From toddlers in daycare to seedlings in forests, young organisms tend to get sick more easily than adults—a phenomenon that has long puzzled parents and scientists alike.
University of Maryland biologists offer new insights into this mysteriously universal pattern in a study published in the journal Proceedings of the National Academy of Sciences on April 4, 2025. The new study on baby plants shows that fighting disease at a young age often comes at a steep cost to growth and future evolutionary fitness—or their ability to reproduce.
"It's a mystery why young organisms don't evolve stronger disease resistance because getting sick early in life can be deadly," said study co-author Emily Bruns, an assistant professor of biology at UMD. "Our findings suggest that a hidden trade-off is involved, stopping them from being able to completely fight off a disease.
The researchers studied a wild plant called Silene latifolia (commonly known as white campion) and its relationship with a fungal disease called anther-smut that infects it. This disease doesn't kill the plants but prevents them from producing pollen, making them unable to reproduce—much like a "plant STD," as Bruns describes it.
By testing 45 different genetic variations of the Silene plant under controlled settings, the team discovered that plants with stronger disease resistance as seedlings produced significantly fewer flowers and seeds over their lifetime when grown in a disease-free field. Meanwhile, plants with stronger resistance as adults showed no such penalty.
"We found that young plants paid a higher 'cost' for fighting the disease compared with adult plants," Bruns said. "Trying to fight off the fungus was more difficult and resource-consuming for these baby plants. They only have so much energy to spend. If baby plants spend it on disease defense, they can't put it toward future growth."
Using their findings, the researchers created a mathematical model showing that these costs of fighting off pathogens are high enough to prevent the evolution of stronger disease resistance in younger plants. Without these costs, plant families with stronger juvenile resistance would theoretically be able to eliminate the disease entirely. But because developing resistance is so impactful for young plants, they remain vulnerable to infection.
"Some young plants 'pay the cost' and survive into adulthood, but they make fewer flowers, meaning they're less able to reproduce," Bruns explained. "But most remain susceptible as babies, allowing the disease a toehold."
The team was surprised that these costs didn't show up right away. Plants that invested in disease resistance as seedlings looked fine at first but produced dramatically fewer flowers in their second year when reproduction would normally peak.
Interestingly, the researchers also found that male plants suffered much higher costs for disease resistance than female plants. Bruns noted that this may be because male plants produce many more flowers than females to spread their pollen as widely as possible—making the cost of diverting resources to disease resistance especially steep for males.
Bruns believes that the team's findings have implications beyond wild plants. Because juvenile susceptibility drives disease epidemics across many species, understanding the evolutionary mechanisms behind this pattern could inform disease management strategies in agriculture, conservation and public health.
Next, Bruns and the team hope to investigate whether disease resistance costs can be reduced by introducing pathogens to plants slightly later in life when plants establish their first true leaves and no longer rely on stored energy. They also plan to explore whether adult plants with higher disease resistance might protect nearby seedlings by reducing the overall presence of disease presence in a specific area.
"Nature is full of infectious diseases," Bruns said. "Understanding the different checks and balances between hosts and pathogens helps us understand how evolution has shaped these relationships over millions of years."
