From Scalar Summaries to Functional Comparisons: A Framework for Analyzing Plant Disease Progress Curves

Disease progress curves (DPCs) are central to evaluating disease management strategies, including host plant resistance. Although widely used and often appropriate, scalar summaries such as the area under the disease progress curve (AUDPC) may obscure meaningful differences in epidemic timing and trajectory shape. Here, I introduce a curve-based framework for comparing plant disease epidemics that treats DPCs as epidemic phenotypes, enabling trajectory-based comparisons beyond conventional scalar summaries. Using a hierarchical generalized additive model, environment-adjusted mean epidemic curves were estimated for each treatment (corn hybrid) while accounting for repeated assessments and environmental heterogeneity. Similarity among hybrids was quantified using a functional distance defined over the epidemic time domain, and hierarchical clustering was used to identify epidemic phenotypes based on differences in curve shape. Applied to multi-environment field data (6 environments; 74 DPCs) for southern corn leaf blight in 13 hybrids, this approach identified distinct epidemic phenotypes that were not fully reflected by AUDPC-based comparisons, despite similar overall disease levels. In addition, a distance-based permutation test indicated that breeder-defined resistance classes (moderately resistant versus resistant), established independently of the curve analysis, were associated with systematic differences in epidemic trajectory shape across environments. By shifting emphasis from scalar summaries to curve-based epidemic representations, this framework provides a complementary tool for host resistance phenotyping and comparative epidemiology and establishes a foundation for hierarchical synthesis and trajectory-based inference across environments.