| Peach gummosis is one of the most important and damaging diseases of peach in the south peach production area of the Yangtze River of China. Peach gummosis causes significant growth stunting, yield losses, and tree death. It has been become a restriction factor for healthy and sustainable development of peach production. The three species of Botryosphaeria, L. theobromae, B. dothidea, and Diplodia seriata were the predominant causes of the disease. The objective of this study was to investigate the histopathology, biochemical changes, carbohydrate metabolism, and expression patterns during peach gummosis progression caused by L. theobromae. In addition, we also investigate the mycotoxin of L. theobromae, effect of ethephon on gum formation, and the disease control of zinc sulfate. The main results are as follows:1. The biochemical changes associated with the interaction between peach shoots and L. theobromae suggest that peach plants respond to the fungus by increasing anthocyanin levels, PAL activity, H2O2concentrations, and the expression of defence-related genes. However, the fungal infection triggered cell death and gum formation, as indicated by the increased MDA content, the expression of cell wall degrading-related genes, and reduced chlorophyll a and b content.2. These changes in carbohydrate metabolism were directly associated with the symptom of peach gummosis. Soluble sugars, glucose, mannose, arabinose, and xylose significantly increased in inoculated tissues of peach shoots compared with control tissues at different times after inoculation. Analysis using qRT-PCR revealed that the abundance of key transcripts on the synthesis pathway of UDP-D-glucuronate, UDP-D-galactose, and UDP-D-arabinose increased, but the synthesis of L-galactose and GDP-L-galactose were inhibited. Accumulation of polysaccharides was also observed by section observation and periodic acid Schiff’s reagent staining during infection. After inoculation, the transcript levels of sugar transport-related genes, namely, SUT, SOT, GMT, and UGT, were induced.3. In this study, we investigated the effect of chemical treatments on peach gummosis after inoculation with Lasiodiplodia theobromae. The results showed that ethephon significantly decreased the gum formation rate. Gum formation was promoted by ethephon treatment prior to pathogen inoculation, but inhibited by ethephon applied after the pathogen. Our results show that ethephon has a dual function in regulating gum formation by affecting the peach shoots and the pathogen. For the ehtephon post-treatment, the gum formation was inhibited by ethephon with lower gum weight, gum formation percent, and lesion diameter. The relative expression of neutral invertase was induced by1-MCP, but inhibited by ethephon. The inhibitory effect was counteracted by1-methylcyclopropane, which is an ethylene signal inhibitor.1-Methylcyclopropane also promoted gum formation. Exposure of the three stains of Botryosphaeria to ethephon inhibited mycelial growth. For the ethephon pre-treatment, the gum formation was promoted by ethephon with more gum weight, gum formation percent, and lesion diameter. Both treatment methods increased the sugar content at12and24hpi. However, Lower levels of sucrose, glucose, and fructose were detected in the ethephon-pretreated shoots than those in the post-treated shoots, with significant differences at24hpi for sucrose,48and72hpi for glucose, and24,48, and72hpi for fructose.4. An experiment system was established to obtain the mycotoxins of L. theobromae strain of JMB-122.9kinds of metabolite compounds of L. theobromae strain of JMB-122, namely,9-octadecenamide,13-docosenamide,2,4-bis(1,1-dimethylethyl)-Phenol, butylated hydroxytoluene,3,5-bis(1,1-dimethylethyl)-4-hydroxybenzenepropanoic acid methyl ester, octadecanoic acid, hexadecanoic acid, octadecanoic acid,2,3-dihydroxypropyl ester and hexadecanoic acid2,3-dihydroxypropyl ester, were identified by GC-MS analysis. The bioassay of those compounds will be determined in the next step. By investigating the effect of inorganic compounds on phytotoxin of L. theobromae, zinc sulfate and KMnO4inhibited the pathogenicity casuing by L. theobromae phytotoxin. The gum wight was significantly decreased after application with KMnO4on inoculated peach shoots.5. Zinc sulfate not only alleviated the development of peach gummosis, but also affected the growth, development, and pathogenicity of L. theobromae. Zinc content was reduced during the development of peach gummosis. For the application of zinc sulfate on inoculated peach current shoots, the lesion diameter, gum weight and both was siginificantly decreased by zinc sulfate. Zinc sulfate treatments significantly reduced disease severity and gum formation and effectively controlled peach gummosis under phytotoxin stress of L. theobromae. Zinc sulfate also induced the expression of defense-related genes of CHI, PR4, GNS3, PGIP, and PAL by qRT-PCR analysis. Those results might be responsible for the resistance increase. Results showed that mycelial growth was significantly inhibited by zinc sulfate. With the increase of concentration of zinc sulfate, the black colony exhibits an increase trend at PDA. Zinc sulfate caused abnormal hyphae at25mM and swelling hyphal tips at50mM by morphologic observation. In contrast to the control the black septate hypha increased with an increase in zinc sulfate concentration. |
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