In California's Changing Climate, Latent Pathogens Drive Novel Woody Plant Diebacks on a Large Geographic Scale

The detection of emergent biotic and abiotic threats to plant health is challenging in an interconnected world undergoing climate change. Here, we present data generated by a diagnostic laboratory at the request of landowners, land managers and farm advisors. The laboratory was tasked with identifying the causes of synchronous and previously unreported diebacks of 10 woody plant species in six ecosystems located hundreds of kilometres apart. In all but one plant species, cankers were identified in close association with the diebacks, and mortality was recorded. All fungi associated with cankers were latent pathogens in the orders Botryosphaeriales and Diaporthales, including Botryosphaeria dothidea and species in the genera Cytospora , Diaporthe , Diplodia , Dothiorella and Neofusicoccum . Distribution of latent pathogens was not random: there was a dominance of one or two species per host species, suggesting plants are experiencing fungus‐specific infectious outbreaks. Fifteen novel host × pathogen combinations were identified, and pathogenicity tests were conducted on potted plants for 12 combinations. Inoculations were run in well‐watered and minimally watered conditions to test the role played by water stress. Pathogenicity was confirmed for nine fungal species, and in five cases, lesion size was larger in plants minimally watered. No primary pathogens were isolated from lesions or baited from the soil; thus, our results show that latent pathogens, likely triggered by climate change, are playing an ever‐increasing role in the emergence of novel diseases in woody plants. Ecosystem resilience may be increased through restoration efforts employing plant germplasm adapted to harsher climates.