A Standard Area Diagram Set to Aid Assessment of Ring Spot Severity in Sugarcane

Abstract

Sugarcane is an important global crop for fuel and energy production, and its harvested area and production have increased in Brazil over the last decade. The method of harvesting recently shifted from manual to mechanized, and it is suspected that the unburned straw left on the soil after harvesting has changed the epidemiology of diseases caused by necrotrophic fungi, increasing their importance. Ring spot (caused by Leptosphaeria sacchari) was considered of secondary importance, but is now more frequently observed on both old and new leaves. This study aimed to develop and validate a standard area diagram (SAD) set to help assess ring spot severity in sugarcane leaves. A nine-level SAD set was developed and subsequently validated by 16 inexperienced raters. Accuracy and precision of the estimates by each rating, with and without the SADs, were determined based on Lin’s concordance correlation coefficient. Inter-rater reliability was measured using the coefficient of determination (R2) and intraclass correlation. The proposed SADs improved the accuracy and precision of the estimates, in addition to reducing absolute error and increasing inter-rater reliability. The SAD set studied here is a useful tool in providing accurate, precise, and reliable assessments of ring spot severity in sugarcane for epidemiological research, when comparing treatments, or in breeding programs.