Climatic Fingerprint of the 2023 Wheat Head Blast Epidemic Season: Historical and Future Climate Analogs in Southern South America

Wheat head blast is a major threat to wheat in South America, but epidemics vary across space and time because they depend on seasonal weather. In subtropical and temperate regions, epidemics occur only when winter conditions allow inoculum survival and weather during heading favors infection. As a result, epidemiological data from these regions remain limited. In 2023, unusual weather led to widespread disease occurrence across parts of southern South America. In this study, we adopted a climate-analog framework to define the climatic fingerprint of the 2023 epidemic season and assess whether similar conditions occurred historically (1992-2023) or recur under mid-century climate projections (2024-2055). Two biologically informed indices were developed for each year: winter thermal favorability, representing conditions for inoculum carryover, and hygrothermal suitability during heading. Historical years were compared with 2023 conditions at each site and with the 2023 Passo Fundo epidemic. Analog years were rare in the historical recor, confirming the exceptional nature of 2023. Future projections did not show a uniform increase in exact 2023 analogs; the clearest increases occurred under RCP2.6 at Santa Rosa (BR), Passo Fundo (BR), and Pergamino (AR). In contrast, years jointly exceeding both benchmark indices increased at every site in the within-site analysis and generally increased more under RCP8.5. In the cross-site analysis, only Brazilian sites reproduced or frequently exceeded the Passo Fundo benchmark, whereas the Argentine sites remained climatically distinct. Climate change projections therefore shifted conditions beyond the 2023 benchmarks more consistently than it increased the recurrence of years closely resembling 2023.