Trees and woody plants are essential for life on Earth. They support biodiversity, improve air and water quality, protect soil, provide renewable products, regulate regional climate, and help manage the carbon cycle. They do this on their own or in partnership with other plants, animals, insects, and microbes.
As our climate changes rapidly, trees, being stationary and slow to reproduce, are highly vulnerable to hotter, drier, or wetter conditions, probably even more so than animals that can move. At the same time, our increasingly connected world allows nonnative pathogens and pests to travel long distances, sometimes causing serious harm to trees that have never encountered them before.
In Northern California, during cycles of recurring drought, ten tree and shrub species in multiple ecosystems developed disease symptoms that had never been seen before. Concerned about a possible nonnative pathogen, landowners and managers turned to the U.C. Berkeley Forest Pathology Laboratory. The study found latent fungal pathogens in nine out of ten species. Most of these fungi had never been reported before in California on the plant species experiencing dieback.
Latent pathogens are fungi that normally live harmlessly inside plants but can become harmful when the plant is stressed, especially due to a lack of water. To test whether these fungi were causing the diebacks, researchers exposed healthy potted plants, and in one case, mature trees, to the fungi. In nine cases, the plants developed lesions similar to those seen in the field. Fungi were consistently recovered from the diseased plants, confirming that the latent pathogens were indeed responsible.
Why were these diseases appearing at the same time in different species and locations? The researchers hypothesised that drought triggers these fungi to become pathogenic. Experiments confirmed this: minimally watered plants developed worse lesions and foliage damage than well-watered ones. Revisiting the study sites later showed that conditions had worsened in about half the locations, even after the drought ended, and mortality was widespread.
These results show that climate-linked drought and latent pathogens are driving new tree diseases and large-scale tree mortality. In one case, climate alone, even without pathogens, was harming trees. Many affected species are now growing in areas that, due to climate change, are no longer suitable. Reforesting these areas with species or ecotypes adapted to hotter, drier conditions can help reduce disease and mortality.
Matteo Garbelotto, Edoardo Scali, Steven Swain, Daniel Macon, Igor Lacan, Douglas Schmidt, Tina Popenuck, Ilaria Martino, Vladimiro Guarnaccia published this study in Plant Pathology Journal:
