Blue Stain Fungi and Quarantine Risk

What are blue stain fungi?

Blue staining found in timber is caused by a number of groups of fungi.

The major groups of fungi that cause blue stain are species of Ceratocystis, Grosmannia, Ophiostoma, Pesotum and Leptographium. These fungi are often restricted to the sapwood and are often associated with bark beetles which help to distribute their spores. Some species of Botryosphaeria, Diplodia and Sphaeropsis also cause blue stain but these infections may also extend into the heartwood.

Australia does not have many of these fungi.

What are the symptoms of blue stain fungi?

Blue stain fungi cause bluish, gray or black staining of the sapwood (Fig. 1a and 1b). Sometimes shades of yellow, orange, purple and red may occur. Blue stain fungi permanently stain the timber so it can be difficult to tell if the fungus is alive or dead. In a cross section of stained timber, you will see wedge-shaped staining. This is caused by the pigmented hyphae growing along the wood rays which radiate out from the centre of the trunk. Infection starts by fungal spores being deposited on the cut or wounded surface of the timber. These spores germinate and produce the thread-like mass of the fungus that grows in the nutrient-conducting vessels and the sapwood, eventually blocking the water-conducting columns of the tree. In longitudinal section, blue stains appear as darkened areas often scattered throughout the length of the timber (Fig. 2a).

Figure 1a and 1b. Blue stain fungi on timber.

Figure 2. The symptoms of Blue stain fungi (a) and Mould (b).

What is the difference between blue stain fungi, mould and decay fungi?

It is important to know how to distinguish between blue stain and mould or decay fungi. Blue stain is predominantly blue, greyish or black and penetrates deep into the wood. In contrast, mould fungi are greenish or greyish and superficial (Fig. 2b). Moulds can grow only in wet wood. Mould can easily be brushed off with a light rub on the surface of the infected wood, whereas blue stain is not removable because it is in the sapwood. Wood decay fungi are often white or brown and are often associated with breakdown of infected wood.

Identification of the fungi and clearance of your commodity

In most cases, there is no clear-cut way to tell the difference between mould and blue stain fungi. The most obvious difference is the depth of discoloration. Unlike mould fungus, typical blue stain fungus penetrates deeply into the sapwood and cannot be removed by either sanding or using chemical cleaners. Identification and viability testing of blue stain fungi may be time consuming and expensive and therefore may affect clearance of your commodity.

What is the quarantine risk of blue stain fungi?

Blue stain may result in a reduction in the value of timber or timber products by discolouring the wood. The fungi do not affect wood strength or cause decay. Blue stained timber may have to undergo bleaching before use, increasing production costs. Some blue stain fungi are pathogenic and can kill trees (Krokene & Solheim 1998).

How do these fungi spread?

Most blue stain fungi are carried by bark beetles and other wood inhabiting insects. Some may be dispersed by rain and wind.

How to facilitate clearance of your timber

• Use unstained timber for your commodity or packaging.
• Keep your timber dry.
• Stop insect infestation (i.e. bark beetles).
• Keep the temperatures above or below ideal growing temperatures.

Kiln drying timber and then protecting it from moisture gives the best protection against Blue stain fungi. Chemical fungicides or biocides may also assist in the control of Blue stain fungi.

Most blue stain fungi favour an environment with 20% or more wood moisture content (Techline 2005), and a temperature of 10–25^oC. Generally, temperatures higher than 65^oC at the core kill blue stain fungi (Techline 2005). The chance of post treatment infection can be reduced by storing treated timber at <20% moisture content and avoiding exposure to rain or the open environment.

What are the Quarantine treatments for blue stain fungi?

For full details of these treatments please see ICON.

If timber and timber products are found to be infected with blue stain fungi during inspection, the AQIS officer will place your consignment on hold and contact an AQIS Plant Pathologist to undertake a risk assessment.  If the fungi is alive, the consignment will be directed for one of the following treatments or re-exported or destroyed at the importer’s expense:

• T9569 Heat Treatment – 85^oC for 8 hours once the core temperature has been reached
• T9651 Gamma irradiation – 25 kGray (2.5 Mrad)
• T9020 Ethylene Oxide – 1200g/m³ for 5 hours at 50^oC or 1500g/m? for 24 hours at 21^oC; or
• T9912 Kiln drying – Please refer to ICON for details.

For treatment options, please refer to Quarantine Risk Materials (QRM) Matrix.

References

Krokene P. & Solheim, H. (1998). Pathogenicity of four blue stain fungi associated with aggressive and nonaggressive bark beetles. Phytopathology 88, 39–44.

Techline (2005). Blue Stain. U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. Issue 05/02.