Induction of Disease Symptoms in Barley by Powdery Mildew

Abstract

Several methods were used to determine if diffusible substances are responsible for any of the symptoms produced by Erysiphe graminis on barley leaves. Phosphate buffer extracts of mildewed and healthy barley leaves were tested for ability to increase respiration of healthy barley tissues. Some extracts were active in these tests, but mildewed and healthy tissues produced extracts of equal activity. Tests for induction of necrosis and chlorosis by diffusible substances were made by bringing mildewed tissues into contact with healthy tissues in leaf pairs. Neither chlorosis nor necrosis was induced consistently regardless of what combinations of resistant and susceptible hosts were used. A search was then made for diffusible substances which might be responsible for the localized retention of green pigment (green island), a characteristic symptom in tissues surrounding mildew colonies on detached susceptible leaves. Excised mildewed tissues (1.1 mm square) did not induce green islands. On the other hand, green islands were induced by water extracts of mildew spores. In these tests, the extract of about 75 mg spores/ml was applied to the surface of detached leaves in single 5 [mu]l drops. The extracts could be boiled, dried, frozen, partitioned against diethyl ether or treated with ion-exchange resins without loss in activity. The extracts also induced localized starch accumulation similar to the starch deposits found around yound mildew colonies. Growing mildew colonies may induce green islands and starch accumulation through the action of the type of substance found in the mildew spore extracts.