Severity	Description
0.001%	1 lesion per quadrat
0.01%	2-5 leaves per 10 plants affected. About 5 large lesions per quadrat
0.1%	About 5-10 infected leaflets / plant; OR about 2 affected leaves / plant
1.0%	General light infection. About 20 lesions / plant OR 10 leaves affected / plant; 1 in 20 leaves affected severely
5.0%	About 100 lesions / plant; 1 in 10 leaflets affected (up to 50 leaves affected)
25%	Nearly every leaflet infected but plants retain normal form; plants may smell of blight. Field looks green although every plant is affected
50%	Every plant is affected and about 50% of the leaf area is destroyed. Field appears green flecked with brown
75%	About 75% of the leaf area destroyed; field appears neither predominantly green nor brown
95%	Only a few leaves on plants, but stems are green
100%	All leaves dead, stems dead or dying

Adapted from: James, C. 1971. A manual of assessment keys for plant diseases. Canada Department of Agriculture. Publication No. 1458.