Farm Fungicides Keep Aspergillus Drug Resistance Alive, Study Finds

"As medical mycologists, we had no idea that such closely related substances were being used in such enormous quantities," she said. "It turned out to be tons, under slightly different names and with slightly different chemical structures. That was a real revelation."

The agricultural and medical products target the same enzyme in the fungus. Building on that insight, Ph.D. candidate Hylke Kortenbosch, supervised by Snelders, mapped where two resistant variants of Aspergillus — labeled TR34 and TR46 — live in the air across the Netherlands. TR34 was already present throughout the country. TR46 clustered in agricultural areas with many flower bulb fields and greenhouses.

Then came a further cause for concern. These resistant variants often also prove resistant to fungicides from two other chemical classes used in agriculture — compounds that are not used in medicine at all.

"In agriculture, we are creating an environment in which natural selection favors the fittest fungi, those carrying multiple forms of resistance," Kortenbosch said. "The genes hitchhike together."

How did the backyard experiment work?

To test whether different fungicides could maintain resistance, Kortenbosch used mown, unsprayed grass from his own backyard. In glass bottles, he grew resistant and nonresistant Aspergillus fungi on the grass, with and without the addition of different fungicides.

The results were unambiguous. Even fungicides that are not azoles gave azole-resistant fungi an advantage. The researchers conclude that banning azoles in agriculture alone would not solve the resistance problem quickly, because other fungicides can indirectly preserve resistance to medical treatments.

The findings carry a cautionary weight at a moment when new antifungals are entering both fields. Snelders is deeply concerned about the development and use of new fungicides, including a new class that inhibits an enzyme called DHODH. The antifungal drug olorofim, now being developed to treat serious fungal infections including those caused by Aspergillus, targets that enzyme. So does the agricultural fungicide ipflufenoquin.

"What you really want is to have a separate class of medicines reserved for treating Aspergillus infections in vulnerable patients, without that same class also being used in agriculture," she said.

What happens next?

Snelders argues that tackling resistance requires a broad approach in which every party involved takes responsibility. She acknowledges the difficulty of changing course.

"Stopping the use of these products is not easy. It is a complex problem, and we are locked into a system," she said. "But we want to keep drawing attention to it and continue urging everyone involved to take responsibility. The problem will not go away on its own, and reducing the use of fungicides in agriculture is important."

The study appears as Hylke H. Kortenbosch et al., "Non-triazole agricultural fungicides indirectly select for triazole resistance in the human pathogen Aspergillus fumigatus," in Current Biology (2026). Like much preliminary laboratory work, the grass-bottle experiments model one setting; the broader dynamics of resistance in fields and clinics remain to be tracked. For now, the message from Wageningen is stark: resistance genes travel in company, and retiring one chemical will not send them home.