If you’re drinking a good cup of coffee as you read this, take a moment to savor it. Hug the cup, if you’re so inclined. After all, coffee may not always be so easy to come by.
Coffee rust is a significant problem in almost every coffee growing region in the world, and in recent years, countries in Central and South America have been hit particularly hard. One of these is Colombia, which cultivates around one million hectares of coffee plants to produce more than 65,000 tons of coffee each year. Consequently, Colombia is one of the biggest coffee producers in the world.
Colombia’s primary coffee crop is the highly valued Coffea arabica. Indeed, Arabica beans are the most popular and widely consumed type of coffee in the world. The combined global production of Arabica accounts for around 70% of the world’s coffee.
Unfortunately, it’s not just coffee drinkers who like it. A particular species of Coffee Leaf Rust (CLR) also has an affinity for C. arabica, and with devastating effect.
What are rust fungi?
They are notorious plant pathogens that disrupt the growth and reproduction of healthy plants. The airborne spores make infection hard to control. Once rust has entered a region, it’s incredibly hard to get rid of.
Most rust fungi are quite host-specific because each has evolved ways to evade its host’s particular defenses. With rare exceptions, most rust fungi harm only a select group of closely related hosts.
Coffea arabica’s particular nemesis is the rust fungus Hemileia vastatrix. It doesn’t usually kill the coffee plant, though if the infection is severe enough the plant may never recover. The rust primarily damages the leaves of the plant, rather than attacking the fruit. At first glance that doesn’t sound so bad given that the fruit is where the coffee beans come from. They are essentially stone-fruit, much like cherries. The ‘pits’ (hard seeds) inside the fruit are what become your cherished coffee beans.
The problem is that a rust infection in the leaves means they can no longer photosynthesize efficiently. Because the plant relies on leaves for food production, the disruption of photosynthesis has a long list of knock-on effects, including the shut down of growth, flowering, and fruiting. In other words, no more coffee beans.
Past outbreaks of H. vastatrix in Colombia alone have reduced coffee production by up to 40% at a given time. If not controlled, the rust has the ability to wipe out coffee production of an entire region. It’s happened before.
Sri Lanka, for example, was the most prominent coffee producer in the world during in the mid-19th century. At the time, Sri Lanka was a British colony, named British Ceylon, and its coffee plantations exported almost one hundred million pounds of coffee each year. That is until a ‘coffee leaf disease’ appeared in the late 1860s and eventually wiped out coffee production. The disease became known as H. vastatrix and the no longer viable coffee crops were replaced with tea crops. Today we associate the word ‘Ceylon’ with tea, not coffee.
So has coffee rust always been a problem? It certainly sounds that way, but it depends on where you ask that question, and the answer has a lot to do with the way plants and their pathogens co-evolve. C. arabica and H. vastatrix both originated in what is now Ethiopia. This is not a coincidence; they evolved together. Like many rust fungi, H. vastatrix is an obligate biotroph. When not in their hardy spore form, these fungi need a host in order to survive. Moreover, they can’t just germinate and take up residence on just any plant because the immune systems of most plants will recognize the presence of an invading pathogen and will quickly mount a defense.
Rust fungi can only invade a host if it isn't recognized. To this end, every species of rust fungi has evolved a form of biochemical camouflage in order to do this, and because plant immune systems can vary from species to species, that means most rust fungi must become specific, as well. Thus, coffee rust has probably always been a problem in Ethiopia, while coffee plants that were then cultivated elsewhere in the world had a good run, until it finally caught up.
Exactly how H. vastatrix got to Sri Lanka from Ethiopia isn’t clear, though trade routes are a likely culprit. Biocontrol efforts kept the rust fungus out of the Americas for another century but even that couldn’t last. By the 1970s it had arrived. We are now in a situation were rust outbreaks are a regrettably familiar story in many coffee growing regions around the world, including Colombia. Increasing global temperatures are unlikely to help the global coffee supply. C. arabica prefers to grow at mild temperatures (preferably 18°C to 22°C), and it’s been estimated that an increase in global temperature of just 1°C would reduce coffee production by about a quarter, and would shrink the number of places in the world coffee could be grown.
Currently, as we cling to our coffee cups, the primary strategy is prevention. Strict biocontrol measures involve early identification of outbreaks, containment measures, quarantine, and so forth. Fungicide is another option, but this is expensive and can be controversial. Moreover, rust spores — the dry powdery substance that forms on infected leaves — are quite hardy and difficult to contain.
But before you kiss that cup of coffee a tearful goodbye, there is a drop or two of hope.
Rust resistant forms of Coffea species have evolved. The only problem is that they don’t taste very good. Typical, right? The upshot though, is that researchers have been able to identify at least nine genes that confer resistance to rust fungi, moreover these species can be bred with C. arabica to improve the taste, which researchers in Colombia have done. The result has had good uptake in Colombia, but the coffee rust is unfortunately evolving, too, and adapting.
Now the race is on to find a way to develop a form of coffee that has enough genetic diversity and resistance genes to truly outsmart H. vastatrix once and for all. The good news is that researchers from the University of California, Davis, announced earlier this year that they had sequenced the genome of C. arabica. This genetic information will no doubt make a big difference.
I’ll leave you with that, and let you get back to your coffee.
Footnote:
In addition to the references indicated by links, I’m also grateful to Dr Louise Morin, a biocontrol expert with CSIRO in Australia, for conversations about rust fungi in general.
