It is one of the deadliest coffee diseases and has been known to wipe out coffee farms completely.
But what is coffee leaf rust? How does it affect farms? Is there anything farmers can do to prevent it? Let’s take a look.
What Is Coffee Leaf Rust?
Coffee leaf rust; or Hemileia vastatrix to give it its botanical name, is a type of parasitic fungus. This means the fungus takes its energy and nutrients from a live host, which in this case is a coffee plant.
Generally, Arabica coffee plants are more susceptible to this fungus, however, Robusta plants can also be affected by it.
Time for a quick biology lesson. The organisms within the fungal life cycle asexually produce thousands of tiny spores. These small spores can travel vast distances through water, rain and air. Once the spores begin to land on coffee leaves, they will wait there until the conditions are just right for them. Factors such as temperature will affect the fungus.
The optimal temperature for the fungus to begin germinating (growing) is around 21-25 degrees celsius. If the temperature is too cold (15 degrees Celsius or lower) or too high (35 degrees celsius or higher) the growth of the fungus will be slowed. When the conditions are right, the spores will enter the leaves through the stoma of the plant. This is a tiny opening within the plant which is used for plant gas exchange.
If given the chance, a single spot of rust on a coffee leaf produces 4-6 generations of spores over a 5 month period. If left untreated, this can result in hundreds of thousands of individual spores which will restart the process over again.
I’m sure this gives you an idea of just how destructive and deadly this disease is.
What Does Coffee Leaf Rust Do To A Coffee Plant?
When a coffee plant has been infected by this fungus, the plant’s leaves will have small spots of discolouration. Starting off as an off white colour, these spots will quickly turn yellow and from there will turn a rusty orange colour. This is where the name ‘Coffee Leaf Rust’ comes from. At this stage, the spores have fully matured.
The most common symptom of coffee leaf rust is the leaves of the coffee plant will start to fall off. Although this may seem quite trivial, without any leaves a plant will no longer have the ability to gain energy through photosynthesis. It also will not be able to store the necessary resources to produce fruit.
This is why often there is a loss of yield the year after a farm has been infected by coffee leaf rust.
The History of The Disease
No one knows for sure when this fungus began infecting coffee farms. However, the first recorded infection was in the 1850s.
A large outbreak in Ceylon (now Sri Lanka) completely destroyed the coffee industry on the island, which caused coffee trade to be replaced by tea. By the early 1960s, the disease had spread throughout Indonesia again, putting a stop to coffee trade in some areas.
In 1970 coffee leaf rust made its way to Central and South America. Despite efforts made to stop the fungus, it spread throughout Central and South America over a 15 year period. This was one of the worst outbreaks of coffee leaf rust ever recorded.
We mentioned earlier how the spores could travel through air and wind. Scientists believe this is how it travelled from continent to continent.
How It Can Be Prevented
Unfortunately, due to lack of research, there is no way of completely eradicating coffee leaf rust. However, farmers have their own methods to tackle the fungus and stop it from spreading.
Using Copper-based fungicides can be effective against coffee rust. However, using too much can cause the copper to build up in the soil until it reaches a level too toxic for coffee to grow. For this reason, farmers will only use Copper-based fungicides when the infection has become large enough to justify using it.
It’s often the case that due to the price of Copper-based fungicides, farmers will wait to see how much of their farm is being infected before they invest. However, the problem with this is that by the time the farmer has realised how severe the infection is, it is often too late.
A common mixture to tackle coffee leaf rust is copper sulphate and hydrated calcium. This is known as Bordeaux mixture or Bordo Mix. It is recommended that rather than using this mixture on just the infected areas, it is used throughout the farm. Farmers will often spray an infected area and will then bring the disease to the rest of the farm as they walk through it. By spraying the whole farm, it doesn’t matter if they carried it.
When copper-based fungicides aren’t strong enough to control an outbreak, systemic fungicides are used instead. The plant will absorb this fungicide, which will allow it to move around the plant and fight off the infection. This makes them very effective for helping to treat an infection.
Although systemic fungicides are not organic, they do not affect the coffee’s profile.
However, they do come with some disadvantages. Due to their high performance, systemic fungicides have always been very expensive. Another big disadvantage of using these fungicides is that they can build up in water and soil and can damage ecosystems. Lots of farmers often struggle to decide if they should use systemic fungicides as although they are very effective, their cost and damage to wildlife is also something to think about.
A few years ago, there was a huge outbreak of coffee leaf rust in El Salvador. To tackle this problem, farmers discovered that another fungus, Lecanicillium lecanii (catchy right?) can actually eat the spores on the leaves and destroy the infection. However, the problem with this is that the new fungus can only get to work after the coffee leaf rust fungus has infected a plant.
So although this method does stop the infection, it most likely won’t save the plant.
Unfortunately, coffee leaf rust is still a huge problem for coffee farms around the world. Although currently, research on this fungus is lacking, more people are starting to become aware of it. This is definitely a step in the right direction to finding a cure for this deadly disease.
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