THE EFFECTS OF HOST CARBON-DIOXIDE, NITROGEN AND WATER-SUPPLY ON THE INFECTION OF WHEAT BY POWDERY MILDEW AND APHIDS

In two experiments, winter wheat (Triticum aestivum cv. Cerco) was gro wn in 350 (ambient) and 700 mumol mol-1 (elevated) atmospheric CO2 con centrations. In the first experiment, plants were grown at five levels of nitrogen fertilization, and in the second experiment, plants were grown at three levels of water supply. All plants were infected with p owdery mildew, caused by the fungus Erysiphe graminis. Plants grown in elevated atmospheric CO2 concentrations had significantly reduced % s hoot nitrogen contents and significantly increased % shoot water conte nts. At elevated atmospheric CO2 concentrations, where plant nitrogen content was significantly reduced, the severity of mildew infection wa s significantly reduced, and where host water content was significantl y increased, the severity of mildew infection was significantly increa sed. In a moderate water supply treatment, the plants grown in elevate d atmospheric CO2 concentrations had significantly reduced nitrogen co ntents (9.9%) and significantly increased water content (4%), the amou nt of mildew infection was unchanged. The severity of mildew infection appeared to be more sensitive to host water content than to host nitr ogen content.