Table 3 Best practices for maximizing accuracy of visual estimates of severity of plant disease

1. Training in the process of assessing and estimating plant disease severity.

2. Detailed instruction or understanding of the specific symptoms and what constitutes healthy and diseased tissue.

3. Use of SADs

 • Ensure the  SAD set is developed and validated appropriately (Del Ponte et al. 2017)

 • Instruction on how to use SADs (generally to interpolate to the nearest percentage point, occasionally as an ordinal scale).

4. Emphasize the risks of estimates falling on preferred, characterized values. Endeavor to estimate to the nearest percent, which is not necessarily at a 5% or 10% step value on the 100% point scale.

5. Appropriate sample sizes – usually 30 (Chiang et al. 2016b), but less or more depending on the errors of the estimates which can vary with actual severity and disease distribution.