Blue stain fungi – an important part of the mountain pine beetle epidemic
May 9, 2008 1:00 PM
Filed Under: Mountain Pine Beetle
Searching for images of the MPB infestation yields a plethora of pictures of dying trees with their characteristic red needles, or those of the beetles themselves which are as small as a grain of rice. We cannot, however, forget the importance of our other player, the blue stain fungi. Though we don’t see them at work, fungi play a crucial role in the epidemic. Therefore, knowing the biology of the fungi is another key component to understanding the epidemic itself.
First a little bit about the types of fungi associated with the MPB. Many different species have been found associated with MPB. Interestingly, individual MPB can be associated with multiple fungal species. If you have an interest in taxonomy, many of these fungi are from the phylum Ascomycota, order Ophiostomatales. For reference, humans (while in a different Kingdom altogether) are in the phylum Chordata, order Primates.
A scanning electron microscope image of a cryo-prepared Grosmannia clavigera conidiophore with multiple conidia (spores)
These fungi cannot move from tree to tree without help. When adult beetles emerge in the summer to look for a new host tree, they transport fungal spores with them. This process, also known as vectoring, is a major benefit that the fungi gain from their association with the MPB. These spores can be transported by beetles in a couple of ways. Spores can be picked up from gallery walls as adult beetles prepare to emerge. They are carried as passengers on the beetle’s exoskeleton and inoculate new trees once beetles have bored in. In addition, and more interestingly, beetles actively collect fungal spores and store them in structures called mycangia. These mycangia are basically exoskeletal pouches on the beetle’s mouth used to carry fungal spores. The spores are then released when the MPB colonize new trees. Once underneath the bark, the fungi germinate and will grow rapidly along the MPB gallery walls and also through the host tree’s phloem and sapwood. The fungi also produce a compound called melanin which discolours the wood and results in the characteristic stain we associate with MPB killed trees. With enough growth, the fungi can block the tree’s water transport system, helping to kill the tree. This is where the fungi return the favour, as fungal attack increases the potential for successful colonization of new trees by MPB. Another point to consider is that the fungi need to survive the cold winter temperatures, just as the beetles do. This becomes important as MPB feed on the fungi. Differences in overwinter survival between fungal species may play a role in both beetle health and the effectiveness of the next years’ colonization efforts. The following spring, the fungi resume their growth and eventually produce new spores (either sexually or asexually) and the cycle begins again.
Once again you can see that knowing the underlying biology of the organisms and how they interact with one another is crucial to understanding the epidemic as a whole. While I have now commented on the beetle (About a beetle – insert website link here) and fungi, we still haven’t looked at the pine tree, host to both organisms and not without defences of its own.
Thank you to Dr Adrianne Rice, Mycologist with Natural Resources Canada for providing fungal information used in this posting.
Posted by Matt Bryman at May 9, 2008 1:00 PM
Comments
John email -
What benefit do the fungus and beetle derive from killing their host tree?
email -
thanks this has helped me with a science project!!!!
Dental Roseville email - www.dentalroseville.com
Grosmannia clavigera, is a species of sac fungus. It spreads to Lodgepole, Ponderosa, Douglas-fir, and Whitebark pine trees from the mouth of the Mountain Pine Beetle. The blue stain fungus has evolved a relationship with mountain pine beetles that allow them to travel from tree to tree on a special structure in the beetle’s mouthparts and stops the tree from producing resin to pitch out or kill the beetle, encouraging the pine beetle invasion occurring all along the Rocky Mountains from Mexico up to Canada. The beetles are able to mine and lay eggs while avoiding the tree’s defenses. The 33 Mb genome of this fungus was sequenced in 2009.
The blue stain fungus spores germinate and produce a thread like mass (mycelium) that colonizes the phloem and sapwood. Spores are usually blown away by wind but blue-stain spores are “sticky”. This process eventually blocks the water conducting columns of the tree draining the trees of their nutrients eventually causing the tree to starve to death. The symptoms and signs of blue stain fungus are a blue-gray discoloration of sapwood in wedge shapes of recently killed trees.
Preventing the blue stain would require that unfavorable conditions for the fungi be maintained like: keeping the wood dry, protecting the wood from mountain pine beetles, and maintaining temperatures above or below ideal growing temperatures. The prevention of the spread of blue stain fungus can only be controlled by protecting the trees from the mountain pine beetle. There are various methods of preventing the spread of the pine beetle but they are highly ineffective and typically very costly.