The objective of this study was to develop a reliable and high throughput screening method to evaluate the response of St. Augustine grass (Stenotaphrum secundatum) genotypes to the grey leaf spot (GLS) caused by Magnaporthe oryzae infection. Whole plant, detached stolon and detached leaf assays under growth chamber conditions were compared to field conditions on eight commercial and nine advanced breeding lines of St. Augustine grass. Disease was assessed using two variables, lesion size (LS) and overall plant disease severity (SEV). LS and SEV were highly correlated for field and growth chamber screening methods using the whole plant assay (LS r2 = 0·79; SEV r2 = 0·83; P ≤ 0·001), the detached stolon assay (LS r2 = 0·75; SEV r2 = 0·72; P ≤ 0·001), and the detached leaf assay (LS r2 = 0·46; SEV r2 = 0·60; P ≤ 0·001). Genotypic variation for resistance in 17 St. Augustine grass genotypes was identified using all screening methods for LS (P < 0·05) and SEV (P < 0·05). The rank-sum method was used to classify St. Augustine grass genotypes into highly resistant (HR), resistant (R), moderately resistant (MR), moderately susceptible (MS), susceptible (S) and highly susceptible (HS) classes based on the rank-sum values of LS and SEV. Two introduced African polyploids used as parents, and two F1 interploid progeny obtained using an in vitro embryo rescue technique, were classified as highly resistant (HR), or resistant (R), across all screening methods.