Ecophysiological responses of olive trees upon infection by Neofusicoccum species

Olive branch dieback, which is caused mainly by fungal species in Neofusicoccum genus (Botryosphaeriaceae), leads to branch death and irreversible damage. This re-emerging disease in rainfed olive orchards in Spain is linked to crop intensification, the prohibition of the burning of pruning residues, and drought. Lepra disease, caused by Phlyctema vagabunda, is another re-emerging disease associated with shoot dieback in high-density olive orchards. In this study, the behavior of N. mediterraneum, N. parvum, and P. vagabunda was evaluated under laboratory and field conditions. Mycelial growth was measured across different temperatures and water potentials. Neofusicoccum species showed maximum growth at 25 °C but tolerated 30 °C and low water potentials (-15 MPa). Phlyctema vagabunda grew better at 25 °C but failed to grow at higher temperatures or under water stress. To investigate the impact on plants, branches of 10 of the main Spanish olive cultivars were inoculated with the pathogens in a 13-year-old orchard. Canker length, branch mortality, vegetative growth, stomatal conductance, chlorophyll fluorescence, and water potential were monitored upon inoculation. In a second pot experiment, using the most susceptible cultivar, 'Gordal Sevillana', we tested the effect of water stress on disease progression. Neofusicoccum species significantly reduced vegetative growth (30-70%), stomatal conductance (gsw) (5-42%), and water potential (9-16%). Despite the two species exhibiting similar virulence levels, according to canker length and branch mortality, their underlying mechanisms require further study. On the other hand, P. vagabunda inoculations were unsuccessful, and no canker symptoms were observed. In the pot experiment, water stress affected plant growth and gsw but did not influence pathogen behavior. Overall, compared with P. vagabunda, Neofusicoccum species are better adapted to high temperatures and low water availability. Their infections impair water relations in olive branches, reducing water potential and gsw and leading to branch mortality. This study highlights the complex dynamics linking woody pathogens and abiotic stress in perennial crops.