Study On Latent Infection,Pathogenic Mechanism Of Botryodiplodia Theobromae And Control For Stem-end Rot Of Mango Fruit

Mango (Mangifera indica Linn.), a valuable fruit crop growing throughout the tropics. However, mango is highly susceptible to postharvest decay during storage and transportation, which leads to huge economic losses. The pathogens can infect the mango flowers and immature fruit and remain quiescent until storage, during which the lesions progressively appear and symptoms become more severe along with the extension of time. It has been well known that the stem-end rot caused by Botryodiplodia theobromae is the one of most destructive postharvest disease in mango fruit.Using B. theobromae and mango fruit as interaction system in this study, we investigated the occurrence in latent infection of pathogenic fungi in six provinces of China according to the tissue isolation method, explored the pathogenic mechanism of B. theobromae, studied the tolerance of B. theobromae to several common fungicides as well as tested some technologies to control postharvest disease in mango. The results were presented as follows:The results showed that the latent infection of fungi comprehensively existed in the healthy mango fruits. There were significant differences in latent infection rate among the mango fruits in different provinces (F=3.59, P=0.0246), but not significant differences in latent infection rate in different years(F=1.57, P=0.2372). The rates of latent infection in the shoulder and the stem end were significantly higher than those in the other parts (F=46.44, P <0.0001). There were11pathogenic fungi to be identified, including Colletotrichum gloeosporioides (Penz.) Penz.&Sacc、Botryodiplodia theobromae Pat.、Colletotrichum acutatum Simmonds、Phomopsis mangiferae Ahmad.、Pestalotiopsis mangiferae (P. Henn.) Sutton.、Dothiorella dominicana Pet. et Cif、Macrophoma mangiferae Hing.&O. P. Sharma、 Alternaria alternata(Fr.)Keissl.、Aspergillus sp.、Curvularia sp. and Fusarrium spp.. In these isolated fungi, both C. gloeosporioides and B. theobromae were the main fungi leading to latent infection in mango. Moreover, M. mangiferae was first reported as one of latent infection fungi in mango fruit.Four tested pathogens including B. theobromae, C. gloeosporioides, D. dominicana and P. mangiferae had different pathogenicity that cause stem end rot in mango fruit, among which the B. theobromae showed the strongest pathogenicity. The growth rate of B. theobromae in culture medium containing pectin or cellulose was extremely higher than that in other three pathogens. Furthermore, the tested results using congo red staining method showed that the activity of pectinase generated by B. theobromae was higher than that in other three pathogens, and cellulase activity produced by B. theobromae was higher than that in D. dominicana and P. mangiferae, but was similar as gloeosporioides. The results by colorimetric method of DNS demonstrated that B. theobromae may release polygalacturonase (PG), pectinmethylgalacturonase (PMG), polygalacturonic acid trans-eliminase (PGTE), pectin methyltrans-eliminase (PMTE) and cellulase (Cx.) when cultured in vitro or in vivo. Among three primary CWDEs, PG and Cx appeared activity peak earlier and activity peak of PMG was lagged.B. theobromae in PA liquid culture produced a specific toxin that can result in browning of mango young leaves, wilting in branches and decay in fruits. The toxin had a strong thermal and acid-tolerance stability but not sensitive to light. The appropriate low temperature was favorable to toxin preservation.B. theobromae produced ethylene in vitro, and it induced the mango fruit to produce large quantities of1-aminocyclopropane-1-carboxylic acid (ACC), which might be responsible for ethylene evolution. The ethylene began to generate increased prior to appearance in symptom of stem-end rot, followed by a rapid increase in production rate accompanying with stem-end rot development. Climacteric ethylene with a short period was associated with rapid softening and rot. Within3to5days of inoculation with B. theobromae, the decay and rot symptom completely appeared in fruit.There are numbers of B. theobromae species that had a resistance to carbendazim. The toxicity of23fungicides to4isolates of B. theobromae was tested by mycelium growth rate methods. The toxicity results were analyzed by the EC50value, the EC90value and the cross-resistance to each other. The mango stem end rot could be effectively controlled by sporgon, propiconazole, flusilazole, prochloraz, iprodione, difenoconazole, tebuconazole, myclobutanil, pyraclostrobin and validamycin A. In addition to carbendazim, the B. theobromae also showed the resistance to other fungicides such as thiophanate-methyl, thiabendazole, kresoxim-methyl, and diniconazole. The results of cross-resistance showed that carbendazim, thiabendazole, kresoxim-methyl, and diniconazole had a positive cross-resistance for each other, while kresoxim-methyl and validamycin existed in a negative cross-resistance for each other.Iprodione, prochloraz and iprodione.prochloraz were effective for controlling stem end rot of mango. According to overall evaluation for efficiency of controlling stem-end rot and anthracnose, iprodione.prochloraz exhibited the best capability, and the prochloraz was in the next place.There was a significant difference (P<0.01) in toxicity of18food additives against6postharvest pathogens (C. gloeosporioidesS, C. gloeosporioidesR, B. theobromaeS, B.theobromaeR, P. mangiferae and D. dominicana) in mango fruit. The sodium o-phenylphenate was the strongest toxicity aganist6pathogens, with showing the lowest values of EC50(7.21μg/mL) and EC90value (39.53μg/mL), followed by sodium dehydroacetate and ferulic acid. The effects of sodium dehydroacetate for controlling anthracnose and stem end rot were the best among the all tested food additives, with the efficacy reaching83.52%and59.32%, respectively, followed by ferulic acid and cinnamaldehyde. The storage time of ferulic acid-treated fruit was15.51d which was the longest among the tested food additives, and followed by sodium dehydroacetate and licorice antioxidants.The method using food additives combining with chitosan coating has been considered as a promising postharvest technology for controlling stem-end rot and anthracnose. The present results showed that the control incidences of three combination treatments including1000μ.g/mL sodium dehydroacetate+1.5%chitosan,1000μg/mL ferulic acid+1.5%chitosan and500μg/mL sodium dehydroacetate+1.5%chitosan against stem-end rot were99.34%and96.97%and90.28%respectively.