Armed and Dangerous
Researchers are working hard on countermeasures to the fungi, weeds, and viruses that are among the more serious biological threats to food security.
Pest: Puccinia graminis Ug99
Whereabouts: Fifty years ago, stem rust led to the resistant wheat varieties that fueled the Green Revolution—leading many farmers to believe they were done with Puccinia graminis. But in 1998, a dangerous new strain named Ug99 appeared in Uganda (Science, 30 March 2007, p. 1786). By 2004, its spread prompted Green Revolution pioneer Norman Borlaug to launch a global research initiative to address the threat. Ug99 has since shown up in Yemen and Iran and threatens wheat crops throughout the Middle East and West Asia. The big fear: Ug99 could cause famine in Pakistan and India, where small farmers can't afford the fungicides used to control the disease.
Symptoms: The fungus infiltrates stems and plugs up vascular tissue. Of the three common rust diseases, stem rust is the worst because it causes the plant to fall over, so the entire harvest is lost.
Losses: Heavy infections can reduce yields by 40% or more. If it reaches India's Punjab region, losses could reach $3 billion per year; if it reaches the United States, the toll could be $10 billion annually.
Countermeasures: The International Maize and Wheat Improvement Center in Mexico has created 15 resistant wheat varieties, but Ug99 is infamous for quickly overcoming resistance.
Pest: Phytophthora infestans
Crops: Potatoes; also tomatoes and other solanaceous crops
Whereabouts: This funguslike organism occurs wherever farmers grow potatoes.
Symptoms: Most notorious for causing the Irish potato-famine of 1845 to 1851, late blight still ranks as the world's most dangerous potato disease. Spread by spores or by planting infected tubers, it first appears as gray splotches on leaves. In high humidity and moderate temperatures, it can destroy a whole field in a week.
Losses: The International Potato Center in Peru reports that yield losses in developing countries are about $2.75 billion annually. Fungicide applications can total 10% of overall production costs.
Countermeasures: Fungicides work but can be harmful to human health and too costly for poor farmers.
Pest: Mycosphaerella fijiensis
Crops: Bananas, plantains
Whereabouts: This fungus, first detected in Fiji in 1964, is now found in 100 countries in the Americas, Africa and South Asia.
Symptoms: The fungus starts as small flecks on the under-sides of the youngest leaves. They expand into brown streaks that can eventually destroy the leaf, decreasing photosynthesis. Fruit from diseased trees can ripen prematurely during shipping, causing further losses.
Losses: Yields reduced up to 50%.
Countermeasures: Commercial plantations frequently apply cocktails of fungicides, sometimes from airplanes, and remove leaves at a cost of 15% to 50% of the fruit's final retail price.
Pest: Striga hermonthica
Crops: Corn, sorghum, sugarcane, millet, native grasses
Whereabouts: Striga originated in Africa and has since become widespread in the tropics.
Symptoms: This parasitic plant attaches to the host's roots, where it siphons off nutrients and water, stunting the host's growth and causing it to wither. When Striga emerges aboveground, it makes a substance toxic to the host. One plant can produce 50,000 tiny seeds that stick to people and their tools or settle in the soil. Seeds can stay dormant for 15 years.
Losses: In sub-Saharan Africa, Striga infects 20 million to 40 million hectares, reducing yields by 20% to 100%. Losses total about $1 billion per year and affect 100 million people.
Countermeasures: Some Striga-tolerant maize can produce small ears despite being parasitized. But farmers must scramble to destroy plants before they produce seed and plant nonhost crops in affected soils. Another approach is to plant a legume called Desmodium, which secretes a chemical that kills Striga, but that requires using livestock to control the Desmodium. Researchers are looking into applying a fungus to kill the seeds.
Pest: Magnaporthe oryzae
Crops: Rice, 50 species of grasses and sedges
Symptoms: Spores infect plants, particularly when humidity is high, often killing young plants. In older plants, the fungus can spread and prevent seed formation.
Losses: Destruction can be extremely fast but variable, with up to 100% loss in some paddies. Some analysts estimate that each year blast destroys harvests that could feed 60 million people, at a cost of some $66 billion.
Countermeasures: Rice blast is a formidable foe, persisting despite the best control efforts. Farmers can manage the disease by rotating crops, maintaining water levels (too little water promotes infection), and using fertilizers prudently. Resistant cultivars help, but no cultivar can withstand all races of the fungus, and blast tends to overcome resistance in two or three growing seasons. Farmers can also use fungicides.
Pest: Phakopsora pachyrhizi
Crops: At least 31 legume species, notably soybeans
Whereabouts: Native to Asia, soybean rust spread to Australia in the 1980s and reached Africa a decade later. It hit South America in 2001, and Hurricane Ivan carried spores into the United States in 2004. It's now found throughout the Southeastern United States and Mexico (Science, 3 December 2004, p. ).
Symptoms: Infected plants develop small pustules on the under-sides of leaves that spread throughout the plant. In the United States, the invasive vine kudzu is the primary host and vector for soybean rust.
Losses: Yields reduced 10% to 80%.
Countermeasures: Early detection and multiple applications of fungicide.
Crops: Cassava, also called yucca, manioc, and mandioca
Whereabouts: East and Central Africa
Symptoms: This virus is emerging as a major threat to a crop already under siege from cassava mosaic virus. Spread by whiteflies and by cuttings, brown streak virus is more insidious than the mosaic virus because the plant can look healthy even as the disease destroys the edible root. Once confined to lowlands in East Africa, it appeared in Uganda in 2004 and has become a threat throughout sub-Saharan Africa. Disease often appears where farmers have planted cassava varieties resistant to mosaic virus.
Losses: Yields drop by up to 100%. In 2003, economic losses totaled more than $100 million per year. This virus and cassava mosaic virus have been called Africa's biggest threat to food security.
Countermeasures: The International Institute of Tropical Agriculture, based in Nigeria, is developing tolerant varieties whose leaves become diseased but whose roots stay healthy. Early-warning monitoring programs and early harvesting can help reduce the impact of the diseases.