Spurred by climate change and global trade, pathogens that cause plant diseases and crop-destroying pests are increasing and spreading far and wide. Higher temperatures are leading to a dramatic expansion in the range of pathogens and pests harmful to food crops, spreading them farther north in the northern hemisphere, and farther south in the southern hemisphere, and into higher altitudes.

A team of crop scientists publishing in Environmental Research Letters aptly wrote: “These range expansions could have substantial economic impacts through increased seed and insecticide costs, decreased yields, and the downstream effects of changes in crop yields variability.” Increasing incidences of pest infestation together with the severity of plant disease outbreaks have led to significant and growing risks to primary productivity, global food insecurity and biodiversity loss in many vulnerable areas of the world. These events have also caused ecological loss. The situation is exacerbated by post-harvest loss caused by pathogenic microorganisms. It is apprehended that any potential yield gains in the next five decades will be offset by climate change-induced pressure caused by both the known as well as the unknown emerging pathogens and pests.

However, to all intents and purposes, plant pests and pathogens have now emerged as a global threat to food security. A diverse range of pathogens, including bacteria, fungi, oomycetes, viruses and nematodes infect plants. They differ in their lifestyles, infection strategies and target plant tissues. Theoretically, climate change may facilitate plant infections in multiple ways including by alternating pathogen evolution, changing hosts-pathogen interaction, vector physiology, and even by facilitating the emergence of new strains of pathogens, which in turn can break down host-plant resistance.

Climate change can also result in the range shifts of pathogens and hosts, which could increase the spread of plant diseases in new areas in a particular zone. While climate warming significantly impacts aspects of the population dynamics of pathogens, such as overwintering and survival, population growth rates or the number of generations of polycyclic species, globalisation and international trades have intensified movement of crop pathogens between continents in the past few decades, increasing the risk of transmissions from disease-prevalent to disease-free regions. A glaring example of trade and transport as drivers of pathogens emergence is the wilt disease of banana, also known as Panama disease or Fusarium Wilt. It is caused by the soil-borne fungus Fusarium oxysporum. The wilt infection has ruined banana plantations across the world for well over a century now. It was first reported in Central America in 1890. By 1960, it rooted itself in tropical America, the Caribbean and West Africa impacting around 40,000 hectare of then dominant variety Gros Michel.

The threat was mitigated to some extent with a new resistant cultivar. But in the 1990s a new strain of the fungus, called Tropical race 4 or TR4, emerged from Taiwan. It proved lethal to the 1000- odd banana varieties available worldwide, including those developed for resistance. Initially, it was restricted to East Asia and some parts of Southeast Asia for two decades, but the disease aggressively hopped continents and spread to 20 countries including India ~ the largest producer and consumer of the fruit. The TR4 Task Force, created by the UN Food and Agriculture Organisation (FAO) in 2013 to manage the outbreak called the strain ‘one of the most aggressive and destructive fungi in the history of agriculture (and) the world’s greatest threat to banana production.’ The Task Force says the most effective approach is to contain the fungus as soon as it is detected and prevent its spread. Currently, fungicides and other chemical and biological control agents have been proven fairly unsuccessful.

In 2018, Australia tried putting in place biosecurity measures to halt the spread of TR4. It introduced strict rules to prevent foreign soil from entering the country. Infested farms were fenced and quarantined. It worked well, but only for a while. Crops developed through genetic modification were also unsuccessful. The most commonly used practices include sanitation and quarantine practices to prevent spread of TR4. Indeed, we are on a slippery peel so far as banana cultivation is concerned. Climate warming can significantly impact the population dynamics of pathogens.

Reduced diurnal temperature decreases the latency period of coffee leaf rust pathogen (fungi) Hemileia vastatrix, promoting rust epidemics in Central America. The rust is a devastating foliar disease which reduces photosynthetic capacity and weakens the tree. It can reduce coffee production from between 30 to 50 per cent. Countries affected include two in North America (1981-2020s), seven countries of Central America (1978-2020s), 11 countries of Africa (1860- 1890s, 1910-1960s, 2020s), four countries of Oceania (1870s, 1890-1910s), eight countries of Asia (1869-1890s, 1940s and 2020s) and US (2020s). Higher temperature along with high humidity is linked to the enhanced disease severity of potato blight pathogen (Phytophthora infestans).

