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Chitosan, a given name to a deacetylated form of chitin, is a natural biodegradable compound derived from crustaceous shells such as crabs and shrimps, whose main attributes corresponds to its polycationic nature. Chitosan has been proven to control numerous pre and postharvest diseases on various horticultural commodities. It has been reported that both soil and foliar plant pathogens fungal, bacterial and viral may be controlled by chitosan application. Microscopical observations indicate that chitosan has a direct effect on the morphology of the chitosan-treated microorganism reflecting its fungistatic or fungicidal potential. In addition to its direct microbial activity, other studies strongly suggest that chitosan induces a series of defence reactions correlated with enzymatic activities. Chitosan has been shown to increase the production of glucanohydrolases, phenolic compounds and synthesis of specific phytoalexins with antifungal activity, and also reduces macerating enzymes such as polygalacturonases, pectin metil esterase etc. In addition, chitosan induces structural barriers for example inducing the synthesis of lignin-like material. For some horticultural and ornamental commodities, chitosan increased harvested yield. Due to its ability to form a semipermeable coating, chitosan extends the shelf life of treated fruit and vegetables by minimizing the rate of respiration and reducing water loss. As a nontoxic biodegradable material, as well as an elicitor, chitosan has the potential to become a new class of plant protectant, assisting towards the goal of sustainable agriculture.

The possibility of inducing resistance to bitter rot ( Colletotrichum gloeosporioides ), brown rot ( Monilinia fructicola ), and green mold ( Penicillium digitatum ) in apples, peaches, and tangerines, respectively, by treating them with ultraviolet light-C (UV-C light) at the stem end in a stationary position without rotation was investigated. This approach was compared with the conventional procedure where fruits were rotated four times, thereby exposing the entire surface area to the full effects of the UV-C light. Results revealed that when the stem ends of apples, peaches, and tangerines were exposed in a stationary position to dosages of 7.5, 7.5 and 1.3 kJ m −2 of UV-C light, respectively, induced host resistance to postharvest decay which was equal to, or slightly better than when fruits were rotated four different times. When fruits were rotated, exposing only one or two different sides to UV-C light, the percent infection appeared to increase, compared to treating only the stem ends or when fruits were rotated four times.

Low-dose application of hormetic (adj. of hormetin, the agent of hormesis e.g. UV-C) ultraviolet light-C (UV-C) to ‘Better Boy’ and ‘Floradade’ tomatoes at maturity resulted in fruits that were significantly, firmer than non-irradiated control fruits at the same stage of maturity. As, firmness increased following UV-C treatment, polygalacturonase (PG) activity decreased. The enzyme activity was lower for UV-C treated fruits than the control. The PG activity in crude extract from decayed tomato tissue infected with Rhizopons stolonifer was lower than non-decayed fruits. UV-C treated tomatoes showed a 40% reduction in PG activity in decayed tissue compared to the control at 72 h after treatment. The lesion diameter, and percent infection of non-treated and UV-C treated tomatoes at 72 h after treatment, was 13.4 mm (100% infection), and 5.3 mm (47% infection), respectively.

The objectives of this research were to evaluate the in vitro fungicidal effect of chitosan and aqueous extracts of custard apple leaves, papaya leaves and papaya seeds, and the combination of chitosan and plant extracts on the development of Colletotrichum gloeosporioides, which causes anthracnose on papaya. Chitosan at 2.0% and 3.0% had a fungicidal effect on C. gloeosporioides. Extracts alone did not show any fungicidal effect while the combination of 2.5% chitosan with all the tested extracts had a fungistatic rather than fungicidal effect. Changes in the conidial morphology of C. gloeosporioides were observed with 1.5% chitosan concentration after 7 h incubation. For in situ studies, control of anthracnose disease was obtained with 1.5% chitosan applied before C. gloeosporioides inoculation. Phomopsis was most frequently isolated from the non-inoculated fruit. Chitosan applications did not influence the content of total solid solubles or percentage weight loss during the storage of papaya fruit. However, there was a tendency toward greater firmness in fruit treated with the papaya seed extract alone or combined with chitosan.

