From toddlers battling colds in daycare to seedlings facing fungal infections in forests, young organisms, across species, tend to fall sick more easily than their adult counterparts. This curious phenomenon has long confounded both parents and scientists. Why is it that the very beings who are still growing and developing seem to be more vulnerable to illnesses than fully mature organisms? Now, a groundbreaking study by biologists at the University of Maryland offers new insights into this age-old question. Their research, published in the Proceedings of the National Academy of Sciences on April 4, 2025, explores why young plants, in particular, are so prone to diseases—and why this vulnerability is not easily overcome.
The Puzzle of Juvenile Disease Vulnerability
It’s an undeniable fact that young organisms—whether they are animals, plants, or even microscopic life forms—seem to fall ill more often and with greater severity than adults. This is a trend observed across the natural world. For instance, baby humans, like infants in daycare, often seem to catch every virus that passes through their environment. Similarly, young trees and plants in forests are often the first to fall victim to pests and diseases. So, why is it that organisms don’t evolve stronger disease defenses early in life to combat these threats?
This question has puzzled scientists for decades. The answer lies in a hidden trade-off, as recent research suggests. For young plants, in particular, fighting off disease may come at a steep cost—one that affects their growth, reproductive success, and future evolutionary fitness. The delicate balance between defense and growth helps explain why young organisms remain vulnerable, despite the potential dangers of disease.
The Study: Silene latifolia and Anther-Smut Disease
The University of Maryland team, led by biologist Emily Bruns, turned their attention to the plant world to explore this mystery. They studied Silene latifolia, a wild plant also known as white campion, and its relationship with a fungal pathogen called anther-smut. This disease doesn’t directly kill the plant but prevents it from producing pollen, rendering it infertile. Think of it as a “plant STD,” as Bruns describes it—an infection that prevents reproduction but doesn’t lead to the plant’s immediate demise.
The researchers wanted to understand how the plant’s ability to fight off this pathogen varied at different stages of life, particularly in its early, vulnerable stages. To investigate this, they tested 45 different genetic variations of Silene latifolia under controlled conditions. What they found was a surprising and enlightening pattern: while plants with stronger disease resistance as seedlings showed resilience against the fungal infection, they paid a significant “cost” to their growth and future reproductive capacity.
The Hidden Cost of Early Disease Resistance
The findings of the study were revealing. Plants that invested heavily in disease resistance as seedlings produced fewer flowers and seeds over their lifetime when grown in a disease-free environment. In contrast, plants that developed disease resistance as adults did not exhibit this same penalty.
Bruns explained, “We found that young plants paid a higher ‘cost’ for fighting the disease compared to adult plants. Trying to fend off the fungus was resource-intensive and draining for these baby plants. They only have a limited amount of energy. If they expend that energy on disease defense, they don’t have enough left to invest in future growth or reproduction.”
This trade-off—between disease defense and growth—is a key factor that prevents young plants from evolving stronger disease resistance. The energy and resources required to fight off pathogens early in life are simply too high, preventing these plants from developing robust immune systems. Over time, this creates a cycle where younger plants remain vulnerable to disease despite the evolutionary pressures to adapt.
The Mathematical Model: Explaining the Trade-Off
Building on their observations, the researchers developed a mathematical model to further understand the dynamics of disease resistance in young plants. This model showed that the costs of early disease resistance were high enough to prevent stronger disease defenses from evolving in juvenile plants. In theory, if there were no costs associated with early-life disease resistance, plant populations with stronger juvenile resistance could theoretically eliminate the disease entirely. However, the heavy toll these defenses exacted on the young plants kept them susceptible, creating a persistent vulnerability.
Some plants do manage to survive the early stages of infection and make it to adulthood, but their reproductive capacity is compromised. “They may survive, but they pay the price in terms of fewer flowers and seeds, which means they’re less able to reproduce,” Bruns said. “On the other hand, many plants fail to survive as seedlings, allowing the disease to continue thriving and infecting new generations.”
The Gendered Impact of Disease Resistance
An unexpected finding in the study was the discovery that male plants suffered more severely from the costs of disease resistance than female plants. This finding sheds new light on the differential roles of male and female plants in the evolutionary process. Male plants produce many more flowers than female plants in order to spread their pollen as widely as possible. This reproductive strategy makes males especially vulnerable to the cost of disease resistance, as diverting resources to fighting pathogens takes away from their ability to produce flowers. As a result, the cost of disease defense is disproportionately high for male plants, making them even more susceptible to disease in their early years.
Implications Beyond the Plant World
While the study focused on Silene latifolia and its interaction with a fungal pathogen, the broader implications of the findings extend far beyond wild plants. Juvenile susceptibility to disease is a pattern seen across many species, including animals and humans. The insights gained from studying plants could help explain why juvenile disease vulnerability is so widespread in nature and how it affects the evolutionary strategies of organisms.
Understanding the evolutionary dynamics behind disease resistance in young organisms could have significant implications for disease management in various fields. For example, in agriculture, the knowledge gained could inform strategies to protect crops during their early stages of growth, when they are most vulnerable to pathogens. In conservation biology, this information could help improve efforts to protect young plants and animals in the wild. And in public health, understanding the mechanisms that make juveniles more susceptible to diseases could lead to better disease control strategies for infants, children, and young animals.
Future Directions: Reducing the Costs of Disease Resistance
Looking ahead, Bruns and her team plan to explore ways to reduce the costs associated with early-life disease resistance. One promising avenue of research is introducing pathogens to plants slightly later in life—after they have established their first true leaves and no longer rely on stored energy. This could help reduce the toll that fighting off disease takes on young plants and potentially improve their survival rates.
Additionally, the team is investigating whether adult plants with higher disease resistance can protect nearby seedlings by reducing the overall presence of pathogens in their environment. This research could lead to new strategies for promoting disease resistance in plant populations and, in the future, potentially benefit agriculture and ecological conservation efforts.
The Larger Picture: Evolutionary Checks and Balances
In the end, Bruns emphasizes that the study’s findings offer a deeper understanding of the evolutionary relationship between hosts and pathogens. “Nature is full of infectious diseases,” she noted. “Understanding the different checks and balances between hosts and pathogens helps us understand how evolution has shaped these relationships over millions of years.”
The findings of this study challenge us to think differently about how disease resistance evolves. It’s not just about survival—it’s about balance. Organisms, particularly those in their early stages of life, face trade-offs between survival, growth, and reproduction. As we continue to explore these trade-offs in different species, we may uncover even more hidden dynamics that influence how life on Earth adapts to the threats of disease.
As the world grapples with emerging pathogens, particularly in agriculture and healthcare, understanding the evolutionary mechanisms behind juvenile vulnerability could be key to shaping more effective disease management strategies in the future.
The study, “Disease resistance is more costly at younger ages: An explanation for the maintenance of juvenile susceptibility in a wild plant,” was published in the journal Proceedings of the National Academy of Sciences on April 4, 2025.
Reference: Slowinski, Samuel, Disease resistance is more costly at younger ages: An explanation for the maintenance of juvenile susceptibility in a wild plant, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2419192122. doi.org/10.1073/pnas.2419192122