Sclerotinia sclerotiorum causes yield losses in a broad range of plant hosts, including carrot, for which a disease risk forecasting model has been previously developed. Detecting and quantifying airborne ascospores as a component of a sclerotinia rot of carrot (SRC) forecast model is currently accomplished using a Sclerotinia semi-selective medium (SSM) in the blue plate test (BPT) which has a threshold of 5 ascospores per plate. To improve the efficacy of applying the SRC forecast model, data from nine years of air sampling were analysed to determine the number of sampling sites needed to accurately estimate the concentration of airborne ascospores in a region (c. 40 km2) of intense carrot production. In six years, ascospore counts were correlated between sites in 20–100% of the possible pairwise comparisons among air sampling sites. The most consistent relationships among the sites were observed during periods of no detection or detection below the BPT threshold. In most years, periods when ascospore counts surpassed the BPT threshold were consistent among sites, but there were some fields and years when these periods were inconsistent. Based on the observations, one air sampling site should be sufficient to detect ascospores when counts are low, increasing to two or more sites during periods when ascospores are detected near or above threshold levels, and cropping and environmental conditions are conducive to disease.