Food security has long been recognised as a global priority. Included as part of their “Zero Hunger” Sustainable Development Goal, the United Nations hopes to achieve food security and bring an end to malnutrition by 2030. Achieving this is no mean feat. It will involve an almost doubling in agricultural output whilst protecting the natural environment.
Plant diseases pose a major threat to crop productivity worldwide. Estimates vary, but around 20% of crops are lost to disease globally. This loss is not felt equally: poorer regions have fewer resources to monitor and control for disease, and if an epidemic occurs, there are usually fewer alternative crops to fall back on.
Like diseases affecting humans and animals, plant pathogens have adapted to certain environmental conditions. Because of this specialisation, a changing climate could affect the incidence, spread and severity of plant disease epidemics.
In some cases, the changing climate may actually be beneficial to farmers in some regions. Studies of Brazilian banana crops have shown that the favourable habitat for the fungus that causes black Sigatoka disease could decrease by 2050. Black Sigatoka can create losses in yield of up to 40%, so even a small decrease in disease incidence is significant.
Taken from Ghini et al (2007). Maps of Brazil showing the favourable (grey) and unfavourable (white) conditions for Pseudocercospora fijiensis, the fungus which causes black Sigatoka, from present-day ('atual') to 2080.
However, in a meta-study of 70 different papers modelling the links between climate change and disease, such cases – where climate change reduced disease outbreaks – were the minority. Over 60% of the studies in the analysis predicted a worsening of disease outbreaks in several major crops.
Assessing how climate change will affect crop yields is a tricky process. It very much depends on what we choose to measure. For example, studies show that some crops may benefit from more carbon dioxide in the atmosphere as it will increase the rate of photosynthesis, generally increasing growth rates. However, this is a very narrow metric and ignores the other changes that come alongside increased levels of atmospheric carbon. Droughts, increased humidity and warmer air temperatures have all been linked to decreased yield.
Our forests are also threatened by climate change. Increased average winter temperatures and heavier winter rains are believed to be behind the high incidence of root rot caused by Phytophthora subspecies in North America. The loss of these forests is two-fold: as well as providing a habitat for thousands of species, the forests also act as a carbon sink. Without them, climate change is accelerated.
Atmospheric carbon doesn't only affect how diseases spread, but how the plants themselves respond to those diseases. Studies of wheat show that increased carbon dioxide not only increases their susceptibility to disease but increases the virulence of Zymoseptoria tritici, one of the most damaging wheat pathogens. Yet other studies have shown that in different host-disease systems (for example, between soybeans and Peronospora manshurica), disease severity is actually decreased with increased carbon dioxide levels.
Wheat shoots with septoria tritici blotch, caused by the Zymoseptoria tritici fungus. The disease poses a significant threat to global wheat production.
Wheat and soybeans are two of the most widely grown crops around the world. Uncertainty in the links between climate change and crop diseases could have major implications for future food security. Due to the interconnectedness of the agriculture industry, the consequences of an epidemic anywhere are likely to be felt worldwide.