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Pathogens are at an advantage in this era of unparalleled human move- ment, transportation and interaction. The COVID-19 pandemic has exposed this vulnerability of the globalised world. In just four months, the novel coronavirus (SARS-CoV-2) travelled from China to 200 countries and killed more than 2 million people. Now, imagine a scenario where multiple pathogens make the rounds of the world and cause outbreaks simultaneously. The global food system faces this predicament.
“One can safely assume that multiple plant diseases are going to spread across the world because of globalisation and climate change impacts. This is devastating as climate change is already exacerbating the food crisis across the world,” says Nick Talbot of The Sainsbury Laboratory, UK, that conducts research on plant diseases and resistance. What’s alarming is that these pathogens are fast mutating to invade previously untouched geographies, infect new hosts and evade resistant varieties.
Pushing bananas to verge of extinction
Caused by soil fungus Fusarium oxysporum, wilt infection has ruined banana plantations across the world for well over a century now. It has managed to wipe out almost all major resistant varieties developed so far.
Fusarium oxysporum was first reported in Central America in 1890. By 1960, it rooted itself in tropical America, the Caribbean and West Africa impacting 40,000 hectares (ha) of then dominant variety Gros Michel. The threat was mitigated with a new resistant cultivar, Cavendish, that now represents the image of banana and provides half of the global supply. However, in the 1990s a new strain of the fungus, known as Tropical race 4 or TR4, emerged from Taiwan. It has proved lethal to over 80 per cent of the 1,000-odd banana varieties available worldwide, including Cavendish. Initially restricted to East Asia and some parts of Southeast Asia for two decades, the disease has aggressively hopped continents and spread to 20 countries, including India—the largest producer and consumer of the fruit. The TR4 Task Force, created by UN Food and Agriculture Organization (FAO) in 2013 to manage the outbreak, calls 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 infection is particularly of concern for East African countries such as Burundi, the Democratic Republic of Congo, Rwanda, Tanzania and Uganda where banana is a staple food as well as cash crop. The region has the world’s highest per capita banana consumption of 400 kg to 600 kg.
Hits breadbasket of Africa
Despite suffering from food shortages, last year the Malawi government banned import of unmilled maize from neighbouring Kenya and Tanzania, and for a strong reason. Both countries are affected by maize lethal necrosis, which is difficult to contain. The country’s agriculture ministry says that the disease has no treatment and can cause up to 100 per cent yield loss. The decision forced the World Food Pro-gram, which offers humanitarian assistance to those affected by conflicts in Malawi, to mill maize from Tanzania before supplying it.
The disease gains its virulence from the combination of two viruses—the maize chlorotic mottle virus (MCMV) and sugarcane mosaic virus (SCMV)—which get transmitted by insects and contaminated seeds. According to the Kenya Agricultural and Livestock Research Organization (KALRO), the disease was first reported in the country’s Bomet county in 2011. It has since spread to 12 counties in the Rift Valley region, which is known as the breadbasket of Africa. Martin Mutai, a farmer in Bomet county, narrates the ferocity of the disease. “In 2014, I planted maize on my 0.5 ha farm. The crop stopped growing after a few weeks, despite regular rains in the region. Over the next few weeks the stalks shrivelled and blotches appeared on the leaves. Eventually, the entire crop withered away,” says Mutai, adding that he did uproot sections of the affected plantation to spare the rest, but all was in vain.
“Only fall armyworms can ravage the plantation like that,” says Paul Birir, another farmer from Bomet, who also lost his entire crop to the disease. “It has not spared anyone in the region,” Birir adds.
Puts a dent in export earnings
Caused by fungus Hemileia vastatrix, coffee leaf rust has been around for a century. It is now established in all coffee cultivation areas worldwide and causes large-scale outbreaks in Asia, the Americas and Africa.
