POSTHARVEST DISEASE CONTROL OF MELONS USING SYSTEMIC ACQUIRED RESISTANCE AND OTHER (2008)

The goal of this research was to test commercially viable alternative methods to fungicides for controlling storage diseases of melons that are safe for human health and the environment. Initially, experiments were conducted on melons to develop a protocol for optimum conditions of disease development during storage for different pathogens and for different stages of fruit maturity. For all pathogens tested, the study found that humidity greater than 90% and temperatures above 20° C support infection and rapid growth of disease. Differences in the rate of infection and extent of disease development after the inoculation of different storage pathogens was observed between rockmelon and honeydew melons, indicating differences in host pathogen interactions. Among the tested pathogens, Alternaria spp. was the least aggressive in infection and disease severity, growing considerably slower than Fusarium acuminatum which was moderately aggressive and Rhizopus spp. which was very aggressive, in comparison. Green half-slip melons showed greater resistance to pathogen attack than green full-slip fruit, while yellow full-slip melons were highly susceptible to pathogen attack. Therefore, the laboratory experiments for postharvest treatments of rockmelons were performed using green full-slip fruit challenged with F. acuminatum. Evaluation of physical and safe chemical methods of postharvest treatment to control postharvest diseases of melons showed that none of the treatments alone was as effective as the commercially available fungicide. Hot water solutions of safe compounds considerably increase their efficacy against postharvest rots, however, symptoms of phytotoxicity on the rind after dipping made their use unacceptable. Iodine was the only safe chemical tested which did not cause any phytotoxicity on melons. When combined with hot water, iodine showed the best control of storage rots and was as good as the fungicides carbendazim or guazatine. Hot water iodine dipping of fruit also delayed ripening and fruit were firmer during storage for a longer period of time. Systemic acquired resistance (SAR) was evaluated as a method of controlling powdery mildew in glasshouse grown rockmelon seedlings by treating with the activators 2,6-dichloroisonicotinic acid (INA) or benzothiadiazole (BTH) or water. Increased resistance due to application of INA or BTH, was observed by the reduction of powdery mildew on pre-inoculated detached leaves and also on intact leaves from natural infections. Heightened resistance due to spraying with elicitors of SAR, was further evident by the increased activities of the pathogenesis related proteins (PR proteins), peroxidase and accumulation of phenolics or antifungal compounds during and after challenge inoculation. Field grown rockmelons were treated with INA or BTH or BABA (β-aminobutyric acid) or water at various stages of plant growth and evaluated for increased resistance against pre and postharvest diseases. Both powdery mildew and downy mildew were significantly less on the SAR elicitor treated plants. Preharvest treatment with SAR elicitors also reduced storage diseases of the harvested rockmelon fruit. The reduction in postharvest disease was similar whether plants were treated once, three weeks before harvest, or given four sprays during the growing season beginning at anthesis. A further postharvest dip with 500 ppm of guazatine gave substantial reduction of storage rots of melons. Enhanced activities of chitinase and peroxidase, two major PR-proteins, compared to the control, indicated induction of defence had occurred in the foliage and fruit as a result of SAR. Over the course of four field and one glasshouse experiments slight phytotoxicity was observed in plants frequently sprayed with INA or BTH, but no phytotoxicity was seen after a single spray during the late stages of fruit development. The combination of SAR elicitor treatment and use of a safe postharvest dip provided substantial control of storage rots of rockmelons. The best treatment for control of storage rots involved application of BTH (50 ppm) two weeks before harvest, combined with a hot iodine dip (55° C) of fruit, achieving equivalent or better disease control than use of guazatine fungicide dip.