Three citrus species (Citrus sinensis Cadenera, Citrus limon Eureka and Citrus bergamia Castagnaro) are involved in the biological control as an alternative to the use of chemical fungicides (metalaxyl) against aggressive Algerian strains of Phytophthora infestans, which are responsible for potato late blight. The fungal extracts from two potato-producing areas (El Abbadia, Ain Defla and Bourkika, Tipaza), which were respectively purified and identified as A(1) and A(2), were studied by in vitro and in vivo tests. The citrus essential oils varied in their in vitro inhibitory effects on sporulation, mycelial growth and survivability of P.infestans isolates. Meanwhile, the in vivo infection rate of detached potato leaves in soaking treatment was found to be significant and confirmed the fungicidal effect of citrus essential oils, which were in good consistency with in vitro results. Bergamot essential oil performed the best in both in vitro and in vivo tests, followed by the orange and lemon essential oils.