A standard area diagram set to aid estimation of the severity of Asiatic citrus canker on ripe sweet orange fruit

Asiatic citrus canker (ACC) is a major disease of citrus in many tropical and subtropical citrus-growing regions. Severe disease can develop on the fruit, making it unmarketable. Research activities need accurate and reliable methods to estimate the severity of citrus canker, most often done visually by raters. The objective was to validate and compare two standard area diagram sets (SADs) as aids for raters to improve the accuracy and reliability of visual estimates of canker severity on ripe fruit of sweet orange (unripe fruit are green, and have slightly different symptoms compared to ripe fruit, warranting different SADs). A 5-diagram SAD set (0.7, 2.0, 7.0, 21.0 and 39.0 %) and a 6-diagram SAD set (0.7, 2.0, 4.0, 10.0, 21.0 and 39.0 %) were compared; the 6-diagram set had an additional diagram in the most common severity range. Fifteen raters evaluated 40 images of cankered, unripe fruit. Both the 5- and 6-diagram SADs improved the accuracy and reliability of rater estimates. Agreement, measured by Lin’s concordance correlation coefficient (ρ c ) was 0.735 to 0.906 when not using SADs, 0.931 to 0.985 when using 5-diagram SADs, and 0.879 to 0.979 when using 6-diagram SADs. Mean inter-rater reliability (R2) was 0.638 when not using SADs, 0.889 when using 5-diagram SADs, and 0.834 when using 6-diagram SADs. The 5-diagram SADs resulted in significantly more accurate and reliable estimates compared with the 6-diagram set, and is thus recommended for use. The SADs should have application for estimating ACC severity on ripe fruit of other citrus, including grapefruit.