While we are bracing for a resurgence of the Covid-19 pandemic in several countries, the next global disease outbreak could strike the plant kingdom. You heard it right. Although the problem has plagued society forever, ‘plant pandemic’—the newer mainstream term given to the catastrophic spread of diseases among plants— has become a cause for concern in the agricultural world, more so after the apparent menace of infectious diseases post Covid-19.
Our crops are just as susceptible to diseases as we are, says Prof A Vaishampayan, emeritus professor of plant genetics and vice-chairman (IQAC), former director, Institute of Agricultural Sciences, Banaras Hindu University (BHU), Varanasi. When crops fail, it threatens food security rendering masses susceptible to starvation.
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Take the 1845-52 Irish potato famine, for example. The pathogen Phytophthora infestans ravaged potatoes, a staple in Ireland then, followed by food insecurity on a large scale. An estimated one million people died of hunger and the remaining had no choice but to take refuge in other parts of Europe for food and work. Incidentally, the pathogen first emerged in 1843 in the US. There, it had a less severe impact. However, Ireland was badly hit given its “dependence on a single crop”, as per research published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS). A lack of political and social will and the British government’s delay to address the problem, too, played a role, the research paper adds.
The case of Ireland clearly shows how vulnerable dependence on a singular crop can make societies in the case of a disease outbreak, adds Vaishampayan.
“Other such examples include the Bengal famine (1942), the bacterial blight of rice in Bihar (1963), coffee rust in Sri Lanka (1867-1870), Downy mildew of grapes in France (1880s), and southern corn leaf blight in the USA (1970),” says Adesh Kumar, associate professor, and head, Department of Plant Pathology, School of Agriculture, Lovely Professional University (LPU), Punjab.
Fusarium oxysporum f. sp. cubense tropical race 4, a fungus, was responsible for the Panama disease of bananas. “The TR4 strain has moved from Asia into Mozambique, Jordan, and, in 2019, Colombia,” the PNAS research states, adding that the “disease threatens the farms of both subsistence farmers in Asia and Africa and major banana plantations”, which can bring down the availability of bananas.
“Different categories of pathogens, such as viruses, bacteria, and fungi have been the reason for plant pandemics. Viruses have become especially potent over the past few years, an example being the viral disease of cereals—barley yellow dwarf luteovirus, which infects barley, rice, maize, wheat, and oats, which sustain the global population,” explains Kumar of LPU, adding: “Similarly, the potato leafroll polerovirus causes loss of 20 million tonne of potatoes annually, which costs a whopping $100 million.”
Impact on humans
Plant pathogens usually do not infect humans directly, and the harm is generally in the form of food insecurity. “In most cases, a sick plant cannot make people sick,” says Supriya Chakraborty, Professor at the School of Life Sciences, Jawaharlal Nehru University (JNU), New Delhi. “It is because the pathogens that cause plant diseases are different in pathogenicity and host range from those that impact humans,” he explains, adding: “Although the ability of plant pathogens to cause sickness among humans and other animals was considered of minor importance, recent evidence of cross-kingdom infection suggests that plant pathogens may have critical impacts on the health and safety of humans as well as animals.” Examples include species of Burkholderia, Pantoea, Rhizobium, and Pseudomonas genera, all of which are known to be plant disease agents but can cause animal disease, Kumar of LPU points out.
Food insecurity is not the only way through which plant diseases can impact humans. “The crop’s quality also comes down,” explains Puja Pandey, assistant professor at the Department of Plant Pathology, BA College of Agriculture, Anand Agricultural University, Gujarat.
Not just that, diseases in plants can also affect humans “through the secretion of toxic metabolites, or mycotoxins by fungi infecting plant products. These fungi produce these mycotoxins which can have direct effects on human and animal health thereby resulting in diseases and death. Some of the fungal species that produce toxic metabolites are Aspergillus flavus, Fusarium spp and Penicillium spp. Poisoning of food due to plant diseases is evil,” explains Chakraborty of JNU.
The cropping pattern changes, too, in some cases, such as in Sri Lanka, where coffee production was replaced with that of tea due to coffee rust in 1867, explains Pandey of BA College of Agriculture. It also changed people’s beverage choices with coffee being replaced by tea. “Similarly, in the southern US, people switched from wheat to corn due to wheat rust,” she says. It can also lead to large-scale migration such as that witnessed during the Irish potato famine, during which many migrated to North America.
