April 27, 2012 | Noelle Swan

Every year, fungal and fungal-like infections targeting the world’s major crops of rice, corn, wheat, potatoes, and soybeans destroy enough food to feed 600 million mouths per year, says Sarah Gurr, professor of plant pathology at Oxford University. And that figure solely represents low levels of infection. Epidemic infections could drastically compromise the global food system. This news comes at a time when agricultural producers around the world are attempting to intensify food production in order to meet steady population growth.

Gurr published her findings this month in conjunction with researchers from England’s Imperial College, Harvard Medical School, University of Massachusetts Medical School, and the University of California, Santa Barbara, in the scientific journal, Nature, within a broader paper addressing fungal threats to animal, plant, and ecosystem health.

The Role of Fungi in Plant Disease

Most infectious diseases that affect humans and animals stem from viruses and bacteria. In the plant world, fungi and fungi-like creatures known as oomycetes can be blamed for nearly all the diseases that wipe out crops. When these diseases achieve epidemic proportions, the consequences can be catastrophic, especially if they strike in a region where masses of people heavily rely on a single crop. The infamous 19^th Century Irish Potato Famine can be traced back to a particular oomycete that devastated potato crops and led to mass starvation.

Preventing Famine

In recent years, a particular strain of black stem rust known as Ug99 has decimated wheat crops in parts of Africa and Asia and could conceivably spread elsewhere, leading to widespread famine in the region. As background, stem rust fungi typically attacks the parts of the plant which are above ground. Spores that land on green wheat plants, for example, form a pustule that invades the outer layers of the stalk.[1] Infected plants produce fewer tillers and set fewer seed, and in cases of severe infection the plant may die. Infection can reduce what is an apparently healthy crop about three weeks before harvest into a black tangle of broken stems and shriveled grains by harvest.[2]

Researchers have been scrambling to find a variety of wheat that can resist the Ug99 fungus, since it first popped up in Uganda in 1999. American computer mogul and philanthropist Bill Gates pledged $26.8 million dollars to support the search for a resistant cultivar. Geneticists had success breeding a variety of wheat that is able to resist another strain of stem rust, Sr31, by transferring a gene from rye into a new variety of wheat. Farmers around the world have sown this new wheat variety, since the 1980s, in an effort to improve food security. However, new strains of fungi inevitably emerge, such as Ug99. 

Another approach to preventing plant disease is to apply some form of fungicide that directly attacks the fungus. Many synthetic antifungal treatments target specific genes or single enzymes in the fungus. Gurr cautions that the approach of focusing on single genes and enzymes, both in cultivation of resistant cultivars and development of antifungal applications, may be shortsighted. Fungal spores are dispersed by the wind, rest in the soil, and find their way through the groundwater into other ecosystems. Spores reproduce many times within a single growing season and evolution marches on. The sheer volume of spores (as many as 100,000 million per hectare in a season) provides ample opportunity for inocula to build up in the pathogen and potentially produce variations that are immune to those single target defenses.

“Planting hectare upon hectare of genetically uniform crops, for example, wheat carrying a single resistant gene and treating the crop with a single target site fungicide, pushes the arms race towards emergence of fungal strains that overcome disease resistance and are antifungal resistant too,” Gurr says. In a telephone interview, she hypothesized that focusing on chemistries that more generally target and disrupt cells rather than single target site antifungals may be more effective in the long term.

Cultural practices may also affect the likelihood that crops will succumb to disease. The American Phytopathplogical Society (APS), a non-profit organization for scientists working on the control of plant diseases, suggests that the timing of planting may be a crucial factor for some crops. Soybeans, for instance, are susceptible to soybean rust, which thrives in moist conditions. APS suggests that delaying plantings of soybean crops may reduce this likelihood by ensuring that the most vulnerable stages of development occur during drier months. Alternately, planting early in the season could allow the crops to reach harvest before pathogens have a chance to become established. APS speculates that alterations to soil composition could also be a valuable tool in preventing fungal infections, though there has not yet been sufficient research into this area.

Moving Forward

While more attention has been paid to addressing fungal threats to agricultural crops than in natural ecosystems, Gurr maintains that a considerable amount of work remains if we are to secure our food systems. While developing nations are least prepared to cope with the effects of crop loss, no nation is completely shielded from the potential effects of plant disease.

In the United States, strains of stem rust continue to “pop up” in wheat crops around the nation. The temperate climate of the “ breadbasket states” of the Midwest provides some degree of protection with harsh winters that tend to kill off the fungal spores. However, a changing climate combined with irrigation and low-till farming practices could potentially extend the lifespan (and evolutionary opportunity) of stem rust spores. While an American famine is highly unlikely, the U.S. could still suffer severe consequences should a single fungal infection reach epidemic levels. An epidemic compromising corn production could have devastating impact on the national economy. Corn, in one form or another, has found its way into much of the food consumed by Americans. Were one of the many types of diseases affecting corn to become more than a persistent problem, food prices in the United States could skyrocket. Already, the economic climate has resulted in an increase in the percentage of Americans experiencing food insecurity. Sudden jumps in food prices could dramatically increase those numbers.

In order to tackle these global threats, Gurr says, “We need more pubic awareness as to the importance of fungi, more research funding, and more philanthropic gestures, such as Bill Gates.”

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[1] Source: http://www.sciencedirect.com/science/article/pii/S0065211308002058

[2] Source: http://articles.latimes.com/2009/jun/14/science/sci-wheat-rust14