Various subspecies, botanical varieties and cultivars of peanuts (Arachis hypogaea L.) from widely separated areas of South America and Africa were evaluated for resistance to C. personatum and C. arachidicola. All cultivated peanuts tested were highly susceptible to both fungi except one collection from Peru, which showed a high degree of tolerance to defoliation.

Sources of resistance were found in the wild Arachis species. Several immune and many highly resistant collections of Arachis were found in the sections Erectoides, Rhizomatosae, and Extranervosae. In section Axonomorphae, A. chacoense (10602 GKP) was highly resistant to C. arachidicola but susceptible to C. personatum, and C. cardenasii (10017 GKP) was susceptible to C. arachidicola but immune to C. personatum. Both Arachis species are cross-compatible with A. hypogaea.

Host response in terms of pathogen penetration was classified into: immune, moderately and highly susceptible; and the reactions after penetration were: highly resistant, moderately and highly susceptible. On highly susceptible peanuts, the germ-tubes showed directed growth toward open stomata through which these fungi penetrate. On moderately susceptible peanuts a few germ-tubes grew toward the stomata, and on the immune entries no directed growth of the germ-tubes was observed. Resistance after penetration was associated with the formation of a barrier in advance of and around the infection site in the form of cell wall swelling and thickening, and the deposition of pectic substances on the cell walls and in intercellular spaces.