| Disease Severity Scale | Description of Scale |
| 0 | No disease observed. |
| 0.1% | First sporulating lesion in the plot. |
| 1.0% | General light infection. About 5 - 10 lesions/plant. |
| 5.0% | About 50 lesions/plant; 1 in 10 leaflets 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. |