A set of standard area diagrams to assess white mold severity on the leaflets of common beans

The fungus Sclerotinia sclerotiorum is the major pathogen affecting common beans yield worldwide and an adequate disease quantification is demanded in some trials. However, a set of standard area diagrams (SADs) to aid visual assessment of severity of white mold (SWM) is lacking. This study developed SADs consisting of eight color images of diseased leaflets with severity values that ranged from 0.4 to 53.7%. Twenty raters [10 experienced (ER) and 10 inexperienced (IR)] validated the SADs by assessing the same set of 50 images twice, the first without SADs and the second using it as an aid. The SADs significantly improved both accuracy and precision for IR as evidenced by increases from 0.86 to 0.98 in the coefficients of bias (Cb) from 0.93 to 0.98 in correlation coefficient (r) and from 0.86 to 0.96 in overall agreement [Lin's concordance correlation coefficients (ρc)] without and with SADs, respectively, whereas for ER only precision (r) was improved by SADs. The SWM estimates were also more reliable because inter-rater reliability (coefficient of determination, R2) was significantly increased for both ER and IR by using SADs. Therefore, the SADs presented is thought to be a valuable tool to provide accurate, precise and reliable estimates of the SWM on common bean leaflets in epidemiological studies, evaluation of disease control methods, assessment of aggressiveness of pathogen isolates, disease resistance and other types of surveys regarding the common beans-S. sclerotiorum interaction.