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Emerging diseases threaten trees globally, and the trend is escalating

Emerging diseases are a significant threat to the global tree population. Diseases have become major contributors to tree mortality in both forests and urban landscapes worldwide, as highlighted in a new study by Dr. Andrew Gougherty, a research landscape ecologist at the USDA Forest Service

Finding new hosts

Emerging infectious diseases are not only continually being introduced but are also incessantly seeking new hosts, thereby posing a risk to an increasing number of tree species.

The danger of emerging diseases is underscored by their ability to cause unprecedented levels of mortality when they encounter novel hosts, as noted in the research.

Rising mortality from tree diseases

These diseases may wreak havoc in their native ranges, leading to previously unseen mortality rates among tree populations. While not all emerging diseases ultimately prove fatal to their hosts, many exert profound impacts that can significantly damage host populations.

Illustrating the gravity of the situation, the study cites historical examples of disease-driven tree population declines. Notably, the 20th century witnessed the devastating impact of chestnut blight. This infamous disease virtually eradicated the chestnut from its overstory position within its native Appalachian Mountains range. 

The affliction of sudden oak death in California, ash dieback in Europe, and butternut canker in the eastern United States are other notable examples. Each of these diseases harbors the potential to wipe out host tree populations and induce irrevocable changes in their respective ecosystems.

“The continued emergence and accumulation of new diseases increases the likelihood of a particularly detrimental one emerging, and harming host tree populations,” said Dr. Gougherty.

Focus of the study 

The research was focused on identifying regions experiencing rapid accumulations of tree diseases and pinpointing the tree species most adversely affected by these emerging threats.

Dr. Gougherty analyzed over 900 new disease reports involving 284 tree species across 88 countries. “The ‘big data’ approach used in this study helps to characterize the growing threat posed by emergent infectious diseases and how this threat is unequally distributed regionally and by host species.”

Alarming rates of emerging diseases 

Significantly, Dr. Gougherty’s findings reveal an alarming acceleration in the number of emerging diseases over the past two decades. 

This surge highlights the urgent need for comprehensive research and effective management strategies to safeguard tree populations from the burgeoning threat posed by infectious diseases. 

“The accumulation is apparent both where tree species are native and where they are not native, and the number of new disease emergences globally were found to double every ~11 years,” said Dr. Gougherty.

Most affected species

The study revealed that pines have accumulated the most new diseases, followed by oaks and eucalypts. 

“Of the tree genera assessed, Pinus had by far the most new diseases reported over the last several decades, likely reflecting both its large native range in the Northern Hemisphere and its wide use in forestry globally,” explained Dr. Gougherty.

Overall, the greatest total accumulation of new diseases was found in Europe, followed by North America and Asia. The study also revealed that most hosts tend to accumulate more diseases in their native ranges than their non-native ranges.

Implications of these tree diseases

“Unfortunately, there is little evidence of saturation in emergent tree disease accumulation. Global trends show little sign of slowing, suggesting the impact of newly emerged diseases is likely to continue to compound and threaten tree populations globally and into the future,” said Dr.  Gougherty. 

“Climate change is likely also playing a role, both by creating more favorable conditions for pathogens and by stressing host plants.”

More about tree diseases

As global temperatures rise and ecosystems undergo dramatic shifts, our trees face an increasing array of threats. As discussed above, emerging tree diseases are becoming a major concern for ecologists, foresters, and conservationists alike. While we have historically battled tree ailments, the recent upsurge in unfamiliar pathogens poses unique challenges.

Ash Dieback (Hymenoscyphus fraxineus)

Originating from Asia, Ash Dieback first emerged in Europe in the 1990s and has since spread rapidly. It’s a fungal disease affecting ash trees, leading to leaf loss, crown dieback, and often the death of the tree. Afflicted trees display distinct dark lesions on their stems and branches. This disease poses a significant threat, especially to European ash populations.

Sudden Oak Death (Phytophthora ramorum)

First detected in the 1990s in California, Sudden Oak Death affects various species, especially oak trees. It causes dark spots on leaves, cankers on the tree’s bark, and in severe cases, tree death. Beyond oaks, the pathogen can affect other plants, acting as a reservoir for the disease and complicating containment efforts.

Red Bay Ambrosia Beetle and Laurel Wilt

The Red Bay Ambrosia Beetle, native to Southeast Asia, carries the fungus Raffaelea lauricola, responsible for Laurel Wilt. Identified in the U.S. in 2002, this disease primarily impacts the Lauraceae family, which includes the avocado tree. The fungus blocks water transport in the tree, leading to wilting and death.

Emerald Ash Borer (Agrilus planipennis)

While not a disease in the traditional sense, this invasive beetle from East Asia is worth mentioning due to its destructive nature. Since its discovery in North America in 2002, it has decimated millions of ash trees by boring under the bark and feeding on the tree’s inner bark, disrupting the tree’s ability to transport water and nutrients.

Responding to the challenge

Reacting to these emerging diseases requires a multi-pronged approach. It’s imperative to:

Raise Awareness: The public, especially those living near forests or with trees on their property, need to be aware of the symptoms and report suspicious tree health.

Fund Research: Developing resistant tree varieties and effective treatments necessitates robust research efforts.

Strengthen Border Checks: Many tree diseases spread through global trade. Enhanced inspection of imported plants can mitigate the introduction of new pathogens.

Encourage Responsible Gardening: Gardeners can play a significant role by not moving plants from affected areas and by adopting disease-resistant tree varieties.

In summary, our global forests are under siege from a host of emerging diseases. Combating this growing threat will require concerted efforts from scientists, policymakers, and the public. The health of our trees is not just an ecological concern, but it also has profound implications for our economy, culture, and well-being.

The research is published in the journal NeoBiota.

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