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.

Dieback and mortality in wildland plant species due to climate change has been on the rise in recent decades, and latent fungal pathogens may play a significant role in these events. During a severe multi-year drought, canopy dieback associated with latent pathogens in the Botryosphaeriaceae (Bot) family was observed in stands of a dominant shrub species, big berry manzanita (Arctostaphylos glauca), across chaparral landscapes in California. These fungi are significant pathogens of woody agricultural species, especially in hosts experiencing stress, and have become a threat to economically important crops worldwide. However, little is known regarding their occurrence, distribution and impact in wildland systems. We conducted a field survey of 300 A. glauca shrubs across an elevational gradient to identify Bot species infection as it relates to a) A. glauca dieback severity and b) landscape variables associated with plant drought stress. Our results show Bots widely infecting A. glauca across the landscape, and there is a significant correlation between elevation and dieback severity. Dieback severity was significantly higher at lower elevations, suggesting that infected shrubs at lower elevations are at greater risk than those at higher elevations. Furthermore, two Bot species, Neofusicoccum australe and Botryosphaeria dothidea, were most frequently isolated with N. australe being the most common and, based on haplotype analysis, likely the most recently introduced of the two. Our results confirm the wide distribution of latent Bot fungi in a wild shrubland system, and provide valuable insight into areas of greatest risk for future shrub dieback and mortality.