A microscopic study of powdery mildew on barley after application of the systemic compound wepsyn

5-Amino, 3-phenyl, 1-bis (dimethylamido) phosphoryltriazole 1, 2, 4 (wepsyn) was applied to the roots of barley seedlings, var. ‘Balder’ in concentrations of 10, 25, 62 and 156 ppm, and the infection process ofErysiphe graminis f.sp.hordei on these plants studied microscopically. During the first few hours after inoculation a retardation of the germination occurred at the two highest concentrations, thus before the fungus had established contact with the host protoplasm. Penetration and formation of a lignituber was retarded at all wepsyn concentrations. At 10 and 25 ppm the formation of primary haustoria was reduced about thirty fold in relation to that on control plants. Mycelium that developed was often stunted. At higher wepsyn concentrations almost no haustoria were formed. Some leaf tip necrosis occurred, especially at the higher concentrations of wepsyn.