|LOVE AND HATE RELATIONSHIP BETWEEN FOREST TREES AND THEIR MICROBIAL PARTNERS – Annosum root rot as a case study
Fred O. ASIEGBU, Professor of Forest Pathology
A popular science description
Forest trees and their associated products have tremendous economic and ecological value. Timber from forest trees form the basis for one of the largest industries in Europe contributing 100 billion euros yearly and the value on a global scale is estimated at 370 billion dollars (http://english.forestindustries.fi; Asiegbu et al., 2005). According to estimates, the demand for wood and forest products is expected to grow by 20% in the next decade. Forestry is also beneficial not only as a potential alternative bio-energy source but also for conserving biological diversity. In this context, forest trees and the associated microbial partners share overlapping co-evolutionary life histories in the context of a dynamic standoff, each seeking after its own interests. The consequences range from latent endophytic relationship to fatal pathogenic infections or mutualism. These microbes known to be associated to plants/trees are critical to forestry, agriculture and global food security. They have important impacts on forest ecology and evolution and are equally key components in maintaining vital ecosystem processes. Some of these microbes cause diseases to plants and trees while others prevent diseases or enhance plant growth. In trees alone, fungi cause some several kinds of different diseases including root rot infections and oak decline. At the other extreme are beneficial symbiotic mycorrhizal fungi which play important and economically significant roles in the nutrition, growth and health of forest trees, as well as in nutrient cycling.
With respect to forest tree diseases, about 15 - 20% of conifer trees cut in boreal forests are rotted and commercially less valuable, largely caused by a fungus with latin name as Heterobasidion annosum. The economic damage due to H. annosum infection approximate to 1 billion Euros in losses to European forestry industry. Additional losses to forestry industry arise from damage caused by fungi to wood and timber products. The worlwide costs for such damage has also been estimated to cost several billion dollars each year. The level of diseased forest stands and associated production losses are expected to continue to increase for the foreseeable future. This lecture will primarily focus on annosum root and butt rot disease of standing conifer trees. In the Nordic countries, root rot and decay of pine and spruce trees causes annual economic losses of several hundred million dollars to forest industries. The disease was first discovered in 1878 by a German scientist named Robert Hartig. In the forest, the fungus is known to spread with the aid of spores to stump surfaces. Sometimes they can also spread through contacts between rotted roots and healthy roots. The fungus can only grow within the wood but very rarely on the surrounding forest soil. The spores of the fungus spreads very easily during summer time but very rarely in winter, therefore infection is more common in summer. As a result, it is often more advisable to fell trees for timber during winter period.
While inside the tree, the fungus virtually depends on the tree for its food, feeding mainly on the three major components of the wood such as lignin, cellulose and hemicellulose. Lignin helps the tree to defend itself from animals and insects, also from bending and strong winds. But as the fungus eats all the lignin, the tree looses its strength and vigor and can easily crack, fall or bend when blown by a mild or strong wind. The fungus also obtain nutrients by degrading the main carbohydrate (cellulose) part of the tree. Cellulose is used for making paper. By degrading cellulose, the fungus makes the wood not suitable for paper making. It equally obtains sweet sugars by breaking down another carbohydrate component of wood called hemicellulose. Hemicellulose is a very important raw material for many chemical industries especially for making xylitol an important sweetener or Furfural used in oil refining. Since the fungus feeds on hemicellulose, the wood is no longer valuable for chemical industries. When the fungus finally degrades all the three major components of wood (Lignin, cellulose, hemicellulose), the tree at this stage is considered to be decayed and is particularly of no use to timber industries because such rotted woods can not be used for construction purposes or for building houses nor for making furnitures or electric poles. However, the living tree tries to defend itself by producing different kinds of poison called phenolics and other antimicrobial compounds. In some cases they produce lots of lignin to strengthen the tree and to prevent the fungus from gaining entry into inner tissues. Sometimes these defense barriers helps to restrict the fungus from killing the tree but in most cases it succeeds in overcoming the defense barriers and the tree succumbs to the infection and dies.
Diagnostics and control
The process of infection and manifestation of disease symptoms and finally death of the tree can be painfully a very slow process which can last several years (ca 15 - 35 yrs) depending on the age of the tree at the time of infection. Practically, this implies that the forest owner may not be aware that the forest stands are infected until very late in the infection stage. A very simple way for identifying infected trees is the presence of fruit bodies characteristic of Heterobasidion at the base of the tree trunk or roots. By detecting the spread of the infection much earlier, the forest owner will be advised to take adequate protective measures to contain and restrain the spread to nearby forest stands.
In the earlier days, nothing was known about how to stop this fungus from causing devastating damage to forest plantations which often results to huge economic loss to forest owners. But thanks to painstaking research at universities in Great Britain and Scandinavia that have led to very important discovery on how to manage Heterobasidion infection. Trials in several parts of Scandinavia have shown that pre-treatment of stumps with urea reduces significantly the incidence of the disease. Field trials by researchers in the Scandinavia using a biological control agent (Phlebiopsis sp.) popularly called ”Rotstop” has shown high efficacy in controlling the spread and growth of the pathogen. At the moment, the annosum root rot disease of standing trees is controlled by the use of the above chemicals, biocontrol agents, and silvicultural measures. So far, these treatments do not lead to full protection. Ongoing research using biotechnological methods focuses on identifying molecules important in pathogenesis and host defenses. This modern approach has led to the discovery of a number of novel host genes including those encoding antimicrobial peptides that have recently shown potential inhibitory effect against H. annosum. Such novel approach, we anticipate will provide part of the necessary basis for resistance research, molecular breeding and the development of effective environmental friendly control methods.
Further Reading: Asiegbu FO, Adomas A & Stenlid J (2005): Conifer root and butt rot caused by Heterobasidion annosum Bref. s.l. Molecular Plant Pathology 6(4): 395 – 409.