Infectious diseases don't just threaten the health of humans and animals. Trees are also susceptible to new pathogens, and scientists worry a growing number of species could be at risk as climate change makes tree populations more vulnerable.

Butternut canker, for instance, is ravaging trees across Ontario, Quebec, New Brunswick and much of the eastern U.S., while sudden oak death — caused by a fungus-like plant pathogen — is devastating oak populations in California and Oregon. 

"In North America, we've effectively lost chestnut as a major overstory tree over the last century, due to chestnut blight. With new diseases continually emerging, other trees could face similar consequences over the coming decades," said Andrew Gougherty, a research landscape ecologist at the U.S. Forest Service.

Gougherty's latest study, published in the open-access journal NeoBiota, quantifies just how bad things are getting for the planet's largest plants. He looked at more than 900 new disease reports impacting several hundred tree species in dozens of countries and found the number of emerging tree diseases has shot up in recent decades.

"One of the surprising findings from this work was the rapid pace of accumulation, with the number of new emerging diseases doubling every approximately 11 years," Gougherty told CBC.

Tod Ramsfield, a forest pathologist and research scientist with Natural Resources Canada who wasn't involved in the study, said climate change is having major impacts on tree health and helping fuel the rise of deadly diseases. 

"With climate change comes drought, with drought trees become stressed, and when they're stressed, they become more susceptible to pathogens in the environment — even native pathogens," he said. 

"The number of diseases — and the extent of the infections — is going up. So, under hotter or warmer conditions, and in some areas, more humid or even more arid conditions, those infections become more prevalent and more extensive," said Suzanne Simard, a professor of forest ecology at the University of British Columbia and the author of Finding the Mother Tree: Discovering the Wisdom of the Forest.

Ramsfield said human movement is also fuelling the spread of tree pathogens between continents, whether that's through global trade or tourism. And while insect-based threats like mountain pine beetles tend to make the most headlines for rapidly decimating tree populations, diseases tend to work more slowly. 

"You don't see them," Ramsfield said, "until the disease pops up and the pathogen becomes established."

The biggest concern for Gougherty when it comes to global tree health? The diseases we don't even know about yet.

"In an ideal world, we'd be able to identify pathogens quickly and implement effective management strategies soon after any symptoms are observed. But it can sometimes take years to determine the cause of new pest outbreaks — whether they be caused by insects, pathogens or some environmental factor," Gougherty warned. 

"Not knowing how pests spread and are transmitted or other basic biology from the onset means we're often playing catch-up."