Experiments were carried out to study the effect of six selected plant extracts viz., Calotropis procera, Azadirachta indica leaf and seed kernel, Allium sativum bulb, Withania somnifera leaf and Solanum suratteuse green berry on mycelial growth and spore germination of Alternaria brassicae in vitro. Influence of these extracts used as foliar sprays on host defense related biochemical parameters i.e. soluble phenol, sugar and protein content, peroxidase, polyphenol oxidase and phenylalanine ammonia lyase activities in mustard leaves was observed. The effect of plant extracts on intensity of Alternaria blight and seed yield in mustard was also recorded. The results revealed that all the six plant extracts Calotropis procera, Azadirachta indica leaf extract, A. indica seed kernel, Allium sativum bulb, Withania somnifera leaf and Solanum suratteuse green berry prepared in distilled water, acetone or methanol, significantly inhibited the mycelial growth and spore germination of A. brassicae at 5, 10 and 15 per cent concentrations in potato dextrose agar medium. The aqueous extract of A. indica seed kernel was most inhibitory to mycelial growth and spore germination of A. brassicae.