The coffee leaf rust – what causes it and 3 of the best solutions

Update January 2022

Coffee rust, Roja in Spanish or simply called coffee leaf rust, it’s a (fungus) coffee disease (native to Africa) that is hurting almost the whole coffee-producing countries. The pathogenic agent of the coffee rust is a fungus called Hemileia vastatrix that hit especially the arabica coffee plant by infected its leaves (the ones that grow not high enough are more hit as humidity makes a good habitat for the fungus to spread over) (robusta variety, on the other hand, isn’t as susceptible as the arabica one, make it a rust-resistant plant).

In this article we are discussing the disease in general and its history, the coffee leaf rust symptoms and signs, what can be done by the farmer to partially control/limitate the spread, the economic impact and some of the worst cases of coffee leaf rust that has spread in the past.
Let’s start with the latter.

SOME OF THE WORST COFFEE LEAF RUST EPIDEMICS IN HISTORY

• 1800: around the late 1800s Srilanka and Java were destroyed;
• 1861: that’s the first case when coffee rust was identified in Ceylon where more than 90 per cent of the coffee crops were destroyed;
• 1970: at that time the coffee rust disease was spreading for the first time in South America, hitting Brazil;
• 2012-2014: it was the turn of the Central American and the Caribbean area; those years are known to be one of the hardest time ever for the American coffee countries, causing severe economic losses. Farmers’ harvest was severely hit with crops reduced by half.

Coffee leaf rust symptoms and signs

Firstly, the spores of the coffee rust are spread by rain or wind and thanks to the climate change the situation is getting worse year by year (on a small scale also birds and insects contribute to the spread of the disease). The unpredictable weather makes it harder for the farmers to take prompt actions as abundant rainfall and strong winds are more present today than ever.

Anyway, every time it starts from small, yellow spots that turn then, into orange, red, and, at the last stage, brown with yellow borders. When a plant is affected by the coffee rust disease, it presents those spots on the underside of its leaves.

After a few hours, defoliation occurs causing leaves to fall into the ground. At this stage, a plant is not any more able to flourish and the cherries on the plant are not going to ripe enough; besides, the harvest is quite compromised over the following years causing heavy toll on the economy of a country and the farmer’s livelihood.

Coffee leaf rust treatments and control measures

Tackling coffee rust disease is not an easy task. However, prompt intervention can reduce losses considerably.

Here’s the most used:

• Rust-resistant plants: enough, choosing for rust-resistant coffee plants seem the best thing to do to avoid the problem at all for now. Colombia was one of the first countries that swapped out old susceptible coffee plants to rust-resistant hybrids such as Tabi or Castillo, to name a few. At that time around 1970, several plants were known to be rust-resistant.Just bear in mind that nobody can tell you if this is a permanent solution.
  It is more likely that those plants that are now rust-resistant, might no longer be like that soonish, as the diseases evolve quickly over the years.
  It has been said that by 2050, there will be half arable land destined for coffee production and the demand will be double. Scientists think that opting for cross-breeding resistant varieties might be the only way to prevent the disease, shortages and economic crises;
• Crop management: tackling the spread need to be done promptly. The fungus infects the coffee plant when there’s moisture on it so it usually occurs when raining. The good thing is that it takes around two days for the fungus to infect the host so there’s a small window for the farmer to take action.
• Air circulation: improving air circulation is considered quite vital to address the disease. Farmers usually improve air circulation by pruning coffee plants and removing weeds.

Coffee leaf rust management: SHADE-GROWN VS. SUN-GROWN

It’s not clear which method is better/risky to reduce/control the spread of the coffee rust disease as both management are affected by the disease in several different ways and so it requires further evaluations.

If on the one hand, a full sun exposure results in much more stressful for the coffee plants, on the other hand, shade-grown management presents several different microclimates making hard to set a unique way of tackle the disease. Air and leaf temperatures are reduced by shade whereas relative humidity and leaf wetness are increased (SEE RELATED HERE https://link.springer.com/article/10.1007/s12571-015-0446-9 ). Also, germination is usually inhibited by the sunlight and spore germination tend to spread easily in shade-grown management.

Coffee leaf rust germination and infection are strongly dependent upon the presence of liquid water. It is assumed that a constant rate of spore deposition and that ungerminated spores did not accumulate on leaves during dry periods, and so in each hour of a wet period, a constant number of spores begin to germinate (SEE RELATED HERE https://royalsocietypublishing.org/doi/full/10.1098/rstb.2015.0458).

LAST CONSIDERATIONS

What is clear is that predicting the first rainfall is paramount for the farmers as they need to plan a series of measures such as how much fertilizers to use.

Appropriate systemic management with preventive fungicide and fertilizers application has seen a reduced risk of spreading no matter the current farm management.

To properly understand the likely impact of climate change on production, and partition out the effects of weather, disease and other factors on coffee yield, a weather-driven coffee yield model is required. For coffee leaf rust and many other fungal plant diseases, temperature and leaf wetness are the most important determinants of infection risk.

Climate reanalysis is the task of the climate research community, but responsibility for recording disease outbreaks, which are the observational data against which models can be tested, lies with the producers, researchers, agribusinesses and plant protection agencies that monitor agricultural systems. It is evident that despite the threat posed by pathogens, serious data gaps remain of where and when outbreaks occur, particularly in the developing world (Bebber DP, Holmes T, Smith D, Gurr SJ. 2014 Economic and physical determinants of the global distributions of crop pests and pathogens).

Edoardo Zanato

I am an Italian coffee lover that pushed for the love of this “amazing drug”  decided to come to London to study about coffee and its different extraction procedures and tastes.