A rising trend
The outbreak of diseases in plants is on the rise, experts say. The signs of such devastation are already being witnessed “in banana-dependent communities in South America and those dependent on cassava in Central Africa. We also see this problem in Florida, where citrus growers are facing massive financial ruin from citrus greening,” Chakraborty of JNU says. “Let us take the case of bananas, especially the Cavendish variety. Panama banana disease threatens to reduce the availability of the fruit in some parts of the world because it does not show resistance to a specific pathogen called Fusarium odoratissimum Tropical race 4. The pathogen spread rapidly from Asia to Africa, the Middle East, and most recently South America, affecting bananas grown for export in the United States. Another example of a worrying transnational disease outbreak is banana bunchy top virus disease. It currently occurs in Africa, Asia, Australia and the South Pacific Islands and is the most serious viral disease of bananas,” the expert adds.
In the Indian context, the brown spot of rice was said to be responsible for a one-third reduction in the yield of rice in 1942 leading to the Bengal famine of 1943. The death toll due to famine was reported to be 1.5 to 3.5 million,” says Chakraborty of JNU. Similarly, during 1946-47, the wheat rust outbreak caused a food shortage. In 1946, the bacterial leaf blight of paddy in Bihar wiped the crop completely. In 1985-87, the tungrovirus and blast wreaked havoc on the rice crop in Tamil Nadu. During 1938-39, the red rot of sugarcane occurred on a large scale in Bihar, Punjab, and Uttar Pradesh,” explains Pandey of BA College of Agriculture.
Of fertilisers & pesticides
The green revolution in India in the 1960s ushered in modern technology, such as the use of fertilisers and pesticides, which improved crop yield and ensured food security. However, their excessive use has been much talked about, which degrades the soil considerably. But can these render plants more vulnerable to diseases?
While there are contradictory reports on the impact of fertiliser use in making plants more susceptible to diseases, “pesticides can prevent crop losses. When host resistance is not available, pesticides are widely used to produce immediate mitigation effects on pathogens either by killing them or constraining their growth and reproduction. However, the extensive use of pesticides in agricultural production can degrade and damage the community of microorganisms living in the soil—in the case of overuse or misuse,” Chakraborty of JNU points out.
On the other hand, “genetically modified crops are developed to address the issues of pests and diseases,” highlights Pandey. “A powerful tool for enhancing plants’ resistance against pathogens is genetic engineering,” Chakraborty adds.
Although farming techniques are employed to reduce diseases and increase yield, some can increase the risk of diseases. “For example, irrigation farming, where farmers rely on a highly polluted cross-border river source,” Chakraborty says.
Monoculture, dense cropping, and continuous farming without fallowing are some other ways that incr-ease the inoculum of pathogens.
Effects of climate change
Large-scale floods, droughts, rising temperatures and the accelerated melting of the polar ice are a testimony of the massive impacts of climate change that the world is reeling under. The impact naturally affects the flora, too. In fact, climate change has been linked to a rise in diseases in plants.
“Directly increased temperature makes plants susceptible to several plant pathogens,” explains Kumar of LPU. Adding to that, the increased temperatures and carbon dioxide levels cause physiological changes in plants, which might affect their susceptibility and increase the intensity of diseases, Chakraborty of JNU says, adding: “Not just that, certain disease-causing agents tend to thrive in warmer temperatures, which means climate change is a factor in disease spread, and that plant diseases, both endemic and emerging, spread and are aggravated by climate change.”
Certain situations make plants more susceptible to diseases. First comes endemicity. “The endemic diseases remain constantly present, whether in moderate or severe form, in a certain area. Examples are wart disease of potato in Darjeeling, Karnal bunt of wheat, banana bunchy top in Kerala,” explains Pandey of BA College of Agriculture. Monoculturing and dependence on a singular crop also play evils here. “For example, The late blight of the potato could cause historical damage to Ireland in the 1840s-50s as it was the staple crop grown in monoculture then. The brown spot of rice created a pandemic situation during the 1940s as rice is a staple crop in many parts of Asia. In the US, southern corn leaf blight became an epidemic in the 1970s as corn was the staple crop then,” explains Kumar of LPU.
Beyond borders
Like in the case of Covid-19, national barriers play no role in deterring plant diseases. “In fact, global plant disease outbreaks are increasing in frequency, which should be a cause for concern,” highlights Chakraborty. “The expansion of international trade in plants and plant products can be considered one of the avenues for the introduction of menacing plant diseases to parts of the world where these were formerly absent,” he adds.
