Induced Resistance Of The Litchi Fruit To Litchi Downy Blight By Oligosaccharins

Litchi downy blight is an important disease of litchi fruit in the stage of growth andstorage and it could cause fruit drop and rot in quantity which will have bad impact on theproduction and sale of litchi. Oligosaccharins is a kind of marine bio-pesticides and it is theresistance inducer of crop, which has been widely used in the control of plant disease.This paper did the research on the resistance to litchi downy blight whenoligosaccharins induced litchi fruit. The results showed that the incidence degree of litchidowny blight was significantly reduced after spraying oligosaccharins solution topostharvest fruit of litchi. The resistance degree of litchi fruit to litchi downy blight wasrelated to concentration of the oligosaccharins solution and induction time. Specifically,the resistance degree induced by100mg·mL-1oligosaccharins solution was the strongest(The decline rate of disease was60.93%). The resistance effect induced by100mg·mL-1oligosaccharins solution in12hours was the best (The decline rate of disease was67.18%).The activity of peroxidase(POD),polyphenol oxidase(PPO), phenylalanine ammonialyase(PAL), chitinase enzymes and β-1,3-glucanase in the peel of postharvest fruit of litchiwere increased differently after the treating of oligosaccharins. Specifically, the activity ofPOD were higher than the control with21.02%and40.96%in the third day and in the fifthday respectively. The activity of PPO was higher than the control by37.88%in the fifthday.The activity of PAL was higher than the control75.28%in the third day. Chitinaseenzymes's activity reached the highest in the third day, which was1.31times of themaximum value of the control. The activity of β-1,3-glucanase was higher than the controlby27.57%in the third day. The amount of malondialdehyde (MDA) and superoxide anion(O₂-) in the peel of postharvest fruit of litchi were both decreased after treating witholigosaccharins. Specifically, the content of MDA were lower than the control by14.33%and20.57%in the second day and in the fifth day respectively, and the content of O₂-werelower than the control by11.55%and23.15%in the fourth day and in the sixth dayrespectively. Therefore, the improvement of defensive enzymes activities and the declineof the content of peroxide substance was one of disease resistance mechanisms ofoligosaccharins inducing resistance of litchi postharvest fruit to litchi downy blight.The results of spraying oligosaccharins and Bacillus subtilis in field to control litchidowny blight showed that the control effect of oligosaccharins, B. subtilis and their barrelmix preparations to litchi fruit in the growth period were55.05%,32.42%and58.08%respectively, which were significantly lower than the control effect of the control pesticideof dimethomorph thiram. The control effect of taking turns to spraying oligosaccharins and dimethomorph thiram was80.10%, which was close to the control effect of only sprayingthe control pesticide4times. The control effect of spraying the tested pesticides topostharvest in7days on litchi downy blight were78.64%,76.50%,53.46%and76.64%respectively. The safety test showed that oligosaccharins, B. subtilis and barrel mixpreparations had no harm to litchi fruit, and they could extend the length of litchi fruitpostharvest storage by2and3days. So oligosaccharins or oligosaccharins mixed with B.subtilis could be an alternative to chemical fungicide to treat postharvest fruit to controllitchi downy blight during the storage period. However, the effect of oligosaccharins tocontrol litchi downy blight during the period of fruit growth was ineffective, while takingturns to spray oligosaccharins and chemical fungicides could meet the control requirementsduring the fruit growth period.