The pathogenicity of a rust fungus (Puccinia philippinensis Syd.), on purple nutsedges (Cyperus rotundus L.). A research aimed at understanding the pathogenicity of a rust fungus (Puccinia philippinensis Syd.), a potential biological control agent of purple nutsedges (Cyperus rotundus L.) had been conducted in a glasshouse of Faculty of Agriculture the University of Mataram. These factorial experiments designed according to Randomized Completely Design (CRD) and consisted of spore density and time of application. Spore density treatments consisted of 0, 5000, 10 000, 15 000, 20 000, 25 000, or 30 000 spore/mL applied either in the morning or in the afternoon. The results showed that the higher the spore density, the higher the number of pustules formed, the higher the disese intensity, and the faster the disease progressed. Urediniospores applied in the afternoon produced more pustules, higher disease intensity, and faster disease progression compare to those applied in the morning.