Management of Phytophthora diseases of durian

Thailand, Vietnam

Professor David Guest
Phone: 02 9351 2946
Fax: 02 9351 4172
Email: d.guest@usyd.edu.au

• Department of Business, Industry and Resource Development, Northern Territory, Australia
• Southern Fruit Research Institute, Vietnam
• Kasetsart University, Thailand
• Queensland Department of Primary Industries, Australia

$846,380

01/07/1998 - 31/12/2001

01/01/2002 - 31/12/2003

Dr Greg Johnson

Durian is an important cash crop in most of Asia. The unusual-tasting fruit is highly prized in many Asian cultures and prices for good fruit can be high. Worldwide, durian production is worth about $3 billion. As with many tropical tree crops, durian is severely affected by diseases caused by the cosmopolitan soil-borne pathogen Phytophthora. Affected plants show symptom such as root rot, canker, pre- and post-harvest decay of fruit, and even dieback of complete seedlings or trees. Infection is most severe during the rainy season, when fungal spores are dispersed from the soil or from tree canker infections directly onto the ripening fruit. In the warm, humid conditions the fungus spreads, rotting the flesh of the fruit, causing the skin to split, and rendering the valuable fruit unsaleable within a few days.
Thailand, the main durian-growing country, appeared to lose about 20% of its production to Phytophthora, but in particular regions sometimes losses could be much greater. Durian production is under way in Australia, and growers in north Queensland and the Northern Territory identified Phytophthora disease as one of the major constraints to productivity.
Attempts at controlling the fungus have usually only taken place when outbreaks of disease are severe, and have relied on fungicides, biological control and careful hygiene to reduce spread. However, orchard management practices are certain to play a part in affecting the survival and dissemination of the pathogen.

This project sought ways to control diseases of the important tropical fruit durian, caused by the fungus Phytophthora palmivora. The main objective was to improve orchard sustainability and the consistency of fruit supplies and quality in Vietnam, Thailand, and Australia through an integrated program of orchard and postharvest management of the pest fungus.

Scientists first obtained baseline information about durian production, and the incidence and severity of diseases. Fungal pathogens were collected from durian and other hosts in several different regions. At the same time, the ecology and variation of Phytophthora populations in durian plantations were studied. Scientists also evaluated control measures such as improved soil drainage, changed nursery practices and soil pasteurisation.
Expertise in Thailand was used to acquire more knowledge about postharvest disease management and durian cultivation. Researchers also examined the effects of injecting durian trees with the chemical potassium phosphonate. Australian scientists are world leaders in the use of this compound to control plant diseases.
Finally, the project linked up durian experts from Thailand, Malaysia, Vietnam, and Australia (which hopes to enter the market by taking advantage of the reversed production periods in the southern hemisphere).

In line with the primary aim of the project, the key output was a set of recommendations for the integrated control of Phytophthora diseases in durian in each location for disease control, from the nursery through to postharvest handling:
Disease control in nurseries: Pathogen-free potting media is fundamental to producing healthy plants, yet the cost of soil pasteurisation equipment is currently beyond the finances of most private nurseries in Vietnam. The project team recommended a search for alternatives to coconut fibre and rice husk, because the research showed that potting media used in Vietnam harbour Phytophthora. Researchers developed bioassays to screen durian germplasm for disease resistance, and put forward local recommendations for propagation of disease-tolerant varieties.
Disease control in orchards: Integrated disease management (IDM) in orchards is achieved through planting disease-tolerant material (wherever possible), by implementing appropriate orchard management practices, cultural and biological controls and through the judicious use of appropriate chemicals. The project demonstrated the efficacy of composted animal manures in improving tree health and reducing disease incidence, and recommended time and rate of application. Other outcomes included identifying suitable cover crops for durian in Australia's Northern Territory (NT), and identifying local sources of suitable mulch and care in its removal in the wet season. The scientists demonstrated the role of fallen fruit, at all stages of development, as sources of inoculum that harbour Phytophthora and other pathogens. They developed recommendations for the rate and timing of phosphonate trunk injections, which have proven spectacularly successful in ameliorating patch canker in Vietnam.
Postharvest disease control: Controlling P. palmivora in the orchard improves the quality of the harvested fruit. Specific fruit treatments were formulated to further reduce the incidence of fruit rot. Manual harvesting, as conducted in Thailand, was recommended for all regions. However, as harvesting indices were not available for the highly variable Vietnamese cultivars, the scientists recommended that fruit be tied to branches to allow natural abscission without the associated disease and injury problems of allowing the fruit to drop to the ground.
Technology transfer: A top priority of this project was the dissemination of recommendations to the durian industry. This process has been greatly assisted by the fact that the collaborating institutions are either extension agencies or have a strong extension culture. Research findings and recommendations have been communicated through training courses, on-farm demonstrations, the use of electronic and written (newspapers, magazines, pamphlets) media, project website and growers groups.
Capacity building: This project has made a significant contribution to horticultural research capacity of the partner countries, especially Vietnam. The skills and research capacity of collaborating institutions have been significantly enhanced through this project, which has not only had direct positive impact on the results achieved, but will provide a lasting legacy for institutions and scientists.