Plants are equipped with a wide array of constitutive and inducible defenses. Recognition of the pathogen results in a series of localized responses, the main event being the rapid death of cells in the area of infection, known as the hypersensitive response, the accumulation of toxic metabolites, and phytoalexins in the necrotic area. Preformed compounds are only part of what are generally considered as preformed barriers, which may also include the presence of thickened cuticle and high molecular weight proteins with antimicrobial activity. The chapter assesses the importance of some examples in which the modulation of preformed barriers could be used as a tool for disease control. Citrus fruits are resistant to wound pathogens during fruit growth, even though a considerable amount of inoculum of Penicillium is present and considerable wounding occurs naturally in the orchard. The inactivation of pectic enzymes by inhibitors has been hypothesized to be a means of modulating fungal pathogenicity.