06/28/2026
In California, we are seeing a shift in tree diseases. Bark beetle outbreaks, introduced pathogens, and weather favoring fire blight, anthracnose, or certain fungal decays have long been our known issues which come in waves. With our diverse landscape across the state, we rarely see one issue affect all regions at once. That remains true. What is changing is that significant tree health issues are emerging across the state simultaneously, in different species and different habitats, even without a single shared causal agent
To understand what’s happening, start with the fact that trees host diverse microbiomes, both in the rhizosphere and within the endophytic communities of their woody tissues. Opportunistic pathogens have long been present within many tree species, causing no problems in relatively healthy hosts.
Humans are prone to shifting baseline syndrome. We forget, or never closely track, gradual changes in climate. Trees integrate those changes physiologically. As temperatures and moisture shift significantly, the accumulated stress lowers host defenses, and latent pathogens that rarely caused disease before transition from endophytic to pathogenic and produce localized epidemics.
A recent paper from team of plant pathologist and climate scientist strives to tackled the problems emerging as signification local pathogen issues, which previously have cause little issues. Many of these species are not newly introduce, but have been latent weak pathogens that have found a new foothold in trees dealing with the more dramatic variation of climate. Their image below shows the numerous latent pathogens now affecting trees in California’s landscapes.
In my region, with help from colleagues, we are seeing a variety of native and now nonnative oaks suffering from Biscogniauxia mediterranea, including some that were not under any obvious environmental stress beyond the broader climate shifts.
To exemplify the shift: here at my home in the upper foothills/lower montane zone around 3,000 ft, we normally see freezing nights for several months in winter, the occasional heavy snowstorm, and cold temperatures when it rains. This winter we saw almost no freezing temperatures. I lost a cluster of sugar pines to mountain pine beetle in February, which is highly abnormal, followed by a long stretch of warm temperatures, into the 80s here in late winter, which pushed spring growth early. We then had an extended period of warm storms. A long warm spell, rainfall, and fresh growth together drove an increase in fire blight, leaf spots, and general fungal issues, as many of these pathogens benefit from the warm and wet combination
Link to the paper in the comments below
