Fungi, coffee, bananas and climate change

Image: Coffee leaf rust defoliation

Do you start your day with a cup of coffee? Maybe it fuels your entire day, in fact, but remember to thank the farmers who have been in an ongoing battle for over 100 years, fighting the diseases caused by fungi, bacteria, and viruses that could cripple or wipe out production completely.

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Coffee rust (a disease caused by the fungus Hemileia vastatrix) is the biggest threat to your daily cup of coffee: once the coffee bean tree is infected by this fungus, the leaves produce an orange-brown powder, like iron rust, before turning yellow; the tree eventually loses all its leaves and finally the ability to produce beans. Coffee rust was first recorded in 1861, and there is now additional pressure from climate change and wider global trade, which are both responsible for increasing the geographical spread of pathogens, threatening crops (and cups) of coffee further. 

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Why is climate change a factor? The weather becoming warmer and wetter in coffee bean growing areas, creates the perfect environment for fungi to be able to overtake the plant, and stop the production of coffee beans.

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In Colombia, in the 1960s, scientists studied the two varieties of coffee: Coffea arabica and Coffea canephora, with arabica having a delicious flavour that Colombian coffee is world famous for, and canephora (more commonly known as robusta) having a rougher and more bitter taste. Sadly, arabica, is more sensitive to rising temperatures and to fungal disease, and therefore more susceptible to coffee leaf rust, while the robusta is a tougher tree (more robust, in fact!), cheaper to grow and more resistant to disease. Scientists realised this and bred new hybrid varieties that combine both the beautiful taste and aroma of arabica with the resistance genes found in robusta. Realising the risks posed to coffee production, in 2023, Starbucks announced its development of six new varieties of coffee seeds to withstand the effects of climate change and to resist leaf rust.

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Image: Banana and Fusarium wilt

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Likewise, climate change poses a threat to the supply of bananas which are sensitive to temperature rises and disease: the warming world increases the spread of diseases. The fungus causing most concern is Fusarium oxysporum (Fusarium Wilt), a fungal infection that has moved from Australia and Asia, to Africa, and now to South America, and making bananas a more expensive ingredient for your banana bread or banoffee pie. 

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Once a plantation is infected with fusarium wilt, it is very difficult to eradicate and will kill all the banana trees. The spores are extremely resistant: they spread by flooding and strong winds, and abnormal weather patterns allow the spores to move around faster and to cause more destruction.

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The result of fungal infections on bananas, along with the rising price of fertilisers, energy and transport, and the need to grow in a more sustainable way, means the cost of bananas will continue to rise.

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Read more about the disease that could change how we drink coffee, and about how banana prices will go up as global temperatures rise.

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