#049: Coffee Rust [Archived]

Note: This is an archived post. You can find the current version of this post here.

Coffee Rust (la roya in Spanish) is a disease of coffee plants that is caused by the fungus Hemileia vastatrix. If you enjoy a fine cup of coffee, then this is one fungus you should be very interested in.  The Coffee Rust is currently ravaging coffee trees in Central America, where 60% to 75% of the region’s crops are infected with the pathogen.  The result of this has been a 15% drop in Central America’s coffee output and a corresponding loss of more than 100,000 jobs over the last two years.  The high-end Arabica trees are particularly susceptible to the disease.  America’s major coffee producers have been able to find enough coffee to meet demand without a noticeable impact on price*, but smaller, specialty brewers are having a harder time.  And we haven’t seen the worst of it yet: the United States predicts that Central American coffee production could fall by 15% to 40% over the next few years.

Coffee Rust was first recorded in 1861 on wild coffee trees in the Lake Victoria region of Africa.  Eight years later the disease was identified on the leaves of coffee trees in British-owned Ceylon (now Sri Lanka).  By 1879 it had become too well established in the coffee plantations of Ceylon to eradicate.  Eventually coffee was no longer a profitable enterprise on the island and the British planted tea there instead.  It is not known how the disease was able to travel across the Indian Ocean, but humans probably played a crucial role.  Within a few years the disease had spread to India, Sumatra, and Java.  The resulting decline in coffee production led to the rise of coffee grown in Central and South America.  Strict quarantine protocols kept H. vastatrix from colonizing the Americas until 1970, when it managed to get a foothold in Bahia, Brazil.  At that time almost every coffee tree in the Americas was descended from one rust-susceptible cutting, so the disease spread across the region like wildfire.  Thankfully, Coffee Rust was unable to spread to trees grown at high altitudes, like most of the Arabica trees.  Hawaii is currently one of the only rust-free, coffee-growing areas in the world.

To understand what is driving the current Coffee Rust epidemic, you need to know a little bit about the biology of the pathogen.  Hemileia vastatrix (phylum Basidiomycota, class Pucciniomycetes, order Pucciniales, family unassigned) is a rust fungus and as such is an obligate plant pathogen.  Unlike other rust fungi, H. vastatrix has a simple life cycle: infect, produce spores, repeat.  A spore lands on the leaf of a host plant (in this case Coffea arabica, Coffea canephora, or other Coffea species) and grows hyphae that enter the leaf through a stoma (a small air hole on the underside of a leaf).  Inside the leaf, the hyphae extract nutrients from the host cells, which eventually die.  The earliest sign of the disease is a yellow, circular spot on a leaf.  The spot grows larger as the fungal hyphae extend outward to new areas of the leaf.  If too much of the leaf has been colonized by the fungus, the leaf will drop prematurely.  In severe infections, this can cause defoliation and death of twigs or entire plants.  Once the fungus is big enough it begins producing spores on specialized hyphae that grow out of the leaf’s stomata.  The spore masses appear as orange, powdery spots on the undersides of leaves.  Although H. vastatrix produces both sexual** and asexual spores, only the asexual spores can infect coffee leaves.  The spores are disseminated by both wind and rain.  The effect of rain hitting leaves will propel spores a short distance, while wind can carry the spores long distances.  Once the spores land on a new leaf, they need access to liquid water for one to two days in order to successfully germinate.  Because of this, they usually germinate during the rainy season.  This is advantageous for the fungus because most leaf growth occurs during the rainy season.

Global warming is one of the factors that have been driving the recent epidemic of Coffee Rust in Central America.  Because of higher temperatures, the fungus has been able to infect trees at high altitudes that had previously been off-limits.  Central America has also experienced more rainfall than usual, which is being blamed on climate change.  This means it is more likely that the fungus will be able to find enough water to germinate.  Low genetic diversity is another factor contributing to the spread of Coffee Rust.  Resistant cultivars are being developed and planted, but there are over 40 strains of H. vastatrix and there is no one tree that is resistant to every strain.

There are a number of ways to mitigate the effects of H. vastatrix, but so far none has proved successful at eradicating the fungus.  The simplest method involves pruning the trees to increase spacing between the leaves.  This increases air flow, which allows the leaves to dry quicker, and reduces the number of places that water can collect.  As a result, spores have access to water for less time, making it less likely that they will germinate.  Shade-grown trees appear to be less susceptible thanks to their lower bean yields.  A tree that produces fewer beans has more resources available and is therefore healthier in general.  Fungicides are also effective against Coffee Rust.  However, fungicides are too expensive for small farmers, who produce most of the Arabica beans.  There are also rust-resistant cultivars of coffee trees available, but these alone cannot stop the disease.  A factor that is inhibiting the development of new resistant cultivars is the loss of the coffee tree’s natural habitat.  This has resulted in low genetic diversity in wild coffee trees and researchers do not have many naturally-occurring resistance genes to draw from.

In an attempt to mitigate the epidemic, Guatemala gave out fungicide to small farmers last year, but for many it was too little, too late.  The Guatemalan government is also providing low-interest loans to coffee farmers to help them weather this storm.  Recently, the amount of money in that fund was raised to $100 million and the timeframe was extended to 2026.  Unfortunately, poorer countries in Central America do not have the means to support such initiatives.  To provide help to these people, several coffee companies, including Starbucks and Keurig Green Mountain Inc., have joined forces with USAID and pooled $23 million.  This money will go toward financing new coffee crops and toward training farmers to effectively fight the disease.  By supporting small farmers and local economies, these initiatives will hopefully also mitigate hunger, poverty, violence, drugs, and emigration problems that may result from a collapse of the coffee crop.

* The current rise in coffee prices is mostly due to drought in Brazil.

** The role of the sexual spores is unknown.  They may infect a secondary host, but no such host has been identified.  Alternatively, they may be the remnants of an ancestral state and may now be nonfunctional.

See Further:

http://www.apsnet.org/edcenter/intropp/lessons/fungi/Basidiomycetes/Pages/CoffeeRust.aspx

http://www.npr.org/blogs/thesalt/2014/07/28/335293974/rust-devastates-guatemalas-prime-coffee-crop-and-its-farmers

http://www.nytimes.com/2014/05/06/business/international/fungus-cripples-coffee-production-across-central-america.html?_r=0

http://www.pbs.org/newshour/rundown/devastating-coffee-rust-fungus-raises-prices-high-end-blends/

You can also find stories on the disease from the Wall Street Journal, NBC News, National Geographic, and many, many more.