Todd's Mitcham peppermint is grown in north-eastern Victoria for essential oil production. Peppermint rust, caused by Puccinia menthae, reduces oil yields by up to 50%. Disease progress was monitored over 4 seasons (1994–98) on untreated peppermint and peppermint treated each season with propiconazole. The untreated peppermint died out within 4 years. A relationship between disease incidence and disease severity was found that could be described by the equation Y = 0.26 x 10 0.02X where Y is disease severity (%) and X is disease incidence (%), with a linear phase at levels of disease incidence <60%. The relationship was consistent for data from different seasons, two separate geographic regions of Victoria, and for pot- and field-grown peppermint. The use of the incidence–severity relationship to develop an action threshold for the initiation of fungicide treatments is discussed.