Fruit and vegetables are highly perishable and postharvest pathogens are one of the major causes in the early termination of their shelf life. Although synthetic fungicide are effective against postharvest pathogens, they face imminent problems. One alternate approach is the induction of host resistance against postharvest diseases. We tested the efficacy of chitosan, Milsana®, and Bion® on carrot, tomato, potato, green pepper, and strawberries against postharvest fungal and bacterial rots. Chitosan was not only antimicrobial but also induced resistance against postharvest infections of Alternaria, Botrytis, and Erwinia in tomato, green pepper, carrot, and potato. Preharvest sprays of chitosan induced resistance against postharvest infection of Botrytis cinerea in strawberries and improved the storage quality. Ultrastructure and cytochemical investigations showed that chitosan not only induced several morphological alterations in B. cinerea and Alternaria alternata inhibiting the production of pathogenic factors by the fungi, but also stimulated various host defense reactions, such as papilla formation and induction of defense enzymes, phytoalexins, and phenols. Application of Milsana®, a plant extract, and Bion®, an activator of host defenses, protected carrots from postharvest decay by B. cinerea. Milsana® was effective at 4 °C, while Bion® was effective at both 4 and 13 °C storage temperatures. Thus, there is great potential for defense inducers in postharvest disease control of fruit and vegetables.