May 30, 2015


Potatoes, rust and mushrooms: let's talk about coffee diseases

This post is about two of my favourite things in life: coffee and fungi.

Here at Machina Espresso, we’re all hardcore coffee geeks. We love coffee because it gives us opportunities to play with cool machines, create beautiful art, teach people about something fascinating, and experiment with different brewing methods and roasteries. Of course, we also love coffee because it just tastes so damn good.

However, loving fungi is different. Most people recoil in horror when I say that I study fungi. But, like coffee, fungi are fascinating things that need a closer look.

What exactly are fungi?

Let’s start by talking about what they aren’t: fungi are not plants, animals, or bacteria. In fact, they have characteristics of all three. Believe it or not, fungi are more closely related to humans than they are to plants. Plants make their own food, while animals and fungi get their nutrition from other things. Some fungi even capture and eat live animals. Some fungi are single-celled organisms that live in the guts of insects, while, on the other end of the scale, the largest organism on planet Earth is a fungus. They are everywhere--inside the leaves of all plants, in the soil under your feet, and, if you’ve been for a stroll outside lately, you are likely covered in microscopic fungal spores.

Fungi are necessary for life. “Good” fungi are essential for baking bread, brewing beer, making cheese, growing plants, synthesizing antibiotics and the Grateful Dead. Where would we be without fungi? That said, there are many “bad” fungi out there as well. These cause a myriad of human diseases, large-scale frog extinctions, food-moulds, insect insanity (no, really!) and a ridiculous amount of unsightly and economically disastrous plant diseases.

The Coffee Plant

That brings us to genus Coffea--the coffee plant. Coffea is surprisingly able to withstand fungal infection because of the unbelievable number of chemicals in its tissues (if you don’t believe me, check out the “Chemicals” page on Coffee Chemistry). These chemicals were evolved to deter insects, bacteria, and fungi from eating the plant--but they’re also what make coffee taste so interesting when roasted. Beer, wine, tea, chocolate--these delicacies don’t even come close to coffee in number of complex flavor molecules. However, some fungi have evolved ways of getting around coffee’s defences and have caused large-scale destruction of coffee plantations.

Coffee diseases

In some Rwandan and Burundian beans, a mysterious defect causes the coffee to taste... a little odd. A parasite attacks the coffee beans without leaving any visible signs that something is amiss until the coffee is brewed--when the grinds come into contact with water, they release a strong, pungent smell of raw potatoes that is impossible to overlook. For a long time, researchers thought a fungus caused the defect because some fungi produce the same chemical smell. Although there is still controversy on the subject, most researchers now agree that potato defect is caused by a bacterium, not a fungus. So fungi are off the hook for this disease.

However, there are an astounding 38 fungal diseases specific to Coffea. Many of the fungi are parasites and require the plant to complete their life cycle; in other words, if the fungal spore doesn’t land on a coffee plant, it won’t ever be anything but a spore.

Coffee berry disease (CBD) is one of the most common and devastating coffee diseases. It starts out as brown spots on the young berry, which is why it's sometimes called brown blight. It’s caused by a fungus called Colletotrichum and essentially transforms a healthy berry into a black, dried pouch. It originated in Kenya and is most problematic in central Africa, which is why coffee plantations there often spray copper-based fungicides on the young berries more than in other parts of the world.

Most coffee pathogens “rust” fungi, which means their bodies look like spots of rust on the surface of the leaf. These spots prevent the leaf from photosynthesis and eventually destroy the leaf tissues and cause the leaf to drop from the plant.

Healthy Coffea leaves

Rusty Coffea leaves

The most famous rust is Hemileia vastatrix, often called roya. “Vastatrix” means “waster”, or “devastator” in Latin, which makes it an excellent choice of name for this parasite. Over the 150 years since it was named, roya has spread from East Africa (it probably originated in Ethiopia, like coffee itself) to all tropics worldwide. It was responsible for collapse of the coffee industry in Sri Lanka in 1890, a central-American and Carribean epidemic in 2012 (remember when specialty coffees were incredibly expensive a couple years ago?), and in 2013 Guatemala declared a state of emergency due to roya destroying nearly its entire coffee output. Roya is still a huge problem today, although fungi-resistant strains are now grown worldwide.

High-altitude, shade-grown, and disease-resistant

There is a lot of confusion in the coffee community about why shade-grown and high-altitude coffees are so important (and why they’re more expensive). These are two separate things, but they both have an effect on fungal disease. Although all coffee is tropical, high-altitude plants are grown between about 900-1800 meters. At that altitude, the air is cooler, the wind is higher and the plants mature slower (=more complex, flavorful beans). High-altitude coffees generally see less fungal disease because the tropical fungi are less adapted to the colder, drier climate.

Shade-grown coffee is mostly an ecological issue, because coffee grown in the shade of other kinds of trees essentially means the coffee is not coming from a gigantic monoculture plantation, and this is better for forest diversity and health. But shade-grown coffee may be more vulnerable to fungal infection because the fungi need a wet, stable environment to grow. Coffee grown in full-sun will be drier (and easier to spray with fungicides) and less susceptible to fungal infection. However, shade-grown coffee grows more slowly and in better soil, which makes the plant more robust and less prone to infection. After you weigh up the pros and cons, it’s still better to spring for the high-altitude, shade-grown coffee for more sustainability, and leave the fungi issues to the farmers.

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