Low doses of hormetic (n. hormesis) ultraviolet light-C (UV-C) seed treatments were used to elicit host resistance to black rot, and improve the quality and growth response of cabbages in greenhouse studies. Different UV-C doses (1.3 to 7.5 kJ m−2) were tested to determine their ability to induce resistance to black rot. The optimum UV-C dose of 3.6 kJ m−2 was effective in reducing black rot and the population density of Xanthomonas campestris pv. campestris in infected cabbage leaves. Seeds treated with UV-C at 3.6 kJ m−2 produced plants with the most desirable color, highest weight, largest head diameter and delayed maturity. The effect of storage time at room temperature on disease incidence of black rot of cabbage from seeds treated with a low hormetic UV-C dose of 3.6 kJ m−2, was 90%, 40%, 60% and 60% reduction of black rot in plants from UV-C treated seeds stored for 2 days, 1, 5, and 8 months, respectively, 8 weeks after transplanting cabbage plants.

`Jewel’ sweetpotato storage roots previously treated with a low hormetic dose of ultraviolet light-C (UV-C) were stored for 30 days before inoculated artificially with Fusarium solani . Storage roots showed an increase in resistance to Fusarium root rot, as indicated by a reduction in lesion diameter, depth and weight of rotted tissue following UV-C treatment. The rate of decay development around F. solani inoculum plugs on UV-C treated sweetpotato storage roots progressed slowly, and about 55% failed to develop lesions on wounded UV-C treated, compared to 11% of untreated sweetpotato storage roots, 10 days after inoculation. There was a polynomial curvilinear regression relationship between percent incidence of Fusarium root rot and hormetic UV-C doses. The hormetic dose of UV-C which suppressed decay to the greatest degree was 3.6 kJ m −2 . Exposure of sweetpotato storage roots to doses of UV-C promoted phenylalanine ammonia-lyase (PAL) production with the maximum PAL activity occurred to the greatest degree at a UV-C dose of 3.6 kJ m −2 . Similarly, a relationship between PAL activity and the incidence of Fusarium root rot was established. Crude extracts from UV-C treated sweetpotato storage roots reduced germination, germ tube elongation, and growth of F. solani compared to extracts obtained from untreated storage roots.

The application of ultraviolet light-C doses (254 nm, UV-C) was used to determine the germicidal and hormetic effects on reducing brown rot of ‘Elberta’ peaches which were naturally and artificially inoculated with Monilinia fructicola. The results showed that a negative relationship existed between UV-C doses, colony forming units of the fungus, and the number of brown rot lesions. Also, the results of our study showed that the hormetic (beneficial) effect of low UV-C dose of 7.5 kJ m−2 induced host resistance by controlling latent brown rot infection. The hormetic effects of UV-C on peaches was photoreversed with visible light and resulted in the reduction of host resistance to brown rot. Furthermore, the results indicated that UV-C doses increased phenylalanine ammonia-lyase activity, delayed ripening and suppressed ethylene production. Additionally, the population of epiphytic yeast (Debaryomyces hansenii) increased curvilinearly on peach surfaces following UV-C irradiation up to 7.5 kJ m−2. Evaluation of the yeast population on the peach surfaces showed an antagonistic activity against M. fructicola. The possibility of the host peripheral defense by UV-C stimulation of the antagonist activity of a yeast is discussed.

Low hormetic doses of ultraviolet light (254 nm, UV-C) reduced postharvest decay of pome, stone, and citrus fruits. Loring and Elberta peaches, Golden Delicious apples, Marsh Seedless grapefruits, and Dancy tangerines were irradiated with selected low UV-C doses and stored. The application of UV-C reduced postharvest decay which included: brown rot ( Monilinia fructicola ) of peaches; Alternaria rot ( Alternaria spp.), bitter rot ( Colletotrichum gloeosporioides ) of apples and brown rot ( Monilinia spp.) of apples; green mold rot ( Penicillium digitatum ) of grape fruits and tangerines; and stem end rot ( Alternaria citri ) and sour rot ( Geotrichum candidum ) of tangerines.