Modeling yield loss associated with the corn stunt disease complex using an ordinal severity scale

Abstract

Historically considered of secondary importance, the corn stunt disease complex, caused by mollicutes and viruses, has recently become an increasing concern in Brazil. In this study, we quantified the relationship between disease severity, measured using a 1–6 quantitative ordinal scale, and corn yield (kg/ha) using data from 22 independent field trials (72 different hybrids) conducted by IDR-Paraná across 10 municipalities in Paraná State over two growing seasons. These trials were analyzed jointly using a random-coefficients mixed-effects model fitted to yield as a function of disease severity, allowing for both random intercepts and slopes across studies. The model explained 80.4% of the total variance in yield (conditional R2 = 0.80), of which 18.9% was attributable to the fixed effect of severity (marginal R2 = 0.19). The adjusted intraclass correlation coefficient (ICC = 0.76) indicated that most of the variability occurred among studies rather than within studies. The estimated attainable yield was 8,227 kg/ha, and yield declined by 1,043 kg/ha for each unit increase in severity, equivalent to an average 12.7% yield reduction per severity score unit. At the highest severity level, yield losses reached approximately 63% relative to symptom-free plots. The two-level categorical covariates hybrid tolerance and hybrid susceptibility did not significantly affect the slope of the relationship, indicating a consistent yield response to increasing disease severity across hybrids. These findings highlight the substantial and predictable impact of the corn stunt disease complex on corn yield under field conditions in southern Brazil.