The proportion of plant population affected by a disease is known as

The proportion of the plant population affected by a disease is known as Disease Intensity.
Disease intensity is a general term used to denote amount of disease, commonly expressed as either disease incidence (proportion of the total number of plants that are infected) or disease severity (proportion of total plant area that is infected).
The disease severity is estimated by a rater as a value on the interval scale and has been used to determine a disease severity index (DSI) on a percentage basis, where DSI (%) = [sum (class frequency × score of rating class)] / [(total number of plants) × (maximal disease index)] × 100.