| Score | Visual observation | Response |
| 0 | No infection | Resistant |
| 1 | 0.1% - 7.0% | Tolerant |
| 2 | 7.1% - 15.0% | Moderately susceptible |
| 3 | 15.1% - 25.0% | Susceptible |
| 4 | 25.1% - 100.0% | Highly susceptible |