Latent Infection And Induced Resistance Of Major Postharvest Diseases On Muskmelon (Cucumis Melon L.c.v. Yindi)

Latent infection and induced resistance of major postharvest diseases on muskmelon (c. v. Yindi) treated with benzo (1, 2, 3) thiadiazole-7-carbothioic acid S-methylester (BTH) and Harpin were studied in this paper. Alternaria, Trichothecium and Fusarium rots caused by A. alternata, T. roseum and Fusarium sp. respectively, are major postharvest diseases on muskmelons. Fruit could be infected by A. alternate and Fusarium sp during development. The latent infection rate was increased with the development of fruit. A.alternata initially infected the styles and settled in ovary during flowering, then kept quiescence in fruit till after harvest. The pathogens also penetrated the peel through epidermal lenticels and nets in the surface of fruit, and remained latent in the underlying epidermal cells. The infection of Fusarium sp. stayed in the tissue around the nets, and caused lesion after fruit were harvested. The latent infetion rate of fruit was decreased significantly by using BTH at 50mg/L or Harpin at 60mg/L. 4 times of foliar application with BTH or Harpin significantly controlled powder mildew and downy mildew caused by Spherotheca fuliginea and Psudoperonospora cubensis, respectively. Extracts were obtained with ethanol-dichlormethane from the peel of melons at different development stage. One zone with antifungal activity was revealed after separation on thin-layer-chromatography (TLC) plates. The possible antifungal substances were identified with gas chromatography and mass spectroscopy (GC-MS) as bis (2-ethylhexyl) dioctyl ester, 1,2-benzenedicarboxylic acid, 2,6-bis (1,1-methylethyl)-4-methyl phenol, bis (2-methylpr) 1,2-benzenecarboxylic acid, methyl ester hexadecanoic acid, squalene. In vitro test indicated that diisoocytl adipates (500μl/100ml) and squalene (4000μl/ml) inhibited mycelial growth of A.alternata. The components and content of antifungal substances decreased with the fruit development. The young fruit at 20 days after flowering had more components and higher content of antifungal compounds; however, the mature fruit had less. The components or content of antifungal substances increased after being treated with BTH at 100mg/L or Harpin at 60mg/L during flowering and 7 days after flowering.The postharvest resistance of fruit inoculated with T.roseum or Fusarium sp. was induced by treatment of BTH or Harpin before 7 days or 1 day of harvest. BTH at 100mg/L or Harpin at 60mg/L decreased the lesion area to a greater extent in fruit inoculated with T.roseum or Fusarium sp., respectively. The lesion area of fruit inoculated with T.roseum or Fusarium sp. was also cotrolled effectively by postharvest treatments with BTH or Harpin. There was a clear time-dependent response of the fruit to postharvest treatments with BTH or Harpin. Efficacy of reducing decay with BTH or Harpin lasted nearly 8 days when fruit were inoculated with T.roseum and Fusarium sp., respectively. The protection of BTH or Harpin in muskmelon was related to peroxidase (POD). POD activity was induced in treated fruit, and was sustained for at least 10 days at room temperature. These results suggested that BTH or Harpin promoted induced resistance in muskmelons by increasing defence-related enzymes.