Temperature and CO ₂ Interactions Reshape <i>Hemileia vastatrix</i> Urediniospore Germination <i>in vitro</i>

Climate change is expected to heighten the risk of epidemics and outbreaks of coffee leaf rust (CLR), caused by the obligate parasite <i>Hemileia vastatrix</i>, as reported in the literature. While knowledge is available on rising temperatures' effects on CLR epidemiological processes, fewer studies have explored combined temperature-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 <i>H. vastatrix</i> urediniospore germination under varying combinations of temperature and atmospheric CO₂ in controlled conditions (in phytotrons), testing thirty combinations of temperatures (16°C 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 (pmax) below the current CO₂ (424 ppm) at 24°C (pmax=0.45) and 28°C (pmax=0.43), and above 424 ppm at 16°C (pmax=0.34) and 32°C (pmax=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.