Quantifying <i>Clarireedia</i> spp. in creeping bentgrass reveals pathogen concentration increases at a slower rate in tolerant cultivars

Dollar spot, caused by fungi in the Clarireedia genus, is among the most economically important turfgrass diseases. One strategy for controlling dollar spot is the use of tolerant cultivars which typically take longer to develop symptoms and have smaller lesion centers compared to more susceptible cultivars. We previously developed a qPCR assay that can quantify the Clarireedia concentration in asymptomatic and symptomatic turfgrass. The goal of this three-year study was to quantify the concentration of Clarireedia in a tolerant and susceptible cultivar of creeping bentgrass (Agrostis stolonifera) throughout the growing season to determine the effect host tolerance has on dollar spot development in the field. Turfgrass samples were collected weekly in 2019, 2020, and 2021 starting the first week of May and ending the second week of August for a duration of 15 weeks each year. The qPCR assay identified significant differences in Clarireedia concentration between the cultivars in both asymptomatic and symptomatic tissue. Linear model analysis showed the tolerant cultivar 'Declaration' always had a lower Clarireedia concentration compared to the susceptible cultivar 'Independence'. This supported our inhibition hypothesis that, although the concentration of Clarireedia required to cause symptoms was similar for both cultivars, the pathogen increased at a slower rate and therefore took longer to develop in a tolerant cultivar. The ability of the qPCR assay to quantify the concentration of Clarireedia in both asymptomatic and symptomatic tissue in the field during the growing season provides a tool for more in-depth epidemiological studies of dollar spot disease in turfgrass.