Standard area diagrams are important for quantification of disease severity to be used in epidemiological studies, screening for partial resistance and assessment of disease management schemes. Results demonstrated that SADs improve accuracy, precision and reliability of the tomato powdery mildew data collection and will help minimize the risk of erroneous decisions or Type II errors in statistical analysis. SADs have been demonstrated to improve the accuracy and reliability of assessment of diseases in different crops including corn white spot, potato early blight, wheat blast, pecan scab and pepper anthracnose.