Late Leaf Rust Monocyclic Components and Primary Metabolism of Raspberry Plants Facing Water Deficit

Drought reduces host resistance and allows the emergence of pathogens that benefit from changes in plant physiology under water stress. However, for pathogens that penetrate only via the stomata, like Aculeastrum americanum , stomatal closure due to water stress can prevent infection. Here, we test the hypothesis that raspberry ( Rubus idaeus ) late leaf rust severity is intensified when water deficit is imposed after penetration of A. americanum in leaves. We perform epidemiological and quantitative analyses of A. americanum colonisation, as well as measurements of leaf gas exchange, on healthy and inoculated leaves of R. idaeus ‘Heritage’. Plants were either grown under well‐watered conditions throughout the experimental period (control) or submitted to water withholding 2 days after inoculation (water deficit). Our results revealed that the incubation and latent periods, urediniospore production and lesion density did not differ between the two water regimes. However, water deficit significantly reduced rust severity, A. americanum colonisation and leaf gas exchange in both healthy and inoculated plants. Comparing the biotic and abiotic stresses, late leaf rust has a greater impact on leaf CO 2 assimilation than water deficit. In addition, there was a synergism between leaf rust and water deficit, causing further decreases in leaf gas exchange of raspberry plants. Based on these findings, we conclude that the occurrence of water deficit after the onset of pathogen infection reduces rust severity by limiting its colonisation capacity, a consequence of low photoassimilate supply to the pathogen due to impaired photosynthetic activity.