Minimising disease impacts on eucalypts in south east Asia

• Forest Science Institute of Vietnam, Vietnam
• Royal Forest Department, Thailand
• Queensland Forest Research Institute, Australia

Vietnam, Thailand and Laos, were developing their plantations of Australian trees and producing export woodchips, attracting overseas investment, and boosting domestic livelihoods as poles, furniture, building timber and fuel. But these activities have been constrained by the diseases leaf blight and stem canker, particularly in Eucalyptus camaldulensis. Eucalypts planted for oil production in northern Australia have also been infected.
Thailand has established over 100,000 ha of plantations of E. camaldulensis, and many other plantings have occurred along roadsides and around farmer's homesteads. In Vietnam, the government wants to increase plantations from the current 245,000 ha to 10 million ha, increasing by 150,000-200,000 ha each year.
However, since 1989 symptoms of fungal attack of eucalypts (defoliation and stem dieback) have struck many locations. The fungus Cylindrocladium quinqueseptatum appears the main agent, but other pathogens including Cryptosporiopsis sp. have been isolated. This disease could lead to losses worth $A40 million in Vietnam and $A20 million in Thailand.

This project aimed to find out how to manage the eucalypt diseases and restore grower and investor confidence in Australian eucalypts for Southeast Asia.

Using techniques that were already largely known, the scientists identified provenances or species of eucalypts that resist the suite of pathogens, and made preparations to use them in place of those that were susceptible. Once growers knew which genotypes to plant in affected areas, the short growing rotation would enable a relatively rapid recovery of production.
The project took a three-way approach. Researchers continued work already begun to identify the main pathogens and their interaction with specific provenances, species and clones of eucalypts in various appropriate environmental conditions. At the same time, seed orchards of E. camaldulensis and E. tereticornis were established in disease-prone parts of Vietnam to identify those genotypes which appeared to resist the disease. Selected stock was tested for performance in field trials where the disease was currently severe. Thirdly, observations of disease at specific locations in Thailand and Indochina were added into existing computer programs that mapped climatic data and simulated eucalypt growing conditions. The scientists attempted to generalise the observations to make the disease hazard predictable for much of the area.
Local staff in the region were trained and equipped to diagnose, survey and manage disease, and knowledge was spread through workshops, field days and information leaflets.

Pathogens causing moderate to severe foliar disease in both Thailand and Vietnam were identified. These included Cryptosporiopsis eucalypti, Cylindrocladium reteaudii, an un-named species of Pseudocercospora, and the newly-introduced pathogen Phaeophleospora destructans. The relative
importance of these pathogens varied, with C. reteaudii being of major importance in Vietnam but causing damage only in very wet years in Thailand. This difference in severity closely corresponded to predictions made from bio-climatic modelling of this pathogen for Southeast Asia.

The regular monitoring of leaf pathogens in trials in south-eastern Thailand led to detection of a new incursion of P. destructans into mainland Southeast Asia. Several important eucalypt stem diseases were found in Thailand and Vietnam for the first time, including cryphonectria canker, coniothyrium canker, and (in Vietnam) bacterial wilt (Ralstonia solanacearum).

Other records were made for Queensland; the occurrence of these diseases in trials was used as a resource for training overseas staff on attachment to CSIRO Forestry and Forest Products, Canberra. Rapid screening methods suitable for greenhouse or controlled environment (CE) chambers were developed for Cylindrocladium and Cryptosporiopsis. However, these methods were not used in Thailand or Vietnam for determining provenance resistance to pathogens because of high between-tree variation, and a shortage of suitable facilities.

The establishment of 3.5 ha of E. camaldulensis as a provenance-progeny trial at Chon Thanh, Binh Phuoc Province provided the opportunity to select trees for growth, form and disease resistance. Following discussions at the 1999 workshop in Ho Chi Minh City a stratified sample of 72 trees was relocated to Song May, Dong Nai Province (a highly disease-prone environment) to allow further evaluation as clonal mother stock for future plantations. During the last year of the project the team evaluated a 50-clone replicated trial at Song May established from 'plus' trees (i.e. trees with superior performance) selected in the south-east region of Vietnam.

The project team established close relations with the Siam Pulp and Paper Co. (SPPC) and its subsidiary the Siam Forestry Co in Thailand. The company established a series of large replicated trials of over 200 selected clones at 30 locations in western Thailand. Project scientist Mrs Pongpanich monitored three of these for resistance or susceptibility to leaf blight and canker diseases. She also trained company staff to recognise pathogens, score disease impacts and select disease-resistant clones. The selection of superior clones in Thailand has led to the identification of 30 clones of E. camaldulensis for release to farmers for planting in the SPPC resource area. This is an ongoing program with some clones being changed every year to reduce risks from damage by pests and diseases.

In Vietnam, the project has established five replicated trials in southern, central and northern Vietnam, using clones selected from the 50-clone and 72-clone trials at Song May. The trials are testing the performance of the selected clones for growth, form and disease resistance across a range of climatic and edaphic conditions, and are also being used to assess performance of trees against pests and pathogens which were undetected in Vietnam at the outset of the research program.

The activities and outputs of this project have led to major changes in how participating countries in Southeast Asia manage eucalyptus plantation diseases. Scientists, tree improvement technicians, plantation managers, forestry enterprises and commercial forestry companies now have much greater awareness of diseases and their impacts on tree health and productivity by. This has led to changes in selection criteria to include resistance to specific pathogens rather than reliance on gross crown health classes, and to the establishment of a series of replicated clonal trials across climatic and geographical gradients to assess impacts on growth and tree health. These trials have included clones selected for different levels of resistance to known pathogens. Without such clones, impacts of pathogens cannot be assessed and reliable selection of superior resistant clones cannot be made.