Weed–pathogen interactions and elevated CO 2 : growth changes in favour of the biological control agent

Summary In this study, we used P arthenium hysterophorus and one of its biological control agents, the winter rust ( P uccinia abrupta var. partheniicola ) as a model system to investigate how the weed may respond to infection under a climate change scenario involving an elevated atmospheric CO 2 (550  μ mol mol −1 ) concentration. Under such a scenario, P. hysterophorus plants grew significantly taller (52%) and produced more biomass (55%) than under the ambient atmospheric CO 2 concentration (380  μ mol mol −1 ). Following winter rust infection, biomass production was reduced by 17% under the ambient and by 30% under the elevated atmospheric CO 2 concentration. The production of branches and leaf area was significantly increased by 62% and 120%, under the elevated as compared with ambient CO 2 concentration, but unaffected by rust infection under either condition. The photosynthesis and water use efficiency ( WUE ) of P . hysterophorus plants were increased by 94% and 400%, under the elevated as compared with the ambient atmospheric CO 2 concentration. However, in the rust‐infected plants, the photosynthesis and WUE decreased by 18% and 28%, respectively, under the elevated CO 2 and were unaffected by the ambient atmospheric CO 2 concentration. The results suggest that although P . hysterophorus will benefit from a future climate involving an elevation of the atmospheric CO 2 concentration, it is also likely that the winter rust will perform more effectively as a biological control agent under these same conditions.