Development and validation of a set of standard area diagrams to assess severity of gray mold in strawberry fruit

Gray mold (Botrytis cinerea) is one of the main diseases that affect strawberries. Severity assessments of the disease in fruits have been mainly performed without any aid. Although there has been a rating scale already published, there are no standard area diagrams (SADs) that could be used. Therefore, our objective was to develop and validate a set of SADs to assess severity of gray mold in strawberry fruit. The set of SADs has seven levels of disease severity (0, 8, 28, 46, 77, 82 and 100%) and they were validated by five experienced raters and five inexperienced raters in plant disease assessment. Photos of 50 strawberry fruits were provided to raters for gray mold severity assessment, which was performed in three stages: i) without any aid; ii) with a rating scale previously published; and iii) with the SADs. Lin's concordance correlation coefficient (LCCC) of estimated and actual severity was improved with the use of SADs (0.94) compared to the rating scale (0.74) and assessment without aid (-0.07). Peer comparison of all raters showed that 87% of the coefficient of determination from the regression models between actual and estimated severity were greater than 0.81 with the SADs, while only 37% and 9% of the values were higher than 0.81 with the rating scale and without any aid, respectively. Hence, there was greater consistency of disease assessment among raters. The set of SADs proposed in this study can improve accuracy and precision to estimate severity of gray mold in strawberry fruit.