Perhaps you’ve heard of something that’s affecting coffee farms, a devastating disease that will tear down your favorite beverage and make coffee in the future no more or perhaps you’ve heard none of this at all and enjoy your cup of coffee with a passive peace of mind. Well, either way, Hemileia vastatrix, informally known as coffee leaf rust, is a disease that has quite a toll on the livelihood of coffee plants and coffee farmers alike. In order to better understand how this disease functions, we recently went on a trip to the mycology department at Purdue University to learn about the work they are doing to fight off this disease. Their department boasts extensive knowledge on rust fungi and works directly with World Coffee Research. They’re doing incredible work and I thought I’d share some of my findings.

Should you care?

Well, by no means do you have to, but the amount of damage that this disease can do to coffee is astounding and to be educated on a huge disease that’s affecting your cup of coffee, I believe, is a worthwhile pursuit.  

What is leaf rust?

Originating in East Africa, same as where Coffea arabica is native, this fungal disease has travelled the world with prolific efficiency. In recent times, take the harvest season of 2011-2012 in Central America, where the disease was first taking hold and fast forward 5 years later with an estimated 1.7 million coffee workers out of jobs and 3.2 billion dollars in damage, it’s impossible to not notice the orange specks on the leaves of the coffee trees in the region. So what exactly is “rust”?

Rust is a type of fungus that consists of over 7,000 different species in the world today. Rusts have huge impacts on a multitude of different agricultural products including wheat (wheat stem rust), oats (oat crown rust), soy (soy bean rust), coffee (coffee leaf rust), and the list goes on. The way rusts have evolved to spread is quite fascinating, as rusts require two different hosts to sexually reproduce.

As an example, there is a rust that is native to the region we live in called cedar-apple rust. This rust requires both a population of Juniperus virginiana (Eastern Red Cedar) as well as a population of Malus sp. (Apples) in order to reproduce. What happens when you cut down your cedar trees? The rust no longer has half of the equation it needs to reproduce, so your apple trees begin to lose the rust over time.

Spots on apple leaf produced by basidiospores (early June). Photo © 2012 David D. Taylor. Photo © 2012 David D. Taylor  SOURCE

Spots on apple leaf produced by basidiospores (early June). Photo © 2012 David D. Taylor. Photo © 2012 David D. Taylor SOURCE

Cedar Apple Rust (Fresh teliohorns on redcedar (mid-March). Photo © 2012 David D. Taylor  SOURCE

Cedar Apple Rust (Fresh teliohorns on redcedar (mid-March). Photo © 2012 David D. Taylor SOURCE

Here’s the anomaly with coffee leaf rust

We don’t know what the alternative host is. Scientists have been searching for the host in areas around Ethiopia, Yemen, and Kenya, to no avail, and believe the alternate host to be extinct. Yet, interestingly enough, the rust is still propagating around the world. How is this possible?

Rusts produce 5 different types of spores, each a separate compartment for reproducing, and the one we will focus on for coffee leaf rust is what are called urediniospores. These spores are able to utilize mitosis (cellular division) to asexually reproduce, thus the need for an alternative host is no longer necessary. This brings us to the question of why is sexual reproduction necessary at all if division via asexual reproduction works just fine?

The answer is genetic diversity.

Asexual reproduction does not introduce new genetics. Sexual reproduction does. So the mutations via asexual reproduction are all extremely similar in their genetic make up. With this in mind, the strains of fungus are all relatively similar, as the only way to reintroduce new genetics is via sexual reproduction. So how do we know these strains are relatively similar? Mycologists at Purdue are tracing the lineage of genetics via genetic mapping to see what strains of rust are in different parts of the world. So interestingly enough, the strains of rust in Brazil seem to be almost genetically identical to the rust found in Indonesia, as the rust strains found in Cameroon are almost identical to the rust found in Central America, yet, in Colombia, it appears there are several different strains. The less strains prevalent in an area, pardon the pun, the less strain on the farmers as it’s easier to combat one specific strain via fungicides or hybrids that are resistant to that strain. Introduce a new strain and you no longer have the strong resistance that that hybrid portrays.

So, with a little science of the fungus out there now, how does the fungus spread asexually on the plant? The spores inoculate the stomata of the leaves (opening on the underside of the leavings for gaseous exchange) and when rain occurs, the spores are flung upwards. So when you see a coffee plant that has been infected by the rust, you’ll notice that the leaves start to fall from the bottom and move upwards, with green leaves on top and a skeleton of branches below.

Two things to note about rust is that it requires high moisture content to propagate (100% water potential) and at higher temperatures it propagates more frequently.

Is there any hope?

Of course there is! Here at Deeper Roots, we’re striving to help with the livelihood and sustainability of coffee farming communities, and one way of continuing that is by utilizing resistant hybrids. These are typically crosses of two different species of coffee, Coffea arabica (arabica) and Coffea canephora (robusta) giving the taste qualities of arabica mixed with the growing and disease resistance of robusta. As the rust mutates via asexual reproduction, these hybrids slowly lose resistance, but it’s a balancing act of reintroducing new genetics into the hybrids and keeping those plants healthy. After all, a healthier plant has a stronger immune system and is less likely to become vulnerable.

Another option is by utilizing a fungicide and applying it on the leaves of the plant, but this requires a lot of constant maintenance, as it you have to reapply throughout the season. Another issue with this application is that the fungicide is not exclusive and kills off all types of fungus, beneficial and negative all the same, and as the mycologist I spoke to said, “The microflora is the first line of defense.” This concept leads to another alternative, which is to inoculate compost tea with a different genus of fungus to outcompete the rust, that isn’t negative to the coffee plant.

So what can WE do about leaf rust?

As coffee roasters and coffee drinkers, we should be paying attention to the varieties and cultivars of coffee that we’re buying, and keep drinking quality coffee that shows off the work of the coffee farmer and values the effort put into coffee agriculture. Oh, and just drink more coffee! Obviously, we can only do so much when we aren’t farming ourselves, but by putting boots on the ground, sourcing intentionally, and attempting to educate as many people as we can on the triumphs and struggles of coffee farmers, we’re doing what we can on this side of supply chain to help lessen the burden.

Braydon Booher - Training/Education

Coffee wasn’t a part of Braydon’s life in the beginning. He couldn’t even stand the smell of it growing up. When he got a job working at Cincinnati’s Findlay market while in school for horticulture, things changed quickly and drastically. He couldn’t get enough of it. As much as he tried to go a different route, coffee wouldn’t let it happen. As a promoter of the saying, “The more you know” Braydon loves teaching people about the entire line of the coffee industry, from a seed to your cup, to brewing and espresso, to anything and everything in between. Emphasizing the work that goes into coffee before he touches it, Braydon loves to talk and drink coffee, while getting his hands dirty within all facets of the coffee industry.

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