Not just international trade, extreme weather events too can be carriers. Take the case of hurricane Ivan (September, 2004), which aided soybean rust movement from Brazil to the United States, as per research published in PNAS. “Plant pathogens may shift hosts and gain the ability to infect new hosts when introduced into new regions. For example, CMV does not naturally occur on cassava in South America but moved into cassava in Africa from an unknown host following the uptake of cassava as a major food crop there,” the researchers point out. “Newly evolved plant pathogenic species can also occur through interspecific hybridisation or mutations within existing pathogen lineages, as illustrated by the emergence of Phytophthora andina in South America and Phytophthora alni in the United Kingdom,” they add.
The way ahead
Just like Covid-19 emerged out of the blue, the same holds true for a plant pandemic. But “researchers and farmers are keeping an eye out,” says Professor Vaishampayan. Here, the role of climate change needs to be factored in. “Vermin and bacteria tend to thrive in warmer temperatures,” he says, which highlights the role of climate change and global warming in plant disease outbreaks.
The Covid-19 pandemic clearly showed our collective lack of preparedness against a major disease outbreak. It has rightfully taught us to keep our guard up at all times and to watch out for pathogens that can impact us, the plants, and the global food supply.
“We are somewhat ready for seed or soil-borne infections. But, what if it arrives by air, floods, or any other way? In that case, our preparation right now is marginal as if a large amount of inoculum enters and crosses boundaries and meets susceptible cultivars of the plant, it may undoubtedly pose a big threat to food security,” says Kumar. Such a threat can be somewhat lessened by “using resistant cultivars, avoiding monoculture, doing crop rotation, addressing climate change and developing effective but less toxic air and water protectants,” he says, on ways to prevent a disease outbreak and mitigating the chances of a plant pandemic.
Proper surveillance systems that warn us of emerging pathogens is crucial, experts say. Also important is to rapidly come up with “solutions to address the threats. However, we somewhat lack a proper framework for that”, says Vaishampayan.
According to a group of researchers, whose commentary was published in the journal PNAS, surveillance, improved detection systems, and global predictive disease modeling are important to mitigate disease outbreaks in plants and protect our global food supply. The idea is to “detect these plant disease outbreak sources early and stop the spread before it becomes a pandemic,” the lead author of the paper writes. Although some diseases like wheat rust and late blight are under global surveillance, others are not monitored routinely. This will require contributions by experts from across fields like crop scientists, crop disease specialists, geneticists, geographers, economists, statisticians, data analysts, and farmers, the researchers noted.
“Plant pathogens, such as the airborne kind, are not restricted by borders. Hence, surveillance plays a vital role here,” says Chakraborty of JNU.
“Visual survey and remote sensing synchronised through satellite data are some of the ways diseases among plants are monitored,” adds Pandey.
Interestingly, many companies have come up with apps that work as crop doctors, helping farmers to detect diseases and recommend the right inputs their crops need. Artificial intelligence (AI) is a key technology here. Plantix is one such app.
Disease monitoring is especially crucial for a country like India where a majority of the working population is still linked to agriculture. “In countries like India where several commodities are exported, the effect of a plant disease pandemic can be shattering. Therefore, plant disease monitoring plays a vital role in safeguarding India’s agri-based economy,” Chakraborty says. “An early diagnosis is necessary to start with control options to prevent any damage on a large scale. AI-based rapid detection systems can be developed to initiate early management practice,” he adds.
Among the proposed solutions to fight plant pandemic, climate change and end world hunger is the use of genetically modified (GM) crops—crops that have been developed to improve yield through the introduction of resistance to plant diseases or of increased tolerance of herbicides.
But are they truly viable? These can play a pivotal role in warding off a plant pandemic, says Pandey of BA College of Agriculture. “Genetically modified or GM crops are developed to ward off pests and diseases,” she adds. “GM crops possess one or more useful traits such as disease resistance, nutritional improvement, abiotic stress tolerance, and insect resistance,” says Kumar of LPU, adding: “Hence, they help play a role in warding off diseases.”
“Genetic engineering is a powerful tool for enhancing resistance against plant pathogens, making plant varieties less susceptible to diseases. Augmenting host plant resistance is the key to combat plant pathogens,” adds Chakraborty of JNU.