Coffee leaf rust knows no borders—neither does coffee science

A new study that involved the unprecedented collaboration of coffee researchers and collaborators from 15 countries at 23 sites around the world provides insights into how 29 Coffea arabica varieties respond to coffee leaf rust (CLR), a devastating disease affecting coffee crops worldwide. Varieties respond differently to disease and this research, published in Frontiers in Plant Science, is the most extensive evaluation of arabica coffee varieties under diverse environmental conditions to date.

“No single country or institution can solve the complex challenges facing coffee on its own,” says Dr. Tania Humphrey, the scientific director of World Coffee Research. “This trial shows the power of global collaboration—by pooling data, expertise, and resources across continents, we’re able to generate insights that no single program could uncover alone. It’s a model not just for coffee, but for how agricultural science must evolve to meet the demands of a changing world.”

The threat of Coffee Leaf Rust

Coffee leaf rust, caused by the fungus Hemileia vastatrix, poses a significant threat to coffee production. The disease can lead to defoliation, reduced bean quality, and yield losses ranging from 35% to over 75% in severe cases. First identified in the 19th century, coffee leaf rust has since spread to nearly all coffee-growing regions, including a recent emergence in Hawaii in 2020.

"Identifying host resistance for managing diseases like coffee leaf rust is crucial in enhancing the profitability and sustainability of Kenya’s coffee sector, while promoting environmental health,” says Dr Elijah Gichuru, the Institute Director of Coffee Research Institute, Kenya Agricultural and Livestock Research Organization. “Participating in this global research activity strengthens our efforts to combat coffee rust and reminds us that we’re not alone, and that scientists worldwide are working together to solve shared challenges for the benefit of stakeholders."

Genetic armor: rust resistance isn’t one-size-fits-all

Utilizing coffee varieties resistant to rust is among the most sustainable and cost-effective strategies for farmers to minimize economic losses from the disease. Resistance depends on the genetic makeup of both the coffee plant and the pathogen. To date, over 55 races of CLR have been identified, each interacting differently with various coffee varieties.

Arabica coffee's resistance genes, known as SH genes, have been sourced from different species: SH1, SH2, SH4, and SH5 from C. arabica; SH3 from C. liberica; and SH6 through SH9 from C. canephora (via Timor hybrids). Commercial arabica varieties often contain combinations of these genes, but no single variety has all the known resistance genes. Moreover, the pathogen's continuous evolution necessitates ongoing efforts to identify and integrate new sources of resistance.

"Understanding rust can be complex for farmers. By identifying which varieties really work against rust in real-world conditions, this research gives farmers better tools to sustain their crops, their incomes, their dignified life, and their future,” says Albertino Meza Ojedas, Agricultural Research Manager at CENFROCAFE. The Peruvian cooperative represents 2,800 coffee-growing families and hosted one of the global network sites. “It’s an honor to participate in this global study that brings together scientists from around the world to solve a shared problem.”

One global trial, 18 countries, 31 varieties

In 2015, the International Multilocation Variety Trial (IMLVT) was established by World Coffee Research, together with partner institutions around the world. The trial placed 31 arabica coffee varieties from 11 breeding programs worldwide into 29 research plots in 18 countries. Importantly, the trial sites captured a huge diversity of climatic conditions, from hot and dry sites in Zambia to cooler, wetter sites in places like Gambung in Indonesia. This unprecedented global effort aimed, in part, to assess natural coffee leaf rust infections under real-world field conditions. For the rust study, a subset of data from the global trial network was analyzed, based on data availability and quality. (Data from 23 of the 29 original sites, in 15 of the 18 network countries, was used; performance of 29 of the 31 varieties was analyzed.)

Global best-bets and local champions

While some varieties exhibited high resistance across multiple sites, none were entirely immune, and all showed some susceptibility in certain locations.

Some varieties, like Parainema, Kartila 1, and IPR107, offered a promising balance of high resistance to coffee leaf rust and stable performance across environments, making them good all-around choices for rust resistance. Others, like EC16 and Catigua MG2, showed very high resistance in some locations, but weren’t as high-performing in others (i.e., their performance was not as “stable” across sites). These varieties perform best under specific conditions. Varieties with pure arabica backgrounds generally showed higher susceptibility, whereas those with interspecific introgressions, such as Timor hybrid derivatives, exhibited greater resistance.

The study highlights the need for continuous monitoring and adaptation to evolving disease pressures.

The environment tips the scales

Resistance isn’t determined solely by genetics—it’s also influenced by where a variety is grown, which can encompass factors like which rust races are present, how the trees are cared for and managed, and a given site’s climatic conditions that can affect rust prevalence. The study found strong genotype-by-environment (GxE) effects, meaning a variety that thrives in one place may struggle in another. Four major "mega-environments" and pinpointed specific sites within each were most effective in discriminating for resistance. This information can help breeders more efficiently test new varieties for rust susceptibility.

“The enormous global collaboration behind this study allowed the identification of varieties and their genetic backgrounds that perform well under local conditions, across macro-environments, and globally. Although further research is needed to understand the genetic basis of the resistance and its interaction with the environment (GxE), our results allow breeding programs to narrow down on useful variation and inform breeding strategies,” says Dr. Jorge C. Berny Mier y Teran, WCR Research Scientist for Plant Breeding & Genomics.

Science without borders

Beyond its scientific revelations, the IMLVT trial is evidence of something equally powerful: a global, collaborative coffee scientific community. Researchers from over a dozen countries came together—not just to run trials, but to build a shared foundation for the future of coffee. It’s an exciting example of the power of knowledge and purpose to transcend borders.

The trial equipped national research programs with new tools, training, and infrastructure, strengthening local capacity while linking scientists across continents. Many participating countries now have improved ability to monitor disease threats, conduct robust field trials, and adapt to a changing climate. The collaborative framework established by the IMLVT serves as a model for future international agricultural research initiatives that support the coffee sector to thrive. At the same time, the insights gained from this study provide important insights for developing the next generation of resilient coffee varieties to equip farmers with the tools needed to sustain coffee production and quality in the years to come.