Latent Infection And Biological Control Of Main Postharvest Diseases On Muskmelons (Cucumis Melo L.)

Latent infection and biological control of main postharvest diseases on muskmelon were studied during 2001 and 2002.Black spot caused by Alternaria alternata and white mould caused by Fusarium spp. occurred frequently after harvest on 'Hami' melons. Both of two pathogens could infect fruits at preharvest. There were two principal periods to infect melons for A.alternata in the field. One was at flowering period. The fungi initially infected the styles and settled in ovary, then kept quiescence till after harvest. Another happened during development of fruit. The pathogen penetrate the peel through epidermal lenticels and net in the surface of fruit, and remained latent in the underlying epidermal cells. For Fusarium spp., the infection occurred mainly during developing of fruit, and the nets formed in peel were important pathway. Net forming stage was the key period when latent fungi infected muskmelon.The preformed antifungal compounds were extracted by ethanol-dichlormethane and column-chromatography from the peel of 'Hami' melon at different mature stage, and detected by bioassay on PDA. The possible anti-fungal substances were identified with gas chromatography and mass spectroscopy (GC/MS) as bis(2-ethylhexyl) pathalate, 1,2-benzenedicarboxylic acid,dioctyl ester, butyl phthalate, methyl palmitate, methyl linolelaidate, methyl linolenate. In vitro test indicated that diisoocytl adipates inhibited growth of Alternaria alternata directly. The species and content of anti-fungal substances decreased with the development and maturity of the muskmelon, little compounds were detected at harvest time. The results indicated that duration of latent infection could be determined by these compounds.The resistance of 'Hami' melon to postharvest Alternaria rot could be induced by postharvest treatment of chemical inducers BTH and Harpin. BTH at 100mg/L showed moreeffective. Fruits were treated with the chemical, then inoculated with the conidia of Alternaria alternata after 48 hours, the diameter of black spot were measured after stored 7 days at 20癈. The diameter of black spot of l00mg/L BTH treated fruit was smaller than the control significantly.The mechanism of induced resistance by elicitors on melons was due to production of pathogenesis related proteins (PRs). Activities of Chitinase and Peroxidase (POD) increased in treated fruit at lOOmg/L BTH and 90mg/L Harpin.Alternaria rot and Pink rot (caused by Trichothecium roseum} were the main postharvest diseases on 'Huanghemi' melon. Two isolates of Bacillus (B1 and B2) were tested as antagonists for postharvest biological control of Alternaria rot and Pink rot on 'Huanghemi' melon. The inhibition rate of B1 and B2 cell culture to Alternaria alternata were 100% and 94% respectively on PDA, both inhibition rates to Trichothecium roseum were 100%. The cell-free filtrates of B1 and B2 by means of centrifugation and filtration also inhibited A.alternata and T. roseum in vitro compared with control significantly. The results of in vivo test showed that the cell cultures of B1 and B2 inhibited the infection rate and lesion development of A.alternata and T. roseum significantly. The cell-free filtrates of B1 and B2 inhibited T. roseum in vivo significantly.