Temperature and CO₂ Interactions Reshape Hemileia vastatrix Urediniospore Germination In Vitro

Climate change is expected to heighten the risk of epidemics and outbreaks of coffee leaf rust (CLR), caused by the obligate parasite Hemileia vastatrix, as reported in the literature. While knowledge is available on rising temperatures' effects on CLR epidemiological processes, fewer studies have explored combined temperature and atmospheric carbon dioxide (CO₂) impacts, with contrasting findings on CLR incidence and severity. CLR epidemics are multifactorial, and controlled-condition experiments targeting individual components help anticipate pathogen behavior under future climates. Urediniospore germination, a time-efficient indicator, enables exploration of pathogen responses across diverse temperature-CO₂ combinations, providing insights into CLR epidemiological shifts. We investigated H. vastatrix urediniospore germination under varying combinations of temperature and atmospheric CO₂ in controlled conditions (in phytotrons), testing 30 combinations of temperatures (16 to 32°C) and atmospheric CO₂ concentrations (181 to 707 ppm). The fitted generalized linear model with a binomial distribution within an augmented design framework to our data was robust (pseudo-R² = 0.78) and revealed that germination probability followed a quadratic pattern as a function of varying atmospheric CO₂, with optima dependent on both CO₂ and temperature. Urediniospore germination probabilities are optimized (p_max) below the current CO₂ (424 ppm) at 24°C (p_max = 0.45) and 28°C (p_max = 0.43) and above 424 ppm at 16°C (p_max = 0.34) and 32°C (p_max = 0.35). This indicates that elevated CO₂ can partly compensate for less suitable temperatures while reducing germination within the current optimal range. Overall, our results highlight a trade-off in which rising CO₂ reshapes, rather than uniformly increases, infection opportunities under climate change, providing new data for future risk models.