Two mechanisms of the resistance of citrus fruits against pathogens—preformed and induced antifungal materials—were investigated. Flavedo tissue of lemon contains the following preformed antifungal materials: citral, limettin, 5-geranoxy-7-methoxycoumarin, and isopimpinellin, which act as the first line of defense against pathogens. Exogenous application of citral to Penicillium-inoculated lemon fruit prevented development of decay. Being subjected to fungal challenge and/or abiotic stress, citrus fruits are elicited to produce the phytoalexin scoparone, considered as another line of fruit defense. According to median effective dose (ED50) of the inhibition of germ-tube elongation or percent germination, scoparone had higher fungitoxicity against Penicillium digitatum Sacc. than the preformed antifungal materials. Different citrus species (lemon, orange, grapefruit, lime, kumquat) varied in their capacities to produce scoparone responding to the combined Penicillium inoculation and heat treatment or to UV illumination. UV illumination of lemon fruit reduced its susceptibility to P. digitatum. Expression of this effect was directly related to the level of scoparone in illuminated fruit. UV light and citral application were visibly injurious to the flavedo tissues in high dose.