Postharvest handling and disease control in melons in China and Australia

• China Agricultural University, China
• Sydney Postharvest Laboratory, Australia
• Gansu Agricultural University, China
• Xinjiang Department of Agriculture, China
• University of Queensland, Australia
• Xinjiang Agricultural University, China

Melons are one of the most important cash crops grown in the northern-central and north-west provinces of China, accounting for a large percentage of many farmers' incomes. Cultivation of melons in the Xinjiang and Gansu provinces increased from 247,570 ha in 1995 to 311,500 ha in 1998, while the harvest grew from 5 million to just over 6.5 million tonnes. The main markets for melons, however, are not local, but are in the eastern coastal cities such as Beijing, Shanghai and Guangzhou, some 1500 km away. In China, little has been done to control postharvest diseases or maintain melon quality, so high losses (35-50 per cent) have been recorded, particularly when melons are sent over these long distances. Consequently, prevention of postharvest disease and maintenance of melon quality during transport has become a major problem for the farmers and wholesalers.
Plants protect themselves from disease through a range of natural defence mechanisms. Recent research has shown that a range of chemicals which boost the natural defence mechanisms in plants may reduce disease losses in melons. Plants treated with activators of natural defence develop systemic acquired resistance (SAR) and have enhanced protection against infections by viruses, bacteria, fungi and nematodes. Systemic acquired resistance (SAR) has been intensively investigated for disease control in many crops. However, there are very few reports considering SAR for postharvest disease control.
In a preliminary project in 1998, it was found that preflowering application of one such 'defence booster', benzothiadiazole (BTH), reduced postharvest rockmelon fruit losses. This enabled extension of the shelf-life of rockmelons to three weeks, sufficient time to enable export to the Hong Kong and Singapore markets by sea-freight. However, further work was required to assess the efficacy of compounds that elicit systemic resistance in melons, and to fine-tune application strategies in relation to environmental stressors such as heat or water stress.

The aim of this project was to further develop preharvest strategies that boost natural defence mechanisms in melons to protect them from postharvest disease. The project also aimed to develop and test postharvest technologies to enhance disease control and to maintain quality during the long distance transport required in China and Australia to reach retail markets.

The work was divided into three broad subprojects, corresponding with the main objectives:
To develop preharvest strategies which boost natural defence mechanisms in melons and maintain quality. This involved screening a range of regulators and elicitors of systemic induced resistance. Researchers assessed impacts on yield, quality and disease control following short- or long-term storage and transport.
To develop and test postharvest technologies that can be used during storage and transport to minimise disease and improve quality. This work focused on the use of heat and fungicide treatments and on options for cooling fruit before and during transport.
To document the melon supply chains in China and Australia, to assess the production, handling, transport and storage conditions for melons and determine what interventions would be appropriate and feasible. This involved cost-benefit analysis, investigation of the precise nature of transport conditions, and identification of inefficiencies and losses in the logistical supply chain. There was also an evaluation of the impact of the technology on farmers' incomes and on other stakeholders.

Preharvest application of defence elicitors - benzothiadiazole (BTH) and ReZist in Australia and Harpin in China - significantly reduced both foliar and postharvest disease in melon fruit. Application of the plant defence elicitors combined with postharvest dipping of fruit provided better control than elicitor alone or fungicide alone. Markers of enhanced defence in melons were identified as the pathogenesis-related (PR) proteins chitinase and peroxidase. Other markers identified were antifungal phenolic compound in the leaves and fruit, as well as development at the cellular level of physical barriers to fungal infection (such as lignin and callose deposits). The timing of elicitor treatment did not appear to affect the level of protection of fruit against disease, since one, two or there applications of BTH provided good protection against postharvest disease. The plant defence elicitors did not adversely affect the quality of the melons in terms of sweetness, flesh firmness and colour, and rind colour.
The efficacy of a range of Generally Regarded As Safe (GRAS) compounds were tested as postharvest treatments to minimise postharvest disease and to replace fungicide use. A postharvest dip of melon fruit for one minute in 30 ppm of iodine solution at 55C provided excellent postharvest disease control, equivalent to that achieved by conventional fungicides.
The potential economic benefits were assessed for the adoption of the application of plant defence elicitor as a pre-harvest spray, and postharvest dipping of fruit in hot iodine. In preliminary trials of the project technologies, it was found that there was a 12% improvement in marketable fruit through the application of the immune response promoter. The hot iodine treatment was also efficacious in extending the postharvest performance of fruit.
The potential benefits of these technologies were analysed from three levels: firm or grower level, supply chain level and industry level. In analysing returns to growers, three levels of benefits were identified: reduction in average growing costs; improvement in marketable fruit; premium prices paid for improved quality. At the supply chain level, financial benefits that could accrue depend on the performance of individual supply chains, the volumes that the chain partners handle, and retailers' pricing strategies. At the industry level, three sources of benefits were highlighted: extra revenue due to increased quantity in marketable fruit, improvement in quality; savings by sending export fruit by sea rather than air freight.
At all levels of the melon supply chains in China, there is no horizontal integration. A strategy that was employed in this project was to encourage cooperation at the farmer level in product branding, sharing of information and centralising postharvest handling tasks such as cooling, grading, dipping and packaging. Workshops were held to develop quality assurance systems, and develop postharvest interventions such as a cool room. Through cooperation, farmers gained greater control in the supply chain, resulting in greater access to markets and better financial returns.