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MECHANISMS OF ANTIFUNGAL ACTION OF CHITOSAN IN POST-HARVEST TOMATO ALTERNARIA INTERACTION INHIBITING THE PROGRESS OF BLACKMOLD
MV BHASKARA REDDY, J ARUL, P ANGERS, and F CASTAIGNE

Department of Food Science and Nutrition and Horticultural Research Center, Lava University, Ste-Foy, Quebec, Canada, GI K 7P4


Background and objectives
Alternana alternata is a causal agent of blackmold of tomato fruits. Pre and post-harvest treatments are needed to prevent the development of Alternaria rot of tomato. No fungicides have been approved for post-harvest treatment of tomatoes to inhibit the development of Alternaria rot due to residual toxicity and development of resistance in pathogens. Chitosan the deacetylated chitin ~ linked glucosamine polymer) has been shown to be anti fungal against wide range of fungi [1]. Chitosan stimulated chitinase activities in strawberries when applied directly to excised tissue, and structural defense barriers when applied to stem scars of bell pepper fruit. Chitosan coating prolonged the storage life of tomatoes and strawberries, and controlled decay of starawberries. Our previous work showed that chitosan inhibits toxin production by A. Alternata [2] and macerating enzyme production by Erwinia in addition to eliciting phytoalexin production. The objective of this investigation was to study the effect of chitosan on tomato blackmold control and its interference with the virulence factors of the fungus such as cell wall degrading enzymes (CWDE's; polygalacturonase, pectatelyase and pectin methylesterase), organic acids (OA's; oxalic and fumaric acids), host specific toxins (HSt's; alternariol and alternariol monomethylether) involved in the pathogenesis process, as well as elicition of phytoalexin (rishitin)production in tomatoes.


Materials and methods
Chitosan 400 p1, lOmg mI-1) was applied at the stem scars of surface sterilized mature- green tomatoes and inoculated with 50 p1 of spore supension (2.5x105 mI-1) at the vicinity of stem scar. The fruits were incubated at 200C and 95% RH for 28 days. The fruits were assayed for fungal growth, lesion diameter and production of CWDE's, OA's, HSt's and phytoalexin. The activities of CWDE's assayed by spectrophotometer and, OA's, HSt's and phytoalexin by gas chromatography. For each treatment 20 fruits were arranged in a randomized complete block design and the experiment was repeated twice. Pooled data were analyzed by analysis of variance.

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Results and conclusions
Significant reduction in fungal growth and lesion size was recorded at chitosan treatments compared to the control. The lesion was less than 50% on the fruit surface with chitosan treatment after 4 weeks of storage compared to total rot in control within 3 weeks. Correspondingly the activities of CWDE and OA, HSt contents were reduced to >50% and rishitin levels were higher with chitosan treatments . The quantitative assay for rishitin revealed almost 50% increase after 14 days of storage in chitosan treated tomatoes-contributing to tissue resistance. Chitosan in addition to its direct anti fungal activity, also inhibited the production of virulence factors by the fungus while eliciting phytoalexin production in the host. Thus chitosan has a great potential for the control of post harvest pathogens of fruits and vegetables.


References
1.EI Ghaouth A, Arul J, Asselin A, Benhamou N. 1992. Mycological Research 96, 769-779. 2. Bhaskara Reddy MV, Arul J, Ait-Barka E, Angers P, Richard C, Castaigne F. 1997. Biocontrol Science and Technology (In press)