Coffee leaf rust - the fungus that threatens arabica
The cup you hold in your hand is more fragile than you think. Arabica, the source of most of the world good coffee, has a deadly enemy: a tiny fungus called Hemileia vastatrix, that is coffee leaf rust. It is the one that can strip plantations of their leaves in a few seasons, cut yields and ruin farmers. It is the one that once already changed the course of history, destroying coffee in Ceylon and turning the island into a land of tea. And it is the one that, stoked by a warming climate, remains to this day one of the greatest threats to the future of coffee. Here is a guide to coffee leaf rust: what it is, how it works, how it changed the history of coffee and how farmers and scientists try to defend against it.
What coffee leaf rust is
Coffee leaf rust is a disease of the coffee leaf caused by the fungus Hemileia vastatrix. It appears as yellow-orange, powdery spots on the underside of the leaf, resembling rust on metal, hence the name. An infected leaf in time yellows and falls, and a plant stripped of leaves has no way to carry out photosynthesis, so it weakens, gives less fruit, and in extreme cases dies. Among the species of coffee plant it is precisely arabica that is most susceptible to attack, with possible yield losses reaching as much as thirty percent if the disease is not controlled. Rust does not kill at once, but wears the plant down gradually, season by season. It is a quiet, persistent enemy that can turn a lush, healthy plantation into a forest of bare branches.
How the fungus attacks the plant
Rust spreads through spores, that is microscopic seeds of the fungus, carried by wind, rain, and also on hands and tools. When a spore lands on a moist coffee leaf, it germinates and penetrates inside through the tiny breathing pores on the underside of the leaf. There the fungus grows, feeding on the leaf tissue, until it produces new spores, which spill out as the characteristic orange powder and fly off to infect further plants. The cycle repeats in a flash, especially in heat and humidity, so a single outbreak can engulf a whole plantation in a short time. This is why rust is so hard to control: before the farmer notices the first spots, the fungus has often already spread. It is precisely this ability to spread like an avalanche that makes rust so dangerous.
Ceylon - how rust changed history
The most famous example of the power of rust is Ceylon, today Sri Lanka. In the nineteenth century the island was a great producer of coffee, until in 1869 an epidemic of rust broke out there. Within a few decades the fungus practically destroyed the local coffee crops, bringing down a whole branch of the economy. The planters, having no way to save the coffee, switched to another plant that is not subject to rust: tea. This is why Ceylon, once a land of coffee, became one of the most famous tea regions in the world. This history is a dramatic reminder that a single fungus can rewrite the economic map of a whole country. The Ceylonese switch from coffee to tea remains to this day the loudest example of what rust can do.
The march across the world
Rust was first noticed around 1861 near a lake in East Africa, and its first great epidemic was recorded in 1869, precisely in Ceylon. From there the fungus spread inexorably. By the 1920s it had taken over most of Africa and Asia, everywhere arabica grew. For a long time both Americas, the main producer of coffee today, remained free of rust, protected by the ocean. That peace ended in 1970, when rust was detected in Brazil, as the first infected place in the Western Hemisphere. From there it spilled across Latin America. By 1985 the fungus had reached practically every coffee-growing region in the world. Rust proved an enemy from which there is nowhere to flee.
The great crisis in Latin America
The most dangerous in more recent times was the wave that struck Latin America in 2008 to 2013. In Colombia the field incidence of rust rose from below five percent before 2008 to more than forty percent, and yields fell dramatically. The epidemic that flared in Central America and the Caribbean from 2011 ultimately engulfed crops on about seventy percent of farms in Latin America. The total damage was estimated at more than three billion dollars by 2021. For hundreds of thousands of smallholder farmers it meant lost income, debt and abandoned plantations. This crisis showed that rust is not a story from the nineteenth century, but a still living, real threat to the whole coffee industry and the people who make their living from it.
Why it is worse now
Rust has always liked heat and humidity, and there is more of both. The warming climate means the fungus can develop at ever higher altitudes, where the cold once held it back, and it is precisely higher up that the best arabica grows. Warmer nights and variable rainfall create ideal conditions for the development of spores. To this are added economic factors: when coffee prices fall, farmers cannot afford the care and protection of plantations, so the disease has an easier time. The tangle of climate change and poverty means that rust returns in waves, ever more dangerous. This is why today it is treated as one of the main threats to the future of coffee, and not just as a local problem. The fight against rust is a fight against time and climate at once.
How farmers defend themselves
Several lines of defence are used against rust. The first is fungicides, sprayed onto plantations, effective but costly and requiring repetition. The second is the management of shade and ventilation of the plantation, because the fungus likes humidity, so better air circulation makes its life harder. The third, the most important in the long run, is rust-resistant varieties. Breeders have bred varieties, like Catimor, Sarchimor or the Colombian Castillo, which carry resistance genes, most often coming from the resistant robusta. The problem is that the fungus can in time evolve and break this resistance, as happened in Central America. The fight against rust is therefore a constant arms race between the fungus and humans, one that never ends.
The varieties and hybrids of the future
The most promising weapon today is the modern resistant varieties, including F1 hybrids. These are crosses combining the good flavour of a cultivated variety with the resistance of a wild arabica, giving plants that are strong, productive and resistant to rust, and at the same time often outstanding in the cup. Research organisations around the world are working on such varieties to secure the future of coffee. It is a response to the weakness of the classic arabicas, like Typica or Bourbon, which are exceptionally vulnerable to rust. We cover these new varieties more in F1 hybrid coffee, and the classic varieties in arabica varieties. Resistance to rust is today one of the main criteria by which the coffee of the future is bred. On it may depend whether we keep drinking good arabica.
What it means for the drinker
For the ordinary coffee lover, rust seems a distant problem of farmers, but its effects reach all the way to the cup. It is rust, alongside drought and frost, that stands behind the swings in coffee prices and shortages of particular origins. When an epidemic strikes a region, prices rise and some coffees vanish from the offer. More and more often specialty coffees are described outright as rust-resistant varieties, because for the farmer it is a question of survival. Drinking coffee, it is worth remembering that behind its availability and price stands a quiet fight with a fungus waged on the other side of the world. Awareness of rust is part of understanding how fragile and nature-dependent the whole world of coffee is. It is a reminder that good coffee is nothing to take for granted.
The essentials in brief
Let us gather it up. Coffee leaf rust is a disease caused by the fungus Hemileia vastatrix, giving orange spots on the underside of leaves, which yellow and fall, weakening the plant and cutting the yield by as much as thirty percent. Arabica is especially susceptible to it. The fungus spreads through spores, like an avalanche in heat and humidity. Rust destroyed coffee in Ceylon in the nineteenth century, reached Brazil in 1970, and in 2008 to 2013 triggered a crisis in Latin America, engulfing seventy percent of farms and causing more than three billion dollars in damage. The warming climate stokes it. It is fought with resistant varieties, including F1 hybrids, fungicides and shade management. Now you know why arabica is so fragile and what threatens its future.
Note every coffee in GustoNote - the origin, the variety and the profile you sense. Over time you will start to see how much the world of coffee depends on the fight against threats like rust, and understand more deeply the value of a good cup.