The Interaction of Humidity and Resistance Elicitors on Expression of Polygenic Resistance of Barley to Mildew

Elicitor-active glucan was applied to leaves of genotypes of barley expressing different levels of resistance to powdery mildew ( Erysiphe graminis f. sp. hordei )which was assessed by measuring mildew colonies per leaf or speed of papilla induction response to mildew challenge. The influence of humidity was also assessed by treating leaves with glucan, inoculating with mildew and immediately sealing the leaves in Perspex containers in an apparatus which enabled temperature, air speed and humidity to be controlled accurately. In water-treated controls low humidity alone reduced colonies per leaf, and at 60% r.h. genotype 7204 showed significantly less effect of humidity compared with other genotypes, although at 40% it responded as other genotypes. At low humidity glucan stimulated resistance for genotypes Proctor and 7204 but not for Golden Promise or 9855; the effect of humidity or elicitor alone was only significant on genotype 7204, Chitin also proved an effective elicitor, but at high and not at low humidity. Time of pre-treatment before mildew challenge, leaf stage, type of elicitor, concentration of elicitor, post-inoculation light regime, mildew isolate and barley genotype all had major influences on papilla response. Other factors were less important. Based on papilla formation, the barley cultivar Proctor was most and Golden Promise the least responsive to glucan as an elicitor. The potential for exploiting the genetic basis of inducible resistance is discussed.