Coffee Leaf Rust

October 4th, 2010

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. 1 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.  ↩︎

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