Mechanisms of severe dieback and mortality in a classically drought-tolerant shrubland species (Arctostaphylos glauca).

PREMISE: Mortality events involving drought and pathogens in natural plant systems are on the rise due to global climate change. In Santa Barbara, California, United States, big berry manzanita (Arctostaphylos glauca) has experienced canopy dieback related to a multi-year drought and infection from fungal pathogens in the Botryosphaeriaceae family. A greenhouse experiment was conducted using Neofusicoccum australe to test the specific influences of drought and fungal infection on A. glauca. METHODS: A full factorial design was used to compare four treatment groups (drought + inoculation; drought - inoculation; watering + inoculation; and control: watering - inoculation). Data were collected for 10 weeks on stress symptoms, changes in leaf fluorescence and photosynthesis, and mortality. RESULTS: Results indicated significant effects of watering and inoculation treatments on net photosynthesis, dark-adapted fluorescence, and disease symptom severity (P < 0.05), and a strong correlation was found between physiological decline and visible stress (P < 0.0001). Mortality differed between treatments, with all groups except for the control experiencing mortality (43% mortality in drought - inoculation, 83% in watering - inoculation, and 100% in drought + inoculation). A Kaplan-Meier survival analysis showed drought + inoculation to have the least estimated survivorship compared to all other treatment groups. CONCLUSIONS: In addition to a possible synergistic interaction between drought and fungal infection in disease onset and mortality rates in A. glauca, these results indicate that young, non-drought-stressed plants are susceptible to mortality from N. australe infection, with important implications for the future of wildland shrub communities.