Guest post: How climate change could accelerate the threat of crop diseases

§ Devastating impacts of crop pathogens

Staple calorie crops, such as cereals and potatoes, are obviously of key importance to food security. We analysed data from the Food and Agriculture Organization of the United Nations (FAO) to determine which are the most important calorie crops around the world. 

In simple terms – based on the provision of calories per capita per day – rice comes in first place. In other words, rice is the mainstay of more diets across the global population than any other crop. It is followed by wheat, sugarcane, maize, soybean and then potatoes. 

Global yield data indicate that these six crops are also grown in the greatest quantities worldwide. In agricultural settings today, three of these top six – wheat, soybean, and potato – are threatened by emerging fungal pathogens.

For the potato, this may be a familiar story. Today’s threat comes from the same organism that caused the devastating Irish potato famine of 1845-49 (also known as the “Great Famine”), which caused the deaths of a million people in Ireland and pushed another million to emigrate.

This is a fungus-like organism called an oomycete. Oomycetes can infect plants, marine life and animals (including humans). The oomycete responsible for Irish potato famine – commonly known as potato or late “blight” – is called Phytophthora infestans. It is appropriately named: from the Greek for plant (phytón) and destruction (phthorá). Infection with Phytophthora causes plant leaves to shrivel and turn brown and the potatoes to rot (it also has a similar effect on tomato plants). 

In 1845, a single strain of this pathogen arrived from Mexico as a newly emerging pathogen in Europe, where potatoes had never encountered it before and were very susceptible. Today, diverse strains are re-emerging worldwide.

§ Threats to global staple food crops

The main pathogens facing wheat and soybean are, perhaps, less well known than potato blight, but their emergence and the threat they pose is equally dramatic.

Soybean is an incredibly important calorie crop both for food and livestock feed and is threatened by a rust fungus which can travel globally on air currents. On arrival where conditions are suitable, soybean rust may cause 80% yield losses. 

The increased frequency of severe weather events under climate change may make this pathogen harder to predict and mitigate against – it is already thought to have hitched a ride from Colombia to the US on Hurricane Ivan in 2004. 

Wheat, meanwhile, is threatened by a number of fungi. In Europe, for example, the most important is Septoria tritici blotch (STB), which costs UK growers alone around €240m per year in yield losses. STB is caused by the fungus Zymoseptoria tritici and is thought to spread on wind-blown spores.

STB took the top spot from another fungus – Stagonospora nodorum blotch – which has declined in prevalence in the UK. Stagonospora is thought to benefit – indirectly – from acid rain and, therefore, has become less prominent as anti-pollution legislation in the 1970s saw levels of atmospheric sulphur fall.

This rather surprising correlation between a wheat pathogen and atmospheric pollution demonstrates the sensitivity of crop pathogens to anthropogenic changes in the environment. 

Wheat is, however, more urgently threatened by “wheat blast” – a disease that has emerged as a novel pathogen in Brazil and the US, having jumped from other grass species. Wheat blast – caused by a wheat-specific strain of the fungus Pyricularia oryzae – reached Asia recently, causing up to 50% crop losses in Bangladesh.

In this context, it is gravely concerning that the global population relies so heavily for calories on so few crops. Due to global trade and changes in climatic suitability for crops, global food supply has become more homogeneous and populations more interdependent. 

For example, our study shows that the area of soybean cultivation has increased dramatically since 1980 – and it is extensively grown in monoculture (as a single crop). In contrast, areas of crops like millet and sorghum have decreased. Wheat, meanwhile, has seen a net drop in cultivation area alongside a relocation of that area globally. Global yields have not fallen, indicating that the new wheat growing areas may be more efficient than the old. 

The table below shows the largest losses (left side) and gains (right side) in harvested crop areas across the world between 1980 and 2007.

Such increases in yields are a possible silver lining of climate change for agriculture. But some of my colleagues at the University of Exeter have demonstrated that, as crops move to new areas, they are followed by their pathogens – responding to changes in climate and host availability. The increased disease risk then offsets gains in yield.

§ Threats to commodity crops

It is also important to bear in mind that food security is not simply about staple crops. Exported commodity crops are the bedrock of many economies. We analysed the FAO data to determine which crops can be described as the world’s most important commodities.

We found that cassava is the top commodity crop in Africa; coffee and bananas in Central and South America; tomatoes in Asia; grapes in Europe; barley in Oceania; and tomatoes and almonds in North America. 

Many of these crops are also under threat from emerging crop pathogens and may be susceptible to others as the climate warms. 

Bananas, for example, are threatened by both “Panama disease” (Fusarium wilt) and “black sigatoka” (Mycosphaerella fijiensis), both of which are emerging fungal diseases. The spread of Panama disease even necessitated the replacement of the globally dominant variety of banana – called “Gros Michel” – with a resistant variety called “Cavendish” in the 1950s.

However, a new strain of Panama disease (known as “TR4”) subsequently emerged in the 1960s and gradually spread worldwide. As the map below illustrates, this spread accelerated recently and TR4 entered the major banana growing country, Colombia, in 2019. This has triggered media reports of impending “bananageddon” and a state of emergency in that country.

§ Modelling future disease risk

Predicting the behaviour of crop pathogens under climate change is not simple. Disease depends on what is known as the “disease triangle”, in which the pathogen is affected by its host and both are affected by environmental factors such as nutrient availability and climate.

The simplest models seek to correlate observed disease to weather data. Such models can be effective in predicting disease risk where simple weather-related factors control disease. 

For example, STB disease of wheat is known to be limited by hot, dry weather, partly because dispersal of the fungal spores relies upon rain splash from leaf to leaf. Predicted increases in the frequency and duration of such spells could be used to predict reduced STB risk. 

In more complex situations, however, these correlative models tend to break down. This leads to a need for “mechanistic models” that take into account the complicated biological workings of the host and pathogen. 

The improved availability of high-resolution climate reanalysis data – which combines observed data and model output – also allows us to derive information about factors such as the temperature of canopy temperature and leaf wetness duration. This data makes the production of complex models a viable option. 

My colleague, Prof Dan Bebber, has produced such models for several fungal diseases. In coffee leaf rust, for example, the model showed that climate change was likely not responsible for a recent outbreak in Colombia. However, a black sigatoka disease model demonstrates that climate change has increased leaf wetness and made temperatures more favourable for this fungus in banana growing regions, increasing the disease risk by 44%.

However, while models can inform us about the importance of climate variables and allow us to test hypotheses about the causes of observed changes in disease prevalence, accurate predictive models still do not exist for many diseases. 

This is because climate interacts with all other aspects of the disease triangle. It affects not only where and when a crop is grown, but determines whether that plant is stressed or healthy, which in turn affects disease resistance. At the same time, it controls pathogen survival and spread. Meanwhile, socioeconomic effects may be felt in agriculture, which may in turn affect plant health and disease outcomes. 

Even more difficult to predict are complex interactions between climate, human behaviour, and non-climate environmental factors such as air pollution. Just as sulphur gases affected wheat pathogens in the 1970s, atmospheric concentrations of other gases – such as ozone and nitrous oxides – also alter plant defenses and pathogen performance. 

As we have seen, pathogens tend to migrate to follow suitable climates, as long as their hosts are present. This means that as humans respond to climate change with altered agricultural practice, crop diseases are likely to keep pace.