Coffee Leaf Rust

There are many large and embarrassing gaps in my coffee knowledge, and coffee leaf rust is one of those. However, it seems that this is something that is having an increasing impact on producers and the industry so it seemed like time to do some research.

Before having a look at recent changes in global climate, rust resistant varieties and the conflict of crop vs cup we should probably start with exactly what coffee leaf rust is:

Leaf Rust (Roya) is the fungus Hemileia vastatrix. It is likely to originate in East Africa, first documented in 1861 though not studied until it began to affect plants in Sri Lanka in 1869. It likely spurred an increasing preference for tea as a crop on the island as it pretty much destroyed the coffee plantations there over the following ten years.

It didn’t really appear in the Western hemisphere until 1970 when it was spotted in Bahia, Brazil. Articles speculate that it was accidentally brought over from Africa along with cacao seeds. From there it spread quickly into Central America and is an issue in every coffee producing country around the world.

Leaf rust takes its name from the orange coloured lesions visible on the leaves, and the fungus attacks the leaves. The results of the attack are first impaired photosynthesis, then defoliation, followed by decreased yield and ultimately the death of the tree. In terms of crop loss figures range from 20% to 80%.

Certain factors can influence susceptibility to leaf rust. Some varieties are naturally more resistant, and there are some varieties created to have increased resistance to leaf rust (we’ll come to these in a moment). Trees in direct sunlight are more susceptible than those in shade. a The age of the leaves also matter – though whether the younger or older leaves are the more resistant varies with the variety. High yielding varieties are generally more susceptible. The environmental temperature is also directly related to the severity of infection.

I’d been aware of leaf rust from my (limited) visits to producing countries. I remember being shocked as I walked through the section of a farm in Costa Rica that had been affected, seeing the trees stripped bare of all their leaves. It appeared on my radar again over the last year due to its influence on crops – most notably in Colombia and Kenya. The recent increase in crops being damaged is likely linked to the weather – global climate change seems to be the likely reason for increased rain across more of Colombia, and the occurrence of rain in months expected to be dry.

Rain is important because moisture on the leaf helps the fungus spread and infect. It provides moisture for spore germination and also aids in dispersal. Once infection takes hold it is difficult to remove, though there are some options for prevention. The change to the environment means that leaf rust is suddenly appearing in areas that had not previously suffered. You’ll probably be aware that as I write this the price of coffee on the commodities market is very high.

This price spike was initially driven by a significant drop in production in Colombia, caused by weather. The reduced yield wasn’t the result of leaf rust but it seems likely that the effects of leaf rust are likely to continue to damage future production and yields.

Prevention has generally been done by spraying fungicides several months in advance of the harvest, and the most popular appear to be copper fungicides. (I should note again that prevention is very different to a cure.) Spraying should be done during the dry months – the problem is when expected dry months become wet months and the fungicide is washed off.

This brings me to the other solution on offer within the industry: rust resistant varieties. Colombia has recently been promoting a variety called Castillo to its producers, as the new rust resistant variety to come out from its research centre Cenicafe. There has been some debate about the cup quality of Castillo – though I’m told that the winning farm at CoE this year (with one of the highest scores on record) produced a lot of 50% Caturra and 50% Castillo. How a separation of just the Caturra from that farm would taste is a separate and interesting question!

In other countries rust resistant varieties definitely have their detractors – varieties like Riuri 11 or the arabusta hybrid Timor (and its children varieties such as Catimor) often have correlations with poor cup qualities and many remain sceptical about the release of a new rust resistant variety called Batian in Kenya last month.

I’m not sure where we’ll end up, how coffee varieties grown – and the preservation of heirloom varieties – will change and most importantly (and deserving of another worringly serious post) how it will affect the already terrifying growth trend of speciality coffee demand easily outstripping production capacity. I don’t mean to monger fear and doom but I can’t help but be extremely concerned about speciality coffee in the next five years.

*Please note: I wrote this blog post to drive me to learn more about coffee leaf rust. I’ve tried to make sure that I’ve gotten my facts correct – but if you spot a mistake please do post a comment. I will try to update and include any corrections quickly. I hope this helpful and not too dry. If people would like I can post some links to further reading.*

  1. I couldn’t find the explanation for this, but I would conjecture that trees in direct sunlight need to open their stomata (pores) in order to cool and prevent damage. Leaf rust infects by entering the stomata. Furthermore – plants that see more sunlight have more stomata, especially in high moisture areas.  (back)

15 Comments Coffee Leaf Rust

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  2. Paul stack

    At the NBC, we blind cupped Castillo and Tabi (another hybrid) with bourbon and caturra, all Columbian from similar altitude terrain etc. Castillo was at the bottom of everyone’s list. Anecdotal but interesting.

