Introduction: The Importance of Urban Plants
Trees, shrubs, and flowers in urban areas play a vital role in our cities—far beyond just aesthetics.
They improve air quality and living conditions, foster social connections, and contribute to public health.
The World Health Organization emphasizes that urban green spaces offer innovative ways to strengthen city resilience, reduce air and noise pollution, mitigate the effects of heatwaves, and improve residents’ well-being.
Public parks, tree-lined streets, and shared gardens are precious allies in the face of today’s environmental challenges. However, these urban plants also have to cope with ongoing climate disruptions, which weaken their health and promote the spread of new diseases.
Climate Change in Urban Areas (Heat, Humidity, Pollution)
Cities are directly impacted by climate change, with its effects intensified by urban-specific factors. The abundance of concrete and asphalt creates urban heat islands, where city temperatures are significantly higher than in nearby rural areas (source: mdpi.com).
In summer, heatwaves are more intense and frequent, and nights remain warm—exhausting both humans and vegetation. In Paris, for example, there are often several degrees of difference between the city center and the surrounding countryside during heatwaves. This persistent urban heat is sometimes accompanied by reduced ventilation and increased pollution—stagnant air contributes to the formation of ozone and other pollutants.
Moreover, warmer air can hold more humidity. This leads to violent thunderstorms (heavy rain in short periods) alternating with prolonged droughts. This increasingly erratic climate—with drier, hotter summers, unusually mild autumns, and unpredictable rainfall—deeply disrupts the urban ecosystem. Urban plants must endure these abiotic stresses: heat spikes, water shortages or excesses, atmospheric pollution, and more.
In the long run, global warming exacerbates the already challenging conditions of plant life in cities.
Urban Plants Under Stress and More Vulnerable
Scientific studies show that climate stress weakens the health of urban vegetation and makes plants more vulnerable to diseases and pests. Rising temperatures modify biological cycles: longer growing seasons, earlier blooming, and also the spread of new parasites, weeds, and pathogens thriving under new conditions.
Drought periods lead to water stress, which lowers trees’ natural defenses, while excess humidity can promote the spread of fungi.
The urban heat island effect worsens these issues. Research has shown that the hottest parts of cities suffer more frequent pest and disease attacks. For instance, Meineke et al. demonstrated that within the same city, trees located downtown (where it’s hotter) were more stressed and more frequently sick or infested than those in cooler outskirts.
In other words, chronic heat weakens urban plants and increases their vulnerability to infections and infestations.
At the same time, climate change allows certain invasive insects to conquer new urban territories. Milder winters no longer kill these insects as before, enabling them to survive and reproduce in more northern regions or within cities.
Public health agencies note that “climate change promotes the migration of insect pests and diseases,” citing the example of the emerald ash borer, which has severely affected the urban forest in southern Quebec in recent years (inspq.qc.ca).
Similarly, warmer temperatures accelerate the life cycle of many insects: some harmful species can now produce multiple generations per year, which increases plant damage.
Lastly, urban pollution may interact with these stressors—trees weakened by air pollution are less resistant to opportunistic pathogens. ouranos.ca.
Urban Plant Diseases on the Rise with Climate Change
What are the main emerging or spreading diseases in urban environments that are being favored by climate change?
Here are a few notable examples of diseases and pests increasingly affecting city plants:
The Pine Processionary Caterpillar (Thaumetopoea pityocampa) :
This southern insect thrives in warm climates. Its urticating (itch-inducing) larvae are now spreading far beyond their historical range. In the Île-de-France region, nests of pine processionary caterpillars are now appearing in pine trees, whereas they were historically confined to the south.
“The cause of their increasingly early activity? Climate disruption, especially autumn heatwaves,” explains the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) (source: bfmtv.com).
In short, abnormally mild autumns allow these caterpillars to develop earlier in the season. The Regional Health Agency of Île-de-France even warns: “At this rate, the entire region could be colonized within five years” (source: bfmtv.com).
By 2021, 84% of the region was already affected by the pine processionary (bfmtv.com). This defoliating caterpillar weakens urban pines and poses a health risk—its hairs are highly urticating.
Winter warming is the main culprit: it increases larval survival, allowing them to colonize areas previously too cold for their life cycle (source: encyclopedie-environnement.org).
Powdery Mildew :
This common fungal disease is recognizable by the white powdery coating on plant leaves. Recent research suggests that powdery mildew may become more prevalent in cities. A study in St. Louis (USA) found that plants in the city center had significantly more powdery mildew than those in the suburbs or countryside (source: washu.edu).
Scientists are still investigating why cities favor this fungus. Urban microclimates—with warmer spring temperatures that accelerate spore germination—and shaded pockets that remain humid even during hot summers may play a role. Wind and traffic may also help spread the spores.
What is certain is that, although powdery mildew is usually hindered by intense heat, cities still offer ideal conditions for its proliferation—especially on street trees, ornamental shrubs, and urban vegetable gardens. The disease weakens plants by limiting photosynthesis and causing premature leaf drop.
Canker Stain of Plane Trees (Ceratocystis platani) :
This is one of the most devastating diseases for urban trees, especially the ornamental plane trees that line many of our avenues. This invasive fungus enters through wounds in the trunk or roots and causes cankers that block the sap-conducting vessels, eventually killing the tree. Once limited to isolated cases, the disease is spreading more and more across southern European cities due to warming.
Hot and dry summers stress plane trees, making them more vulnerable, while storms can damage bark and facilitate fungal entry.
“In urban areas, fungi and diseases are spreading—including new ones—such as the plane tree canker, which attacks roots, infiltrates vessels, and eventually kills the plant,” says researcher Giorgio Vacchiano about trees in Milan (source: ilgiorno.it).
This deadly pathogen has, for example, ravaged the plane trees along France’s Canal du Midi. Combating it often requires felling and incinerating infected trees, which has a major visual and environmental impact in cities. Climate change further complicates this, as drought- or pollution-weakened trees are more vulnerable to such pathogens.
Emerald Ash Borer (Agrilus planipennis) :
This small metallic green beetle, originally from Asia, is an invasive pest that destroys ash trees by burrowing tunnels under the bark. In North America, it has killed millions of ash trees and has recently spread to Europe.
Warmer climates allow the emerald ash borer to expand its range.
In Quebec, it was first detected in 2008 and has since spread across the Montreal region, threatening nearly a third of the trees on Mount Royal (where ashes represent 25–30% of the population) (source: lemontroyal.qc.ca).
All urban ash trees are vulnerable, and without intervention, an infested ash will die within 5 to 7 years. Montreal has already had to cut down tens of thousands of ashes to contain the outbreak. Climate warming increases the risk by allowing the insect to survive winters and complete its life cycle farther north than before.
Canadian urban forestry managers note that after exceptionally hot summers, the insect’s spread accelerates. This pest is a striking example of the threat posed by exotic species in a changing climate.
Other Examples :
Many other diseases and pests are taking advantage of shifting climate conditions. The sycamore lace bug (Corythucha ciliata), a small sap-sucking insect, proliferates during hot summers and yellows sycamore leaves in cities.
The box tree moth (Cydalima perspectalis), an invasive moth, has destroyed boxwoods even in urban parks, aided by milder winters.
Fire blight (Erwinia amylovora), a serious disease affecting apple, pear, and hawthorn trees, may see an extended active period due to warmer springs.
Even some viral diseases in ornamental plants are being monitored, as climate-related stress can weaken plants and promote the expression of viruses that were previously latent (source: jardinsdefrance.org).
In short, urban plant diagnostics need to be reassessed: pathogens that were once harmless may now become problematic, while entirely new threats are emerging in our climate zone.
Concrete Examples from Several Cities
No urban environment is spared, although the nature of the problems may vary by region.
Here are some real-world cases observed in cities around the world :
Paris and the Île-de-France Region :
In recent years, the French capital has experienced a surge of pine processionary caterpillars. As mentioned earlier, over 80% of the Île-de-France region is now colonized due to increasingly mild winters (source: bfmtv.com).
Parisians are now discovering itchy caterpillar trails in their gardens each spring—something unimaginable a few decades ago in this climate. Paris also faces worsening classic fungal diseases: for instance, powdery mildew on horse chestnuts or linden trees is now very widespread after humid springs followed by hot summers. Another well-known urban threat: the horse chestnut leaf miner (a moth whose larvae burrow into leaves) isn’t caused by climate change, but summer water stress accelerates leaf drop from already damaged trees.
The City of Paris is also closely monitoring canker stain. In 2020, a canker outbreak was discovered in the 15th arrondissement, leading to the preventive felling of several plane trees.
Overall, Paris is focusing on planting diverse species to avoid a repeat of the catastrophe caused by Dutch elm disease in the 1970s (when all the city’s elm trees disappeared due to a fungus).
Unfortunately, climate change makes some previously well-adapted species (like sycamore maples or silver birches) much harder to maintain, as they suffer from drought and become targets for wood-boring insects.
Montreal (Canada) :
Montreal is at the forefront of the battle against the emerald ash borer, as previously mentioned. Since its arrival, the insect has forced the city to cut down thousands of trees in its parks and streets.
To offset these losses, Montreal launched a Climate Plan that includes the planting of 500,000 new trees by 2030 (source: montreal.ca).
Besides the ash borer, Montreal’s summers have grown hotter and drier, weakening the city’s abundant silver maples. After heatwaves, more branches die, and tree cankers become more common.
In 2022, an exceptional drought hit Quebec: many young trees recently planted in Montreal didn’t survive due to lack of watering.
The city is now experimenting with more drought-resistant species (such as bur oak and honey locust). Meanwhile, leaf diseases like tar spot fungus on maples have become more aggressive following wet springs.
Montreal’s urban forestry managers have also noted that winter stress is decreasing (extreme cold spells are now shorter), which could soon encourage the spread of pests like the gypsy moth (Lymantria dispar)—a highly destructive defoliator.
Milan and Cities in Southern Europe :
Cities in Southern Europe are on the front lines of climate change, enduring scorching summers that severely stress urban vegetation.
In 2022, Milan experienced a historic drought combined with extreme heatwaves: nearly 4,000 trees died that summer from lack of water, and many others suffered internal damage (source: iodonna.it).
The following year, in July 2023, a violent storm knocked down hundreds of trees in Milan, including many plane trees and elms already weakened.
Local experts now question whether the traditional tree species planted for over two centuries—like plane trees—are still suited to today’s climate.
As researcher Giorgio Vacchiano explains, “One hundred years isn’t a problem for a tree in normal conditions—but in cities, with the added pressures of climate change, it might be.” (source: ilgiorno.it)
Milan has started replacing some plane trees and elms with hardier alternatives like the European hackberry (Celtis australis, known locally as “bagolaro”), a Mediterranean tree highly tolerant of drought and wind (source: ilgiorno.it).
As for diseases, canker stain of plane trees is wreaking havoc in Northern Italy: several large cities in Lombardy have recently had to remove entire rows of infected plane trees.
In Rome, it’s the Asian lady beetle (Harmonia axyridis), an invasive species from East Asia, that has spread thanks to mild autumns. It threatens ecological balance by transmitting fungal diseases to plants and outcompeting native ladybug species.
Each urban region sees its own “climate-related plant diseases” emerge, but the underlying pattern is the same everywhere: rising temperatures and disrupted water cycles weaken plants and benefit pests.
How to Prevent or Limit Urban Plant Diseases: Practical Advice
Faced with these concerning trends, what can individuals and local governments do to protect urban plants and reduce the impact of diseases?
Here are some practical tips to apply in our gardens, balconies, and streets :
🌱 Choose Resilient Species and Diversify Urban Vegetation :
In a changing climate, it is wise to plant species that are adapted to new conditions. Favor varieties that tolerate heat and drought, especially if your summers are increasingly dry (e.g., albizia, crepe myrtle, or certain Mediterranean oaks do well with low water availability).
Maximize species diversity: avoid planting only one type of tree across an entire neighborhood. A more diverse plant ecosystem is less vulnerable—if one species is targeted by a pest, the damage will be limited if other species nearby are not suitable hosts (source: inspq.qc.ca).
For example, alternating plane trees, lindens, and hackberries along a boulevard helps reduce the risk of mass die-off from a single disease. Likewise, favor native species whenever possible—they support local biodiversity and are generally well adapted to the climate—and avoid invasive exotic plants that may unbalance the local ecosystem.
🌿 Plant and Maintain Wisely :
Whether it’s a street tree or a potted plant on your balcony, a few thoughtful gestures can make a difference.
