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Mycelial growth of Rhizoctonia solani and Phytophora infestans was totally inhibited in vitro conditions by hydrosoluble methanolic extracts from L. tridentata leaves collected from the Sonoran and Chihuahuan deserts of Northern Mexico. An observed, antifungal effect in the bioassay was very clear even at a low dose of 500 ppm evaluated against P. infestans and 2,000 on R. solani; the growth of both fungi was inhibited in the medium. Significant differences among the extracts from both deserts were found on the inhibition of the pathogens, the extracts from the Sonoran Desert being the most effective. A latitudinal effect on mycelial growth was observed: the extracts collected at southern latitudes of the Chihuahuan Desert were more effective against both pathogens than the extracts from north latitudes. An inverse effect was observed with the extracts from the Sonoran Desert: those from northen latitudes had a greater effect against growth of both pathogens than those from southern latitudes. Practically, extracts from the eight sampling sites of both deserts at 4,000 ppm did not allow mycelial growth.