Pests and diseases are important causes of tree mortality in forests and urban landscapes. The continued introduction of new diseases and the ability of pathogens to infect new host species puts all types of trees at increasing risk. The emergence of diseases in previously unaffected host species can cause unprecedented mortality, posing an increasing threat to trees' native habitats.

New research shows a global increase in tree diseases, with species such as pine, oak and eucalyptus having the greatest impact. Research shows climate change could worsen the situation, posing a major threat to global tree populations and ecosystems. Source: Andrew V. Gougherty

History and recent effects of tree diseases

While not all diseases outright kill their hosts, some can have a dramatic impact on host populations. Chestnut blight was probably the most famous tree disease in North America during the 20th century, completely wiping out chestnut trees from their native range in the Appalachian Mountains. Recently, we've seen sudden oak death disease in California, ash dieback in Europe, and mountainside rot in the eastern United States, each of which has the potential to wipe out host tree populations and alter the ecosystems in which they exist.

"The continued emergence and accumulation of new diseases increases the likelihood that particularly harmful diseases will emerge and harm host tree populations," said Dr. Andrew Gugerty, a landscape ecology research specialist with the USDA Forest Service. Recently, he has been exploring where tree diseases accumulate most quickly and which trees are most affected by new diseases. This information can help researchers and land managers better predict where new diseases are most likely to emerge."

Tar spots on maple trees in Frostburg, Maryland, USA. Source: Andrew V. Gougherty

Comprehensive study of tree diseases

The study analyzed more than 900 new disease reports in 284 tree species from 88 countries and quantified how emerging infectious diseases accumulate in different geographies and on different hosts. "The 'big data' approach used in this study helps characterize the growing threat posed by emerging infectious diseases and how this threat is unevenly distributed across regions and across host species," the authors write.

Powdery mildew on maple trees in Vancouver, British Columbia, Canada. Image source: Andrew V. Gougherty

Research findings and global trends

Dr. Gougherty found that over the past two decades, the number of emerging diseases has increased rapidly around the world. "This accumulation is evident in both native and non-native habitats of tree species, with the number of new emerging diseases doubling every 11 years globally," he explains. Among the trees he assessed, pine accumulated the most new diseases, followed by oak and eucalyptus. He explains that this may be due to the pine's widespread native distribution in the northern hemisphere, as well as the planting of pine forests across the globe. Overall, Europe has the largest cumulative total of new diseases, but North America and Asia are close behind.

Additionally, he found that emerging tree diseases were more common in areas where trees were native than non-native, except in Latin America and the Caribbean, possibly because most of the trees he evaluated were not native to the region.

"Unfortunately, there is little evidence that the accumulation of emerging tree diseases has reached saturation," Dr. Gugerty warned. "Global trends show little sign of slowing, suggesting that the impact of emerging diseases is likely to continue to intensify and threaten global and future tree populations. Climate change is likely also at play, creating more favorable conditions for pathogens and stressing host plants."