Kifle Belachew Bekele, secretary of the Ethiopian Coffee Science Society, tells Down To Earth (DTE) that the disease incurs 35-50 per cent yield loss, and the cost of control with fungicide is very high. However, there has been a surge in the incidence rate of the disease in recent years. In Ethiopia, the leading producer of coffee in Africa, the disease incidence rate has increased by 35.3 per cent, according to a May 2023 study published in Plant Health Cases. Shafi Oumer, deputy director general at Ethiopian Coffee and Tea Authority, says the surge has put a dent on the country’s annual exports and revenues. Ethiopia’s coffee export fell short of its annual targets, US $1.8 billion, generating only $1.3 billion in 2023, says a report by the authority.
The disease is also showing a trend of moving from lowland to mid- and highland coffee growing regions, where it reduces the average annual yield by 49.5 per cent, according to the Plant Health Cases study. In 2012, Mexico, Brazil and Guatemala reported the disease for the first time. In Mexico, the disease wiped out 50 per cent of coffee production in three major coffee producing states Chiapas, Veracruz, and Oaxaca that contribute 80 per cent of total exports, states a June 2020 study published in The Science of the Total Environment.
Creating hunger trap
In a way, cassava holds the solution to Africa’s struggle with food insecurity, poverty and malnutrition. The tuber is a staple food in many African countries. It is drought-tolerant and grows well even on poor soil. However, cassava brown streak disease is fast eliminating that possibility. First spotted in Tanzania in 1935, the disease has spread from eastern Africa to central and southern Africa. Its spread has been particularly aggressive in re-cent years.
In Rwanda, export earnings from roots declined by 40 per cent from $7.9 million in 2016 to $4.7 million in 2017, largely because of a severe outbreak of the disease, states a November 2023 study published in Plant Pathology. Geofrey Ng’andu, agriculture extension officer of Kanchibiya district in Zambia, says the disease entered the country in 2017 and has since spread rapidly. “In 2022, about 500 ha was affected by the disease. In 2023, it increased to 700 ha. This year, close to 1,000 ha stands affected,” says Ng’andu. Slyvester Mulenga, a farmer from Kanchibiya district, says, “We would eat maize mealie meal for three months in a year and cassava mealie meal for the rest of the year. It has been our staple food, but not anymore,” he says. The disease infects 97 per cent of the plants and can lead to hunger and death of many cassava-dependent families, Mulenga says.
A persistent threat
Caused by fungus Phytophthora 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 dis-ease spreads quickly in fields and can result in total crop failure if untreated.
In India, a leading potato producer, the disease continues to cause outbreaks since the 1800s. The losses are more in hilly regions where the crop is grown under rainfed conditions. Joginder Singh, a potato farmer from Sehjo Majra village in Ludhiana district of Punjab, grows commercial varieties of potato on his 28 ha farm. He says the infection wiped out one-third of his crop in November and December 2023. He incurred losses worth Rs 70 lakh. Another farmer from the village, Jatinder Singh, grew potatoes on 140 ha. But the tubers did not develop fully, making them unsuitable for commercial buyers. He says it would take him three years to recover the losses, that too if the weather does not turn against him. Originating from Mexico, the disease has spread to almost all potato-growing regions in the world. According to US-based organisation USABlight that monitors outbreaks of late blight, the disease is a major threat to global food security.
Economically damaging
Citrus tristeza disease, caused by a viral species of the genus Closterovirus, is an economically damaging disease that has changed the course of the citrus industry. First recorded in Argentina in the 1930s, and shortly after in Brazil and other South American countries, the virus eradicated 75 per cent orange trees in Brazil’s Sao Paolo state alone in 1959. Simi-lar devastations were also reported in South Africa, West Africa and California.
In the 1980s, infected citrus plants from endemic countries were shipped in vast numbers to unaffected countries, leading to a large-scale infestation and extending the pandemic. The disease is the prime reason for devastating global pandemic in orange, mandarin, grapefruit and lime orchards, crushing entire industries.