Bug Meets Gall: Interactions between Arthropods and Exobasidium fungi in North America

In the hidden world of plants, a quiet drama unfolds between the green leaves and the invisible organisms that live on them. Fungi are a vast kingdom of life, numbering in the millions, and they often interact with the animal kingdom in ways that are rarely seen. Among these interactions, some fungi do not just infect plants; they change the plants' appearance, creating strange bumps, blisters, or colorful patches that look nothing like the normal leaves or flowers. These changes are caused by a specific group of plant pathogens called Exobasidium. While scientists have long known that these fungi cause disease in important crops like blueberries and tea, and that they create unique symptoms on their hosts, the full story of how these fungi spread and survive has remained a mystery. A key question has lingered: do insects or other small animals play a role in moving these fungal spores from one plant to another, or is the spread purely a matter of wind and rain? Understanding this relationship is crucial because it could reveal how these diseases travel through forests and farms, and whether the tiny creatures we often overlook are actually the drivers behind the infection.

For three years, a team of researchers traveled across North America, from the cool forests of the Pacific Northwest to the tropical gardens of the Dominican Republic, to answer this question. They set out to find out exactly which animals were visiting these strange fungal growths. The team collected hundreds of samples of infected plants, carefully photographing every detail of the galls and leaf spots they found. They looked for any sign of life on the surface of the fungi, from tiny insects to larger bugs, and they brought many of these specimens back to the lab for closer inspection. Their goal was simple but ambitious: to map the community of animals that regularly interact with Exobasidium fungi and to see if these animals were merely passing by or if they were actively feeding on the fungus and helping it spread.

The results of this extensive survey revealed a bustling world of activity on the surface of these fungal infections. The researchers found that nearly forty percent of the fungal samples they examined showed clear signs of being eaten or damaged by invertebrates. This damage came in many forms: some animals chewed on the fungal tissue, others pierced it with sharp mouthparts, and some even tunneled through it. The most common type of damage was chewing, which accounted for the vast majority of the observed injuries. While many of these signs were indirect, the team also managed to photograph and collect animals that were directly touching the fungal spore-producing surfaces. They documented a diverse array of creatures, representing more than ten different orders of arthropods, including spiders, mites, and various types of insects. These interactions were not limited to a single region or climate; they occurred in tropical, subtropical, and temperate environments, suggesting that this relationship is a fundamental part of the fungus's life cycle wherever it grows.

One group of animals stood out above all the others: the larvae of a family of tiny flies known as gall midges. These small, worm-like creatures were found on the fungal galls in large numbers, with some samples hosting more than forty individual larvae at once. In a groundbreaking discovery, the researchers were able to raise these larvae in the laboratory until they turned into adult flies, proving that the larvae were using the fungus as a complete food source to survive and grow. This is the first confirmed evidence that these specific fly larvae feed on Exobasidium fungi. The team also found that when they captured other insects, such as plant bugs and grasshoppers, that were sitting on the fungal galls, the bodies of these insects were covered in live fungal spores. When they washed these spores off the insects and placed them on nutrient-rich plates in the lab, the spores grew into new fungal cultures, confirming that these animals were carrying viable, living fungi on their bodies.

The study suggests that these interactions are far more than accidental encounters. The researchers observed that the fungi often produce bright, flower-like symptoms that might be designed to attract animals, similar to how a real flower attracts a pollinator. However, the team found no evidence that these fungal growths were attracting typical pollinators like bees or butterflies. Instead, the primary visitors were the gall midge larvae and other herbivores that feed on plant tissue. The researchers propose that the fungus might be altering the plant's chemistry to make the infected tissue more nutritious and easier to eat, effectively turning the plant into a buffet for these insects. In return, as the insects move from one infected plant to another, they likely pick up spores on their bodies and transfer them to healthy plants, acting as vectors that spread the disease. While the researchers could not definitively prove that every insect they saw was spreading the fungus, the fact that they found live spores on the bodies of so many different species strongly suggests that these animals play a major role in the fungus's ability to move through the environment.

This work opens a new chapter in our understanding of how plant diseases spread. By showing that a wide variety of insects and other invertebrates regularly feed on and carry Exobasidium fungi, the study challenges the old idea that these diseases spread only through the air. It highlights a complex web of relationships where a fungus, a plant, and a host of tiny animals are all connected. The researchers noted that while they found many clues, there is still much to learn about which specific insects are the most important carriers and how exactly the fungus manipulates the plant to attract them. Future studies will need to dig deeper into these relationships, but for now, the picture is clear: the strange, colorful bumps on our plants are not just signs of disease, but bustling hubs of life where the boundaries between fungus, plant, and animal are constantly being crossed.