Quantification of dollar spot inoculum (Clarireedia spp.) in bentgrass clippings using droplet digital polymerase chain reaction (ddPCR)

Abstract

Dollar spot, caused by the fungal pathogen Clarireedia jacksonii, is one of the most economically damaging and globally prevalent turfgrass diseases. Accurate assessment of cultivar resistance is critical for breeding programs and effective disease management. However, evaluating resistance is challenged by the lack of precise tools to quantify pathogen load under field conditions. In this study, we developed and validated a highly sensitive droplet digital polymerase chain reaction (ddPCR) assay for quantifying C. jacksonii DNA in clipping samples collected from three creeping bentgrass cultivars (007XL, Penncross, Oakley) and one colonial bentgrass cultivar (Musket) grown under natural infection in the National Turfgrass Evaluation Program (NTEP) Fairway trial in Maryland. The ddPCR assay, paired with newly designed primers, reliably detected C. jacksonii DNA at concentrations as low as 1 × 10⁻¹⁴ g without requiring standard curves. A strong positive correlation was observed between C. jacksonii DNA quantity and field-assessed disease severity. Notably, resistant cultivars maintained relatively low pathogen levels and delayed symptom development during early-season epidemics, whereas the susceptible cultivar, Penncross, exhibited higher early-season DNA levels and two distinct epidemic cycles. These findings suggest that resistant cultivars may either suppress early C. jacksonii buildup or tolerate higher pathogen loads before symptom onset. This study demonstrates the utility of ddPCR as a precise, field-applicable tool for monitoring C. jacksonii dynamics, offering critical insights for improving cultivar selection, optimizing fungicide timing, and enhancing sustainable disease management strategies in turfgrass systems.