Rain-driven detachment and dispersal of Venturia oleaginea conidia from olive leaf spot lesions

Venturia oleaginea, the causal agent of olive leaf spot (OLS), threatens olive trees worldwide. Although conidia are mainly dispersed by rain, the relationship between rainfall and conidial dispersal has not been quantitatively characterized. This study investigated the role of rainfall in the dispersal of V. oleaginea conidia from sporulating olive leaf lesions and developed a model describing rain-dependent dispersal. In spring and autumn of 2022 and 2023, leaves with OLS lesions were collected from commercial orchards in Italy and Spain, induced to sporulate under controlled conditions, and exposed to simulated rainfall. A rain-generation system was operated for 1, 2, 3, 4, 8, 12, 16, or 20 min at an intensity of 3 mm min⁻¹, producing raindrops of variable size and velocity and generating different rainfall amounts. Despite variation among trials in lesion size and conidial density, rainfall consistently detached approximately 79% of the conidia present on lesions. Cumulative conidial dispersal followed a nonlinear asymptotic pattern, with maximum release occurring at the onset of rainfall, indicating an efficient rain-detachment mechanism. As cumulative rainfall increased from 0 to 20 mm, dispersal rate declined because of the progressive depletion of detachable conidia, util it approached zero between 40 and 180 mm. An exponential saturation function obtained based on trials conducted in spring 2022 and 2023, and autumn 2023 described the relationship between rainfall and conidial dispersal (with concordance correlation coefficient = 0.862 and RMSE = 0.150). These results provide quantitative information on rain-driven conidial dispersal that can improve weather-based models predicting OLS epidemics.