The graph below shows the seasonal variations influence the number of spores in the atmosphere. The weather conditions in summer and autumn favor the growth of certain fungal species and the dispersal of spores. Exceptional high concentrations of spores from the species Alternaria and Cladosporium, as well as from species of the phylum Basidiomycota can be found from June to October (modified after Lacey J, 1996).
Source: National Center for Biotechnology Information, U.S. National Library of Medicine – Mold Allergens in Respiratory Allergy: From Structure to Therapy – Allergy Asthma Immunol Res. 2015 May; 7(3): 205–220. Published online 2015 Mar 11. doi: