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Plant diseases flourish in city landscapes

Urban life seems to provide an ideal environment for plant diseases like powdery mildew. This particular fungal disease affects many plants, including garden vegetables and roadside weeds.

A recent study has shed light on this phenomenon, uncovering patterns that could help us understand why this disease is more prevalent in cities.

Patterns of plant disease in the city

The research, led by Professor Rachel Penczykowski of Washington University in St. Louis, tracked powdery mildew infestations on common broadleaf weeds. The analysis included 22 sites in the St. Louis area, ranging from the rural Shaw Nature Reserve to Forest Park in the heart of the city.

The researchers discovered a surprising trend: city weeds had significantly more mildew than those in the suburbs or countryside. “Now we need to figure out what’s driving those patterns,” said Penczykowski.

While the specific fungus studied is harmless to other plants or animals, different species of powdery mildew pose significant threats to vegetable gardens, flowers, and vineyards. “It’s a big concern in different agricultural industries,” noted Quinn Fox, a recent PhD graduate and the study’s lead author.

Shade and mildew growth

At any given location, mildew thrived in shaded areas provided by trees or buildings. This was confirmed by experiments at an environmental field station, where potted plants in the shade of solar array panels had more mildew than those in full sun.

However, this finding led to a larger question: if mildew prefers shade, why is it so common in sunny urban areas? “That’s the paradox,” noted Penczykowski.

Urban heat and mildew spread

The heat of city life might be a double-edged sword for mildew. Urban areas, including St. Louis, are warmer due to the “urban heat island” effect, where pavement, buildings, and other structures absorb and retain heat.

While extreme summer heat can be deadly to powdery mildew, the additional warmth may help spores grow faster in the spring.

Additionally, human activities and vehicles may facilitate the spread of spores along busy city roads and between parks.

The role of shade in plant diseases

In follow-up studies, the research team sowed seeds from various locations in a greenhouse and transplanted them into gardens across the St. Louis region.

Each garden was divided into full sun and shade treatments, with plants and temperatures monitored over a year.

The preliminary results suggest that while extra heat is generally detrimental to mildew growth, small patches of shade can still promote infestations, even in hot urban environments.

“We’re starting to understand how heat and other aspects of human activity can change the dynamics of disease in plants,” said Fox.

Urban agriculture risks

St. Louis has a vibrant urban agriculture scene, with numerous backyard and community gardens. However, little research has been done to understand the specific risks of plant diseases to urban agriculture.

“There are a lot of backyard gardens and community gardens. But very little work has been done on understanding the particular risks of plant diseases to agriculture in cities,” noted Penczykowski.

This study highlights the need for further research into how city environments affect plant diseases, which could lead to better management practices and healthier urban gardens.

Global warming and plant diseases 

Global warming, characterized by the increase in Earth’s average surface temperature due to rising levels of greenhouse gases, has significant impacts on plant diseases. 

Distribution of plant diseases

As temperatures rise, the distribution of many plant pathogens is altered. Diseases that were once confined to tropical and subtropical regions are now appearing in temperate zones, broadening the range of affected crops and ecosystems. 

Favorable conditions 

Higher temperatures and changes in precipitation patterns create more favorable conditions for certain pathogens, leading to more severe outbreaks of diseases like rusts, blights, and mildews. 

Life cycles

Warmer climates can speed up the life cycles of pathogens, increasing the number of infection cycles per growing season, which can result in more rapid spread and greater severity of plant diseases.

Heat stress

Heat stress and drought conditions, which are becoming more common due to global warming, weaken plants and make them more susceptible to infections. Stressed plants are less capable of mounting effective defenses against pathogens. 


Additionally, global warming affects interactions between plants and pests, which often serve as vectors for diseases. For instance, increased temperatures can lead to higher populations of aphids, which transmit plant viruses.

Examples of these changes include the spread of coffee rust, caused by the fungus Hemileia vastatrix, which has become more prevalent in higher altitudes of coffee-growing regions due to warmer temperatures.

These shifts highlight the interconnectedness of climate change and agricultural health, emphasizing the need for adaptive management strategies to protect crops and ensure food security.

The study is published in the journal Ecology.


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