  3. matt daks

    I asked an agronomy friend about the prevalence of rust on plants in direct sun vs those under shade and her response was:

    plants in direct sun are generally more stressed, which makes them more susceptible to a whole list of diseases including leaf rust.

    just another bit of info to chew on…

  4. thompson

    I think Matt is correct. Plants with more sun exposure will have more leaf and fruit bearing, plants in shade would not produce as much fruit or leaves. Shaded plants have less stress load, less energy directed toward fruit ripening, more energy to combat disease. A note too that Roya affects a lot more than coffee – cacao, flowers, corn … funny to read this article as I am in Colombia and it is everywhere. Much better this year than this same time last year, because (at least Huila) was much drier. Rains last year made for a heavy presence of roast. Carlos Imbachi, where I was today, looked much better, but the best time to combat rust is early, when the first spots appear. BTW I am not 100% sure your photo is rust. The dark spots make it look like Ojo de Gayo (called Mancha de Hierro here)

  5. Jason Coffee

    I too was pretty ignorant on this subject. Thanks for the education on the subject.

    The part of this article that struck me the most was your last point about your concern for the Specialty Coffee industry over the next 5 years. James, I would love it if you could expand on that idea and how different factors might contribute in a future article.

  6. Benjamin Turiano

    James, I have recently been doing a lot of research into the agricultural aspects of coffee, and would greatly appreciate any additional links that you found useful.

  7. Andrea Mitchell

    Hello Jim,

    We’ve selected you as our Foodista Drink Blog of the Day for October 22nd! This blog post will be featured on the Foodista homepage for 24 hours. Besides posting your link on the homepage, we will also post a couple shout outs on our Twitter and Facebook pages.

    Since you are now a part of the Foodista Featured Drink Blogger of The Day Community, we’ve created a special badge for you to display proudly on your blog sidebar. I couldn’t find your email on your blog to send you the access code for the special badge, but I want to make sure you get it if you are interested. Please send me an email and I’ll send it right away.

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  8. Michael

    Hello, Jim and thanks, as always, for your signature mixture of humility and provocation. I am sure you are as in the know as anyone in the industry on this issue, but the varietal pedigree of the winning Colombia COE lot you mentioned here has become a source of controversy and something of a battleground issue between advocates for Castillo and coffee-obsessed roasters advocating for more production of heirloom cultivars. Despite some pretty questionable coverage of “origin” in the past, Sprudge has done a great job covering this. (Again, you are probably among the anonymous expert-blogger sources…)

    This is a very challenging issue – one, incidentally, that I think could have significant media potential and be a great candidate for the coherent industry messaging idea you proposed on 6 October. It is certainly a source of concern at the market end of the chain, as you rightly suggest. But imagine what it means at origin in a place like Colombia, where smallholder farmers with potential to produce the kind of quality the Square Mile Coffees of the world are looking for are losing production to coffee rust and considering Castillo to keep production levels commercially viable. You can’t blame a guy for wanting to keep his kids in shoes and in school, after all. It seems that there are few roasters whose quality premiums can compensate for massive production losses at the farm level. And with subsidized access to resistant varietals, the incentive for farmers to favor yields over cup quality is strong – a temptation that may take them out of differentiated markets altogether.



  9. James Hoffmann

    This is really the heart of the issue. We could be looking at a point in time where speciality production starts to actually decline – instead of simply failing to grow at the rate of current rate of demand.

    This is certainly a different ethical/moral aspect to sourcing and how those with direct relationships will continue to discuss projects and crops.

  10. Michael

    Thanks for the reply and your concern around the future of sourcing in all its dimensions. Have you seen the proposal for the “Global Collaborative Research Program on Coffee Quality” first presented at the SCAA Symposium in Anaheim? Kind of a “green revolution” for specialty coffee that seems to take your concern as a point of departure – how to leverage technology to ensure adequate supply of coffees that make the grade for price differentials and keep the single-serves flowing. If so, I would be very interested to hear your take on it.

    In broad terms, I think you point out why it is necessary. I guess my concern with this project (without having had a chance to look carefully at the proposal) would be the same as my concern about any investment in quality upgrades at origin – will it provide the co-investment necessary for smallholder farmers to make the necessary upgrades, and in the end, what will be the return to growers of the added investment of time, energy and resources in improved quality? Will they benefit, or will it “just” keep quality, value and business up at the retail end of the chain?

    Thanks again for taking up this subject.

  11. MarkP

    I should call your attention to an article, published in the Economist last July, titled “Rust in the Bread Basket.” Wheat Rust provides – not in all ways – a parallel to the current Coffee Leaf Rust dilemma. I’m also reminded of a letter to the editor in a later publication. A Kenyan researcher suggested that through mutation induction, a process by which wheat seeds are exposed to gamma rays, subsequent crops displayed drought and disease resistant characteristics (like to the current strain of Wheat Rust). The process is supposed to mimic natural processes – only accelerated. It is also supposed to be environmentally friendly and so forth. Anyways, I’m not a biologist. I only wanted to draw your attention in that direction.

    By the way, I’ve enjoyed the La Loma…

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