Physiological symptoms induced by drought stress outweigh vascular pathogen infection in walnut

Drought stress can affect the success of xylem-dwelling pathogens due to modifications of the xylem environment as water potential declines. However, the interaction between these abiotic and biotic stresses on plants is complex and requires specific experiments to distinguish between multiple effects. This is especially important in agroecosystems, where monocultures of individuals facilitate disease transmission and water scarcity can lead to deficit irrigation practices to optimize water management, control canopy size and maintain crop productivity. We measured photosynthetic gas exchange, stem xylem water potential, non-structural carbohydrates (NSC), morphology and growth of walnut trees in response to two imposed stress treatments. One was the inoculation with the two cosmopolitan vascular fungal pathogens Diplodia mutila and Neofusicoccum parvum. The other was a manipulation of water availability with well-watered controls compared to deficit irrigation treatments representing 75% and 25% of well-watered controls. We found that deficit irrigation significantly reduced all measures of gas exchange and stem xylem water potential, and most morphological, growth and NSC variables. Signs of severe drought with leaf yellowing and senescing occurred at the end of the experiment when leaf water potential reached -1.6 MPa. In contrast, responses to pathogen inoculation were limited to reduced stem xylem water potential, total plant leaf area and leaf area ratio. There was no reduction in photosynthetic rate per leaf area with pathogen inoculation, but the reduction in whole plant leaf area led to an overall reduction in whole plant photosynthesis. Pathogen-induced effects were independent of the plant water status, yet they were only visible in fully irrigated trees suggesting that drought minimizes the scope of measurable symptoms. Biotic damage was not enhanced under drought stress perhaps indicating that the host had not reached a critical water stress status conducive to pathogen virulence.