This is a fast spreading and the most devastating disease of potatoes. In the last a few years, new scientific research has overturned the long–held view of agricultural experts that in the absence of drought food crops would be relatively unharmed by rising temperatures. But it is now observed that changes in global temperatures can profoundly impact the occurrence of pathogens in agriculture and natural ecosystems, increasing the risk or exposure to new pests and pathogens. Global warming is projected to increase the abundance of many fungal soil-borne plant pathogens, with significant consequences. Warming temperatures can result in the development of new strains of pathogens that are better adapted and more virulent.

The severity of Fusarium head blight of wheat is likely to increase due to the shift from the milder Fusarium culmorum that prefers cool and wet conditions to the more aggressive Fusarium graminearum that prefers warm and humid conditions. It is also assumed that elevated temperatures can supress plant immunity, leading to increased pathogen infection. Many had thought that the higher CO2 levels might fertilize plant growth enough to counterbalance any yield decreases due to heat stress. But unfortunately intensive research designed to confirm that hypothesis now shows that food crop yields are likely to decline more rapidly with higher temperatures than previously believed. Moreover, weeds and pathogens appear to benefit from extra CO2 much more than food crops. Elevated CO2 levels increase the severity of powdery mildew on cucurbits as well as head blight and blotch on wheat ~ all caused by harmful activities of plant pathogens.

Atmospheric CO2 impacts plant-immune responses and hormone levels that can influence plant-pathogen interactions. Variations in relative humidity and soil moisture are among the main drivers of abundance and infectivity of plant pathogens, and therefore climate induced changes in humanity will likely impact future plant disease outbreaks. Many fungal diseases require high humidity for spore germination and infection of their host plants. High humidity generally promotes the virulence of pathogens infecting aerial plant tissues. Undoubtedly, till now we have a limited understanding of the combined effects of multiple environmental factors on plant-pathogen interactions. But it is clear that in the globalised world, plant pathogens are causing multiple outbreaks, hampering food insecurity for millions. A few more examples seem to be quite relevant.

Cassava, which produces carbohydrates-rich tuberous roots, holds the solution to Africa’s struggle with food insecurity, poverty and malnutrition. The tuber is a staple food to as many as 800 million people across the world. It is drought-tolerant and grows well even in poor soil. Cassava Brown Strek is a viral disease that may appear on the streams of the plant. Also, a dry brown–black necrotic rod of the cassava tube exists, which may progress from a small lesion to the whole root. Finally, the root can become constricted, due to tuber rot, stunting growth. The viral disease was first spotted in Tanzania in 1935, and it has spread from eastern Africa to central and southern Africa. Its spread has been particularly aggressive in recent years. The disease which infects 97 per cent of the plants can lead to hunger and starvation death of many dependent families. It appears that plant viruses have set a hunger trap for human beings.

Caused by the fungus Phytophthira infestans, late blight is a potentially devastating disease of tomato and potato that infects all parts of the plant ~ from leaves to stems to the fruit and tubers. The disease spreads quickly and can result in total crop failure if untreated. In India, a leading potato producer, the disease continues to cause outbreaks since the 1800s. Originating from Mexico, the disease has spread to all potato-growing regions in the world. Citrus tristeza disease, caused by a virus species of the genus, is an economically damaging disease that has changed the course of citrus industries. First recorded in Argentina in the 1930s, and shortly after in other South American countries, the virus eradicated 75 per cent of the orange trees in Brazil’s Sao Palo state in 1959.

Pathogens are at an advantage in the era of unparalleled human movements, transportations and interactions. The Covid-19 pandemic exposed this vulnerability of the globalised world. In just four months, the novel coronavirus (SARS-cov2) travelled from China to 200 countries and killed as many as 2 million people. Taking this hard fact into consideration, if we imagine a scenario where multiple pathogens make the rounds of the world and cause outbreaks, food security can hardly be maintained. Acute starvation would prevail worldwide. Alarmingly, these plant pathogens are fast mutating to infest previously untouched geographies. Are we ready for another pandemic?

(The writer is a retired IAS officer)