Natural endophyte reduces disease severity in cork oak caused by Biscogniauxia mediterranea and Diplodia corticola under different watering regimes

Cork oak (Quercus suber) is an economically important species in the Mediterranean region, but its decline due to fungal pathogens such as Biscogniauxia mediterranea and Diplodia corticola poses a major threat to forest sustainability. Climate change is aggravating this problem, and few control measures are currently available. The use of naturally occurring endophytic antagonists as biocontrol agents offers a sustainable strategy to mitigate cork oak decline. In this work, cork oak seedlings subjected to normal irrigation (70% field capacity) and drought (25% field capacity) were inoculated with the endophytic fungus Simplicillium aogashimaense, followed by one of the pathogens (B. mediterranea or D. corticola). The endophyte significantly alleviated the negative impacts of both pathogens, showing a stronger protective effect under drought stress, where the combined pressures of water limitation and pathogen infection were mitigated. In addition, S. aogashimaense modulated key physiological and biochemical responses in the seedlings, including phenolic accumulation, protein expression, peroxidase activity, and oxidative stress markers. The mode of action differed between pathogens: defense responses against D. corticola involved increased hydrogen peroxide (H₂O₂) production and lipid peroxidation, while interactions with B. mediterranea were associated with reduced peroxidase activity. The consistent protective effect under both water regimes highlights the potential of S. aogashimaense in climate change scenarios characterized by increased drought frequency. This study supports the use of endophytes as environmentally sustainable tools in integrated forest management, enhancing cork oak resilience to combined stresses and contributing to a better understanding of plant responses to pathogen attack.