In vitro plants and callus culture of two Vitis vinifera cultivars (cv. Baga and Maria Gomes) and one rootstock (R3309, i.e. Vitis riparia var tomentosa x Vitis rupestris) were inoculated with conidia of Phaeoacremonium angustius and Phaeomoniella chlamydospora. Response to infection was determined in plants grown in vitro by assaying growth rates. malondialdehyde (MDA) production (lipid peroxidation) and chlorophyll content and fluorescence. Growth rate and malondialdehyde production were also used to determine resistance of calluses to infection. Infection reduced growth and increased MDA in infected plants and calluses, and reduced chlorophyll content and fluorescence in infected leaves. Symptoms were more evident in plants infected with P angustius. showing that this species is more virulent to plants and calluses than Ph. chlamydospora. Differences in virulence among strains of Ph. chlamydospora were also found. as 1AS and CAP053 were more virulent (induced more severe decreases of growth and chlorophyll fluorescence. together with higher MDA production in both cultivars) then CAP080. Growth of rootstock plants and calluses was less affected by infection than growth of other cultivars. Contrarily to Baga and Maria Gomes, chlorophyll content and fluorescence of rootstock plants were only affected by P. angustius. Also Baga plants and calluses were more resistant than those of Maria Gomes. These data show different degrees of resistance among genotypes. Reduction of callus production by infection supports the idea that fungus infection may reduce cicatrisation by inhibiting callus formation during grafting Or wounding; and therefore, contribute to the entrance of opportunist pathogens. Implications of using in vitro cultures to assay host/pathogen relationship and virulence/resistance degrees among the different genotypes of fungus and grapevines are discussed. (C) 2004 Published by Elsevier B.V.