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Cures fit for a king

Durian fruit is highly prized in Southeast Asia for its flavour (ACIAR)
Durian fruit is highly prized in Southeast Asia for its flavour

Regarded as "the king of fruits," durian (Durio zibethinus Murr.) is highly prized in Southeast Asia for its flavour as well as the premium price it fetches in the marketplace. Durian is grown throughout Southeast Asia and the large unusual tasting fruits, weighing between one and eight kilogrammes, are used in a variety of culinary dishes. The pulp can be eaten raw, cooked, frozen or dried and the seeds are used to make candy. But severe losses in durian (as well as black pepper, cocoa, coconut, papaya, and rubber) are caused by the pathogen Phytophthora palmivora butl. To help reduce such losses, several initiatives funded by Australian Centre for International Agricultural Research (ACIAR) have achieved success in increasing orchard sustainability and consistency of durian fruit supplies in Vietnam, Thailand and Australia.

Problems with Phytophthora

Originally native to Indonesia, Malaysia, and Borneo, Thailand is currently the largest producer and exporter of durian, where a wide variety of cultivars are grown. In recent years durian production has rapidly taken off in Vietnam, expanding into the northern highlands, displacing rice and other crops which are not as profitable. But here, as elsewhere including for durian growers in Australia, losses to Phytophthora disease is given as one of the major constraints to productivity.

A relative of diatoms, kelps and golden brown algae, the Phytophthora pathogen causes billions of dollars in damage in both temperate and tropical regions of the globe. In durian, Phytophthora manifests disease during all stages of growth, causing leaf blight, seedling and tree dieback, patch canker of the trunk, root rot, and pre- and postharvest fruit rot. Phytophthora is found in all cropping areas of the southern and central Vietnam highlands where moist conditions are common. The incidence and severity of its diseases are increasing, particularly in the Mekong Delta region where waterlogged conditions frequently occur.

An integrated approach

As Phytophthora infects durian trees in many ways, efforts to produce resistant cultivars are slow and complex. In addition, common farming practices used to achieve higher fruit yields, such as flood irrigation, heavy use of nitrogen fertilizer, quick-growing varieties, monocultures and high density planting, increase the severity of Phytophthora disease. To mitigate this, an ACIAR project led by Dr. David Guest has developed integrated disease management (IDM) recommendations for farmers to help control Phytophthora at multiple points in the disease cycle.

The IDM recommendations focus on preventing the spread of Phytophthora from the soil into the tree canopy and from one tree to another. Farmers are encouraged to plant a diverse mix of plants, such as fruit, vegetables and timber trees, which help limit build-up of the pathogen. Additionally, Guest says adding decomposing organic matter, such as wheat straw and chicken manure, releases "ammonia and volatile organic acids which kill Phytophthora, and the residual organic matter stimulates competitive and antagonistic microorganism growth." Farmers are also advised to collect ripe fruit while it still hangs on the tree instead of allowing it to fall to the ground.

Guest says a top priority of the project was the dissemination of recommendations, which have been communicated to farmers through training courses, written media, radio and television, growing associations and demonstrations. For example, at an annual event to judge durian held at the Southern Fruit Research Institute (SOFRI) in the Mekong Delta, "extension officers take the opportunity to speak to the farmers who gather for the popular and highly publicised competition about new recommendations." Guest says although the longer-term impact of only a few projects is being formally followed up, "every durian farmer I have visited is following at least some of our recommendations. The most effective adoption pathway is through converted farmers, and the few hundred farmers we had direct interactions with have obviously been very successful at passing improved farming practices on to their peers."

Economic and effective

Another initiative led by Guest has involved testing a water-soluble and inexpensive chemical solution called phosphonate for its ability to control Phytophthora diseases in durian. The compound is injected directly into the trunk of trees where Guest says "it will move up with the sap flow and move right throughout the tree into all the leaves and the flowers and the developing fruits." The treatment was determined to be successful when tested in Vietnam's Mekong Delta and ACIAR scientists have since determined optimal rates and times for injection.

Phosphonate is also economical. Farmers can share the cost of injectors and drills, as each farmer needs to inject his trees only once or twice a year (depending on which IDM procedures the farmer has adopted). Guest points out that farmers "only need one or two extra durian fruits per tree to cover the cost of injecting." Data shows that over the lifetime of the tree, the value of income from improved yields versus injection costs are in the order of 20:1.

Written by: Treena Hein

Date published: September 2006


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