Simulated elevated atmospheric CO2 and temperature affect the severity of bean and pelargonium rust

Experimental trials were carried out on bean (Phaseolus vulgaris L.)/Uromyces appendiculatus F. Strauss and pelargonium [Pelargonium zonale (L.) L'H,r.ex Aiton]/ Puccinia pelargonii-zonalis Doidge pathosystems, under phytotron conditions, to evaluate the effects of simulated elevated atmospheric CO2 concentrations, ranging from 800 to 850 ppm, compared with standard CO2, ranging from 400 to 450 ppm, and temperatures, ranging from 14 to 18, 18 to 22, 22 to 26 and 26 to 30 A degrees C. A total of eight CO2 and temperature combinations were tested to establish their effects on the development of bean and pelargonium rusts. A doubled concentration of CO2, at the same temperature regime, was found to lead to an increase in U. appendiculatus disease severity on the beans. The combined CO2 and temperature factors significantly influenced the severity (p = 0.049) of the rust caused by U. appendiculatus. Moreover, Puccinia pelargonii-zonalis was observed to be more severe on pelargonium at the lowest tested temperatures, that is, between 14 and 22 A degrees C. A high CO2 regime was shown to significantly increase disease severity at such temperatures. At the highest tested temperatures, that is, between 26 and 30 A degrees C, which are generally not favourable for rust development, the increase in CO2 had no significant effect on disease severity.