Improving accuracy, precision and reliability of severity estimates of Glomerella leaf spot on apple leaves using a new standard area diagram set

Glomerella leaf spot (GLS) causes severe necrotic leaf symptoms on Gala, the most cultivated apple cultivar in Brazil. To evaluate this disease, the standard area diagram set (SADs) developed on the state of Paraná for a leaf severity level up to 30.93% has been used. However, in samples collected in the states of Santa Catarina and Rio Grande do Sul, disease symptoms reach 75% of severity. Therefore, the objective of this study was to develop and to validate new SADs that include higher GLS severity levels. The proposed SADs have 10 color images of diseased leaves with severity values ranging from 0.2 to 75%. These SADs were validated by 12 raters and have significantly improved the disease assessment. Therefore, it is believed that SADs developed in the present study will allow an adequate evaluation in regions or orchards where the disease is more severe.