Suppression of resistance against penetration by Erysiphe graminis f.sp. hordei conferred by the mlo5 barley powdery mildew resistance gene was accomplished by using the glucose analogue, 2-deoxy- D-glucose (DDG) or mannose applied to true leaf tissue of the mlo5 containing Riso-R/barley isoline. Additional suppression of mlo5 penetration resistance against the avirulent E. graminis isolate was achieved by using DDG, mannose, or glucose in combination with the phenylalanine ammonia lyase inhibitor alpha-aminooxy-beta-phenylpropionic acid (AOPP). A mlo virulent isolate of E. graminis was also tested against Riso-R and the isoline Riso-S. The penetration efficiency of the mlo virulent isolate against both isolines was also enhanced by treatment of leaf tissues with DDG or mannose alone or in combination with AOPP. It was concluded that Riso-R penetration resistance utilizes a backup defence involving phenolic compound synthesis, but that phenolics are not responsible for the primary mechanism of mlo5 penetration resistance. Sequestration of phosphate ions caused by complexing with DDG or mannose may lower the energy available for penetration resistance in these barley lines, obviating both inherent and mlo5 based penetration resistance. Barley epidermal cell papillae were associated with penetration resistance in both barley isolines, and all papillae contained callose. However, papillae were significantly smaller and less frequent when tissues were treated with DDG or mannose. The potential inhibitory effects of glucose, DDG and mannose on activity of the enzyme callose synthase in vitro were tested and proved negative. (C) 1997 Academic Press Limited.