Barley plants grown under germ-free conditions have increased susceptibility to two powdery mildew fungi

Germ-free barley plants were inoculated with either Erysiphe gramims DC. f.sp. hordei or E. polygoni, powdery mildew fungi of barley and common buckwheat, respectively, and the infection course of the fungus was observed. The purpose was to clarify the effect on the plants' defence reactions of the presence of the microbes which surround the plants in nature. The percentage of penetrations was significantly higher in germ-free plants than it was in those not grown under germ-free conditions in both fungus-plant combinations. In the case of leaves inoculated with E. graminis f.sp. hordei, which was studied in more detail, the increase in the percentage of penetrations appeared to be due to a decrease in the percentage of conidia which (a) stopped development at or prior to appressorium formation, and (b) failed to penetrate because of papilla formation in host plants. These results suggest that, in plants grown in nature, general or non-host resistance—involving morphological, physiological and biochemical changes—may be induced or activated by the interaction of plants with surface microbes surrounding them.