Effect Of Chitosan And Sodium Silicate On Control Of Dry Rot Of Potato Tuber And Possible Mechanisms Involved

Potato dry rot caused by Fusarium sulphureum is one of the most important postharvest diseases restricting potato production and incidence of disease is up to 30% during storage. Synthetic fungicides treatment is primary and effective method for controlling potato dry rot, but there is growing concern over the indiscriminate use of synthetic fungicides on crops because of the possible harmful effects on human health and the emergence of pathogen resistance to fungicides. So there is a worldwide trend to explore new alternatives in order to replace or reduce the use of synthetic fungicides. Mechanism of chitosan and silicate, known as alternatives to, has become hot subject for phytopathologist, but few study were reported about application of chitosan and silicate on postharvest disease control and mechanism. In this paper, fungistatic characteristic and induced disease resistance of chitosan and sodium silicate on dry rot of potato tuber (cvs. Atlantic) were studied by the techniques of pathology, biochcmistry and histopathology. The results showed:1. Effect of different concentration chitosan treatment on disease development of potato tuber wound inoculated with Fusarium sulphureum was studied, the result showed 0.5% and 1% chitosan dissolved in lactic acid could effectively control dry rot of potato tuber, but 1% chitosan caused phytotoxicity. In vitro tests, mycelium growth, dry weight of mycelium and spore germination of Fusarium sulphureum were inhibited by chitosan. Scanning electron microscopy observations showed that chitosan induced morphological change, including intertwist, distortion and swelling, inflated mycelium became blasted, wizened and cupped. Structural alteration of hyphae were also found under transmission electron microscope, these changes included abnormally distribution cytoplasma, non-membraneous inclusion bodies assembling in cytoplasma, considerable thickening of the hyphal cell walls, very frequent septation with malformed septa of 0.5% chitoan treated hyphae. Higher concentration of chitosan cause fungal hyphae cell to be seriously damaged, including cell membrane disorganization, cell wall disruption and outbreaking of inner cytoplast. New hyphae (daughter hyphae) inside of the collapsed hyphal cells were often detected in the cytoplasm of chitosan-treated hyphae, the results showed that chitosan treatment could change form and structure of Fusarium sulphureum hyphae.2. Dry rot of potato tuber wound inoculated with Fusarium sulphureum was significantly reduced by sodium silicate dipping, control effect of 100 and 200mM sodium silicate treatment was better, but higher concentration failed to get better effect. Results of in vitro test showed that sodium silicate could effectively inhibit mycelium growth, dry weight of mycelium and spore germination of Fusarium sulphureum. Morphological changes of sodium silicate treated hyphae were observed by Scanning electron microscope, these changes included mycelium sparsity and asymmetry, hyphae swelling, curling and cupped. Transmission electron microscopy observation showed structural alterations of treated hyphae were found including thickening of the hyphal cell walls, cell distortion, cavity or electron-material in hyphal cell. New hyphae (daughter hyphae) inside of the collapsed hyphal cells were often detected in the cytoplasm of sodium silicate treated hyphae.3. Induction of disease resistance to dry rot in potato tuber and its slices by chitosan were studied; the result showed that lesion expansion of potato tuber challenge inoculated 3 days after chitoan treatment and infection ability of Fusarium sulphureum inoculated on potato tuber slices 48 and 72 hours after 0.25% chitosan treatment were significantly reduced. In addition, treatment of chitosan at 0.25% increased activity of peroxidase (POD), polyphenoloxidase (PPO) and the production of H₂O₂ and O₂^- in potato tuber tissue, and decreased activity of superoxide dismutase (SOD); activity ofβ-1,3-glucanase(GLU), phenylalanine ammonia-lyase (PAL) and content of total phenolic, flavanoid increased after chitosan treatment, but there were not significant difference between treatment and control. The challenge with Fusariiim sulphureum enhanced furthermore activity of POD, PAL and PPO in the chitosan treated, more accumulation was also found in total phenolic, flavanoid and H₂O₂ and O₂^- in potato tuber tissue.4. Sodium silicate could induce potato tuber and its slice resistance to dry rot. Sodium silicate at 100 mM significantly reduced lesion expansion of potato tuber challenge inoculated 3 days after treatment and infection ability of Fusarium sulphureum inoculated on potato tuber slices 48 hours after treatment. Further study showed that activity of POD,PPO,PAL and GLU and the production of total phenolic, flavanoid of potato tuber tissue after 100 mM were not higher than that of control, even lower during early treatment period, although the value of them gradually increased with treatment time prolonging. After challenge inoculation with Fusarium sulphureum, activity of POD,PPO,PAL and GLU and the production of total phenolic, flavanoid dramtically increased and their increasing ratio were significantly higher than those of control, this result suggested that sodium silicate could enhanced defence response of potato tuber against pathogen infection.In conclusion, the present study shows that chitosan and sodium silicate could directly inhibit the growth of Fusarium sulphureum in vitro and potently induce defense reactions in potato tuber. This suggests chitosan and sodium are promising as fungicide to reduce potato tuber disease and to partially substitute for the utilization of synthetic fungicides in fruit and vegetables.