Jarrah Decline*
Jarrah forests in Australia are dominated by Euclayptus marginata, known as Jarrah trees. Jarrah decline, caused by Phytophthora cinnamomi, has been a major factor affecting South West Australia forest management and land use practice for over thirty years. An introduced soil-borne fungus, P. cinnamomi is believed to have evolved in the tropical and subtropical regions of South East Asia. Even though it grows and multiplies best under tropical conditions, this fungus has proved to be one of the most widespread plant pathogens known to man. It attacks nearly one thousand different plant species, including Euclayptus marginata, and is a major problem in horticulture, agriculture and forestry throughout the world. The damage caused by P. cinnamomi in Australian forest amounts to the destruction of whole plant communities, eliminating 50-70% of the species present, including both valuable timber trees and a whole array of brilliantly flowering woody shrubs that do not occur outside of Australia. The death of 59 indigenous species in 34 genera and 13 families has been recorded from infested jarrah forests. The overall changes in open forest infected with P. cinnamomi include the reduction of tree canopy and seedling regeneration, the reduction in species diversity and population density of the heath understory and the increase in field-resistant monocotyledons.
Symptoms:
The most common symptom caused by P. cinnamomi is root rot. Necrotic lesions are formed in unsuberized, feeder roots. The pathogen penetrates the epidermis and cortex, grows into the stele killing phloem and cambium and extends along the roots, possibly girdling the collar. Secondary symptoms resemble those of drought. Woody shrubs become chlorotic, die back and collapse; their root systems totally destroyed. Large branches of trees die back, and the crown thins. Death can occur in as little as three years, however, it usually takes 5-10 years to kill eucalyptus trees.
Development:
The pathogen is distributed either by the movement of water and infested soil or gravel or by the planting of infected nursery stock. Infection occurs under warm, moist conditions when free water is available for zoospores production, dispersal and germination.Banksia grandis, a small tree present in Jarrah forests, is usually the first to die after the fungus is introduced. The rapid growth of P. cinnamomi in Banksias trees, combined with the fact that banksias trees have extensive vertical and horizontal roots, means the fungus can spread through the forest without having to move into the soil. Banksia tissue thus provides a buffer from the often hostile (dry) soil environment and allows the fungus to persist in infected areas, even after most of the vegetation has been killed.
The discoveries that Phytophthora cinnamomi could invade woody tissue, that the fungus can occur at high density at depth in the soil of specific site types and that the tree’s vertical root system is vulnerable has given a new perspective on how Phytophthora cinnamomi operates in upland sites of the jarrah forest environment. Jarrah trees (E. marginata) appear normally to be resistant to the fungus. The fine root system is infected and some invasion of major horizontal roots can take place but over large areas of forest, where Phytophthora cinnamomi has killed the banksia understory, the rate of spread of the disease and the death rate of jarrah trees is relatively low.
The ability of Phytophthora cinnamomi to cause rapid destruction of some jarrah forest is the consequence of a bizarre combination of factors. Jarrah trees have evolved to cope with long dry summers by developing an extensive root system to tap groundwater supplies. This allows the trees to continue to use large amounts of water over the summer, but has left them vulnerable to root attack by Phytophthora cinnamomi. On specific site types, where there is a layer of concreted laterite present, vertical movement of water is impeded and lateral flow occurs. These are ideal conditions for spore production and movement just below the surface. Vertical roots of jarrah and banksias pass through depressions in the concreted layer. The presence of infected banksias roots and the fact that the root channels are formed in depressions which accumulate water, cause zoospores to be concentrated around jarrah’s vertical roots. The fungus first infects jarrah’s fine roots, which often proliferate in the root channels, then invades and kills the major vertical roots. Whereas fungal invasion of a horizontal root (unless it takes place near the trunk) appears to have little impact on the tree’s capacity to obtain water, the fungus has only to invade a small distance into a vertical root to cut off the water supply. Once the tree’s vertical roots are destroyed, its unsated demand for water results in desiccation and death.
While climatic conditions, particularly unseasonal rainfall, play an important role in determining disease intensity, it appears that the processes, which cause mass death of jarrah and rapid spread of the fungus, operate only on specific site types. There is little prospect of controlling the disease on these sites. It is not known how much forest, which has not been affected by Phytophthora cinnamomi, is growing on these highly susceptible sites, but there are large areas of forest growing on soils that do not have the properties that permit rapid disease development. The forest growing on these free draining sites has the greatest potential for management and protection from Phytophthora cinnamomi. Scientists are optimistic that on these sites long term control of jarrah dieback disease can be achieved.
Management:
Disease has had an enormous impact in Western Australia where people, including politicians, were afraid that the problem might become statewide. Disease has resulted in reduction in royalties and increased regeneration costs. Current management practices now include reducing the density of the banksias understory, the maintenance of the free draining characteristics by minimizing soil disturbance and maintaining a closed canopy, the employment of stringent hygiene and quarentine techniques, and the promotion of resistant understory species such as legumes. Researchers are also examining inter- and intra-species resistance in eucalyptus.
*Excerpts selected from:
Shea et al. 1984. A new perspective on Jarrah Dieback. Forest Focus 31:3-11.
Weste G. and Marks GC. 1987. The biology of Phytophthora cinnamomi in Australiasian Forests. Annual Review of Phytopathology 25:207-229.