Accuracy and efficiency of assessments of cassava brown leaf spot aided by standard area diagram sets based on whole compound leaves or single central leaflets

Abstract

This study aimed to develop, evaluate and compare standard area diagram sets (SADs) based on whole compound leaves or single central leaflets of cassava as aids to estimate the severity of brown leaf spot (BLS), caused by Passalora henningsii. The proposed SADs illustrated either compound leaves or central leaflets with eight disease severities ranging from 0.1 to 24%. The SADs were validated by eight raters with no previous experience in disease assessment. There was a positive association between actual severity on compound leaves and central leaflets (r = 0.89; p < 0.01). Lin’s concordance correlation analysis of estimated versus actual disease severity (based on image analysis) showed that all statistics (υ, u, Cb, r and ρc) were improved by using both SADs. Similarly, by analysing the coefficient of determination and intra-class correlation coefficient, the estimates of severity were more reliable using these SADs. Further field tests demonstrated both SADs were suitable for assessment of BLS in different growing areas and as an aid to estimate severity in a germplasm collection of cassava. However, using the SADs based on central leaflets resulted in more rapid estimates compared to using that based on compound leaves (a reduction of 25 and 31% of time taken for the assessments in different growing areas and in the germplasm collection, respectively) with no loss of accuracy. This is the first time that assessment efficiency of diseased whole organ and organ subunits has been performed. Based on the results, the SADs using central leaflets is preferred in situations that require a large number of assessments in a short time frame.