Plant in the right place, taking into account the plant’s needs (sun exposure, soil type, root space) to minimize stress.
During heatwaves, it’s better to water thoroughly but less frequently, encouraging deep root growth, rather than small daily sprinklings.
Water early in the morning or in the evening to reduce evaporation. During dry spells, mulch at the base of plants helps retain moisture.
Avoid unnecessary pruning or injury during hot periods—wounds are entry points for disease, and pruning cuts heal poorly in dry weather. For lawns, leave the grass slightly taller in summer to keep the soil cooler.
For street trees, cities are increasingly using watering basins and irrigation collars to make every drop of water count.
In autumn, collect and dispose of fallen diseased leaves (e.g., if your maple has black spot fungus) to reduce the risk of reinfection the following spring.
👀 Monitor and Act Quickly :
Early detection is crucial in controlling plant diseases. Regularly inspect your plants for symptoms: suspicious spots on leaves, branch dieback, insect presence, webbing, etc.
The sooner you act, the greater the chances of saving the plant or preventing the spread. For example, if you spot pine processionary cocoons as early as January, have them professionally removed before they hatch (never handle them yourself without protection—they’re dangerous).
You can also install pheromone or eco-traps for certain insects—like trunk collars to catch descending processionary caterpillars.
If in doubt about a symptom, seek a phytosanitary diagnosis from a nursery, garden center, or municipal green space department.
Isolating a sick plant (especially in pots) can sometimes prevent contagion. Finally, limit excessive nitrogen fertilization—rapidly growing trees produce “soft” tissues that attract aphids and scale insects.
🐞 Encourage Natural Biodiversity Helpers :
In eco-friendly urban gardening, it’s essential to work with nature. Encouraging natural predators of pests can help maintain ecological balance.
For example, planting nectar-rich flowers near garden beds attracts ladybugs, hoverflies, and other beneficial insects that eat aphids and scale insects.
Installing birdhouses for tits in parks can help control caterpillars—birds love them, even if they can’t fully eliminate infestations.
A richer urban biodiversity (mixed hedges, flowering meadows, etc.) creates a more resilient ecosystem, less prone to pest outbreaks.
Similarly, living soils rich in microbes help plants defend themselves more effectively—some mycorrhizal fungi even improve drought and disease resistance.
By applying these good practices, each of us can contribute to healthier urban plants—more resistant to diseases and better suited to a changing climate.
That also means greener, more pleasant cities for everyone.
Innovative Digital Tools for Diagnosing Plant Diseases (VerdiVista, Apps, etc.)
Finally, it’s worth noting that digital tools are now supporting urban gardeners.
Several apps and online platforms can identify plant diseases from a photo or short description and provide appropriate treatment advice.
For example, the VerdiVista app offers guided plant diagnostics: by photographing a diseased leaf or flower, users receive suggestions in seconds about possible diseases, likely causes (fungus, insect, deficiency…), and suitable treatment recommendations.
These kinds of tools are great for anyone wondering “What’s that spot on my rose?” or “Why is my ficus dropping leaves?”—giving fast, reliable insights to help intervene early.
The app also includes helpful articles such as :
Ask All Your Plant Questions with VerdiVista’s AI Chat ! 🌱
Conclusion,
The impact of climate change on urban plant diseases is already visible—and it’s likely to intensify in the coming decades.
Urban plants, so vital to our well-being, are under increasing stress, which makes them more vulnerable to both new and existing pests and pathogens.
From processionary caterpillars to fungal cankers, bark beetles to powdery mildew, a growing number of threats are targeting the green infrastructure of our cities.
That’s why it’s urgent to adapt how we manage urban green spaces, to select species better suited to tomorrow’s climate, and to involve citizens in monitoring and preserving plant health.
Cities like Paris, Montreal, and Milan are already taking action—climate plans, diversified reforestation efforts, stronger plant health surveillance, and more.
Protecting the health of urban vegetation also means protecting our own. Our destinies are intertwined.
By cultivating greener, more resilient cities, we reduce the effects of climate disruption while maintaining healthy and livable spaces.
The challenge is great, but at the local level, each of us has a role to play in helping our urban plant companions continue to thrive—despite a changing climate, and for the benefit of all.
