and can cause trees to collapse in the worst instance.

Giant polypore causes what is known as white-rot. This means it degrades and feeds on both lignin (the substance which makes plants woody) and cellulose (cell walls). Its fruiting bodies (fungal brackets) only survive for a year, and may not form every year. The fruiting body is just one part of the fungus. If the fruiting body is not visible, this does not mean the infection is not present, just as removing the bracket will not stop infection.

Giant polypore usually affects the underside of tree roots. This makes the extent of infection very difficult to ascertain. The severity of infection does not always have a visual affect on the crown of the tree either. For these reasons, it is often unwise to keep an affected tree if it is in falling distance of public or property.

I will now focus on Kretzschmaria deusta, (previously known as Ustilina deusta). This is a particularly dangerous type of decay fungus. It is relatively common, reproducing with spores carried in the wind and can infect most species of tree.

Kretzschmaria deusta is dangerous for two main reasons. Firstly it attacks live trees. It decays the base of the tree, attacking the heartwood. This can often make the extent of decay within the tree difficult or impossible to identify. It can potentially cause stem failure without warning, particularly if the fruiting body has not been identified.

The second reason Kretzschmaria deusta is particular dangerous is because the fruiting body is difficult to spot. It is not an obvious fungal bracket. It instead grows first a white layer at the base of the tree when juvenile, then transforming into a black layer when mature. It can often look like a thin tar covering at the base of the tree. As the fruiting body only grows at the base of the tree, it can often be hidden by long grass, weeds or other plants within a dense understorey. This is one reason you may see experienced tree consultants kicking undergrowth away from the base of trees when carrying out an inspection.

As Kretzschmaria deusta can cause tree failure without warning, infected trees are often condemned as soon as the fungus has been identified, particularly if the tree is in a high risk location such as at a roadside or adjacent to a building. There is no cure for the infection and it can continue living on dead tree stumps after the rest of the tree has been removed. This makes controlling the disease difficult and we must instead be vigilant to spot infection before serious harm is caused.