This is a method used to quantitatively determine the degree of infection caused by a pathogen. Typically, a sample of the pathogen, such as a suspension of bacteria or viruses, is inoculated onto leaves, and the number of lesions that develop is measured. The experimental unit used is typically a half-leaf, as the leaves of an opposite pair differ less from each other than from leaves on different nodes. This is why this method is also referred to as the “half-leaf test” or “opposite leaf test”.