The extract of a medicinal plant Daphne gnidium , was tested for its in vitro and in vivo antifungal activity against Pyrenophora teres Drechs. f. teres the fungi causing Net blotch of barley .The extract obtained by plant decoction is tested in vitro on the growth diameter of five Moroccan isolates of Pyrenophora teres Drechs. f. teres. Twelve concentrations were tested: 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 55 and 60 g /L . The concentration of 40g/L of D.gnidium extract induced complete inhibition of 4 pathogen isolates in vitro conditions . The isolates inhibited by the medicinal plant extract and transferred in PDA media only have not revived. Therefore the effect of D.gnidium on Pyrenophora teres f. Sp. Teres is fungicidal. The most active concentration of the extract in vitro studies was tested, afterwards, in vivo against Net blotch on barley leaves . Incidence of Net blotch was decreased to a rate of 0 on the scale of Tekauz , while control barley plants showed a rate up to 9 on the severity scale of the disease . This study demonstrated that plants extracts have a high potential to control Net blotch of barley. Therefore such natural products represent a sustainable alternative to the use of chemical fungicides.