Objective New and efficient biocontrol suspensions that could prevent fruit postharvest diseases such as litchi downy blight were screened out to reduce the loss of litchi during transportation and delay browning, and reveal the relevant physiological mechanisms of biocontrol efficacy.

Method The bacterial suspensions of Bacillus amyloliquefaciens PP19 and Exiguobacterium acetylicum SI17 were sprayed onto the litchi fruit for pre-treatment at the postharvest in the laboratory. Then, a suspension of Peronophythora litchii(SC18)24 hours after pre-treatment. Subsequently, the disease severity of litchi fruit was observed and the enzyme activity or substance contents corresponding to peel at different time points were detected.

Result The suspensions of PP19 and SI17 significantly reduced the litchi downy blight, and the control efficacy of PP19 was 39.59%-88.76% while that of SI17 was 18.08%-91.01%. Their bacterial suspensions increased the activities of CAT, SOD, GLU, PAL and PPO in peel and the content of anthocyanin while decreased the activity of anthocyanidase and the content of H₂O₂ and total phenol in peel. SI17 increased much more chitinase activity compared with that of PP19.

Conclusion According to the results mentioned above, it is speculated that the effect of two biocontrol suspensions on disease prevention may be related to the change of related enzyme activities or substance contents to disease resistance in peel.