Although the effect of elevated carbon dioxide (CO2) concentration on pollen production has been established in some plant species, impacts on fungal sporulation and antigen production have not been elucidated. Wolf et al. examined the effects of rising atmospheric CO2 concentrations on the quantity and quality of fungal spores produced on timothy (Phleum pratense) leaves. Timothy plants were grown at four CO2 concentrations, and leaves were used as growth substrate for Alternaria alternata and Cladosporium phlei.