Preharvest Chitosan and Postharvest UV Irradiation Treatments Suppress Gray Mold of Table Grapes

Gray mold, caused by Botrytis cinerea Pers. , is the most economically important postharvest disease of table grapes (10). It is responsible for significant losses of table grape berries both before and after harvest, and is a major obstacle to long-distance transport and storage. Control of the disease is especially important in storage because it develops at cold temperatures (–0.5oC) and spreads quickly among berries. Currently in California, gray mold in storage is controlled primarily by sulfur dioxide fumigation (28). However, there is a growing consumer demand for food without chemical preservatives, and its residues can be harmful for people allergic to sulfites. Concerns about residues in grape berries and other foods caused sulfur dioxide to be removed in 1986 from the United States Food and Drug Administration Generally Recognized as Safe (GRAS) compound list (3,50); that shift encouraged research on alternative approaches. Sulfur dioxide applications also can cause phytotoxicity symptoms, including bleaching of the berries and browning of the rachis (35,48). Chitosan, an N-acetylated derivative of the polysaccharide chitin, is a natural polymer with a polycationic nature which has numerous applications in agriculture and agroindustries (34). Chitosan is an ideal preservative coating for fresh fruit and vegetables because it has a diseasesuppressive effect resulting from both physical and biochemical mechanisms (33). The physical properties of the polymer allow it to produce a film on the surface of treated fruit (16,33). Chitosan also inhibits the growth of decay-causing fungi (2,18,40) and induces defense responses in several plant systems, mostly in dicotyledon plants (47). It elicited phytoalexin biosynthesis in pea (23) and induced the production of antifungal hydrolases in groundnut and orange (4,46). The elicitor activity of chitosan seems to arise from the interaction of its polycationic molecule with negatively charged phospholipids of host cell membranes (25). Chitosan treatment prolongs storage life and controls decay of cucumber, carrot, apple, citrus, kiwifruit, peach, pear, strawberry, and sweet cherry (4,6,15,18,19,27,32,37,39,40,53). A commercial formulation based on chitosan (Elexa-4; Glycogenesys Inc., Boston) is available and registered in the United States (Environmental Protection Agency no. 70464-3) for the control of gray mold and powdery mildew of grapes. UV-C (ultraviolet-C) irradiation, with a wavelength of 254 nm, is another method to control postharvest decay of fruit and vegetables. Prestorage treatment of pome fruit, stone fruit, citrus fruit, tomato, mango, and table grapes with low doses of UV-C was reported to reduce postharvest decay (11,20,22,36,49). UV-C irradiation has been shown to induce defense responses in several fruit and vegetable species (20,29,49). Grape berries contain several substances with antifungal activity, such as tannins and phenolic compounds, that are reported to be important for resistance to gray mold (45). Other substances are not preformed, but are produced following infection by the pathogen. Defense reactions by grapevine to B. cinerea infection include synthesis of phytoalexins, mainly stilbenes, such as transand cis-resveratrol, catechin, αand ε-viniferin, and pterostilbene (1,12,26), and the synthesis of pathogenesis-related (PR) proteins, including chitinase, β-1,3 glucanase, phenylalanine ammonia-lyase (PAL), and stilbene synthase (7,13). The property of the plant to produce these compounds and the rapidity of the response are related to the pathogen resistance of the host. Chitosan induced PAL activity in treated berries (41). Nothing has been reported about the influence of chitosan treatment on the induction of catechin, resveratrol, or chitinase synthesis in grape berries; moreover, it is not known if chitosan-treated berries exposed to UV-C irradiation would exhibit an increase in catechin and resveratrol content. The objectives of this study were to investigate (i) effectiveness of vineyard applications of chitosan to control postharvest gray mold and to induce catechin, resveratrol, and chitinase content in table grape berries, and (ii) possible better decay control and enhanced host resistance by irradiation of chitosan-treated berries with UV-C.