The establishment of perennial grasses as biomass crops has increased the production acreage of giant miscanthus (Miscanthus x gigantues Greef et Deu, MXG). Yield loss and establishment failure could be detrimental to the sustainable production of this crop, and therefore, exploitation of differentiation in cultivar response to fungal diseases could be a key management strategy. A study was initiated in 2010 to evaluate MXG cultivars for foliar disease incidence (FDI) and compare to switchgrass (Panicum virgatum L., SG) cultivars, isolate and identify fungi from symptomatic leaf material, and demonstrate through Koch’s postulates the ability of these fungi to incite symptoms observed in the field. Giant miscanthus FDI ratings were similar between MXG cultivars, but significantly lower when compared to SG cultivars. Thirty genera of fungi were identified from fungal collections, and 16 pathogenic genera were isolated. Twelve isolates were selected and four were demonstrated to be pathogenic on Mxg.