Development of heat systems for quarantine disinfestation in tropical fruit

• Bureau of Plant Industry, Philippines
• Thailand Institute of Scientific and Technological Research, Thailand

Fruit flies are a major impediment to export trade in fresh tropical fruit by many Southeast Asian and Southwest Pacific countries, including Australia. Fruit supplied to the major markets of New Zealand, the USA and Japan, in particular, must be disinfested to satisfy the quarantine requirements of these countries.

Traditionally disinfestation has been achieved with ethylene dibromide (EDB) fumigation, but new standards that require much lower EDB residue levels in produce will not enable sufficient fumigation to kill fruit flies. To date, there is no suitable heat treatment that is cost effective yet efficient in destroying fruit fly and pathogens while maintaining fruit quality. It is important to note that heat disinfestation is a quarantine measure, not an alternative to field pest control.

Of the several ways of applying heat to fruit, the systems with the greatest potential are vapour heat, hot air, and hot water treatments. Dipping in hot water, the most efficient and least expensive method of raising fruit temperature, will control postharvest diseases in mango, citrus fruit and papaya, but the longer treatment time needed for disinfestation causes considerable fruit damage. However, recent research by the Queensland Department of Primary Industries (QDPI) indicates that hot-water treatment may still have potential for disinfestation.

The project originated as a result of direct requests from the Philippines and Thailand, following interaction between research groups looking at the postharvest handling of tropical fruit. The research fits well with Australia's own interests in extending exports of tropical and subtropical fruit, currently only at 1-2 per cent of production.

There is no conflict of interest between partner countries and Australia, because their harvest times relate to seasons in the northern hemisphere. Scientists from QDPI with Thai and Philippine colleagues will work through two subprograms to develop commercially applicable heat disinfestation schedules for tropical fruits.

The entomology subprogram will develop heat treatments for removal of fruit fly from key fruits in each participating country, with treatments modified, where necessary, to minimise effects on fruit quality and maximise disease-control effects. Comparisons will be made between the effects of different heat application methods and between representative fruit fly species and fruits in order to determine the extent to which specific disinfestation conditions are independent of the method of application of heat, fruit fly species and fruit.

The fruit-quality subprogram, dealing with fruit quality and disease control aspects, will optimise disinfestation heat treatments for minimum fruit damage and maximum disease control. Fruit tolerance to heat is variable, and these studies will indicate treatments such as preconditioning that will increase that tolerance. All proposed disinfestation protocols will be tested for disease control efficiency, and additional methods will be developed when these treatments are found to be inadequate.

The Australian activities will be conducted by scientists in three branches of QDPI - Entomology, Horticulture and Plant Pathology. QDPI has expertise in fruit fly biology and control acquired during more than 75 years of continuous research. The Department also has 5 years experience in disinfestation research and extensive interaction with Asian countries through past and present ACIAR projects involving Thailand, Malaysia and the Philippines. In this project, collaborating scientists in Thailand and the Philippines will work with fruit fly species and fruit not present in Australia.

The general principles established in the course of the project will allow the development of more efficient disinfestation protocols and of cheaper and more effective disinfestation units by industry. Removal of quarantine barriers by appropriate disinfestation procedures will in turn allow a major escalation in trade in commodities susceptible to fruit fly. New export markets will also assist several of Australia's subtropical and tropical tree-fruit industries, which are fast reaching levels of overproduction for the domestic market.