A monoculture is a large area with exactly the same type of plant. If a farmer plants a field of wheat, technically, it’s a monoculture, since it is only one variety of one species of plant. But when people talk about the dangers of monoculture, the reference is to vast areas — millions of acres — planted with exactly the same variety of plant. This situation is problematic. If the particular variety of plant is susceptible to a particular disease, and that plant disease strikes, it can spread rapidly over a large area and destroy the crop.
For example, in 1970, the corn crop in the U.S. experienced widespread failure due to just this type of situation.
“A fungus called Bipolaris maydis had showed up on the maize in the Philippines in the 1960s…. By 1970, the disease was showing up in Florida, where a lot of corn is grown. When the fungus started infecting corn plants and making them unsuitable for marketing, the price of those that survived rose steeply. Farmers attempted to control the fungus by spraying their fields with fungicide, but that was expensive and only partly successful. By the time the disease had spread through the American South and into the Great Plains, it had caused a full-blown crisis.
“The Southern corn leaf blight, as it became known, cut production of the essential food by 15 percent. It cost U.S. farmers and consumers hundreds of millions of dollars. And, because cattle are fed a diet heavy in corn and the cost of that corn went up, too, the price of beef rose steeply.” 
Since the 1970s, the monoculture problem has only become worse. A few large multinational corporations dominate the world seed market. They heavily promote their seeds over other options, such as saving seed from a previous crop or buying seed from a smaller seed source.
A different sort of problem is occurring with the world supply of bananas. The type of banana you find in supermarkets is the Cavendish. It is a seedless fruit, which is propagated from banana trees by shoots that grow up from the root system. As a result, there are millions of banana trees that are genetically identical. So they are all equally susceptible to pests or plant diseases.
At the present time, this type of banana is under attack by a type of fungus. The Washington Post has the story: Bye, bye, bananas. They say that the Gros Michel, formerly the top banana in the commercial market, was wiped out by a fungus in the early 20th century. It was replaced by the Cavendish.
“Now, half a century later, a new strain of the disease is threatening the existence of the Cavendish, the banana that replaced the Gros Michel as the world’s top banana export, representing 99 percent of the market, along with a number of banana varieties produced and eaten locally around the world. And there is no known way to stop it — or even contain it.” 
Researchers say it is only a matter of time before the disease spreads throughout the banana-producing regions of the world. Bananas will go up in price, and eventually become largely unavailable.
Now you might be thinking: “That’s not so bad. I don’t even like bananas. I’ll eat apples.” But the same principle — the susceptibility of monocultures to pests and disease — applies to the vast majority of staple crops grown in the U.S. and worldwide. If a plant disease were to spread within any one of the top staple crops in the world (corn, wheat, rice, soy, palm oil, barley), the results would be disastrous.
Currently, there is a plant disease spreading in the wheat fields of Africa and the Middle East. The disease is a type of ‘stem rust’ called Ug99. It was first identified in Uganda in 1999. Almost all of the varieties of wheat that are currently planted in North American and Europe have no significant resistance to Ug99. There are a number of wheat varieties, identified or developed since 1999, that are resistant to Ug99. However, these varieties are not being planted by most farmers. The large agribusinesses are not selling these varieties of wheat, and the farmers are not planting them. It’s a serious problem, currently unfolding. But we don’t know when or if it will spread to North America.
We could develop Ug99 resistant varieties of wheat. In fact, agricultural researchers have already made great progress in this area. The problem is that most wheat comes from a few large agribusinesses, and they’ve shown no interest in transitioning to Ug99 resistant varieties. Can we make a quick transition once the problem occurs? Not at all.
It takes a vast acreage of land to produce seed for the 56 million acres of land planted with wheat in the U.S. each year. To take the small amount of resistant wheat available from researchers, and increase the amount to seed 56 million acres would take at least several years. We would have to go through repeated cycles of planting, harvesting, and then replanting the seed from the previous crop to a larger area of land. And the companies doing so would not be paid any money for those crops until the final crop that is sold for seed — at a price that is probably not much different from ordinary wheat. So there is a strong financial disincentive to prepare for this impending agricultural disaster.
Notice that the list of staple crops above includes palm oil. It probably does not strike you as particularly dangerous if some plant disease were to attack the palm oil monoculture the way it is attacking the banana crop. But palm oil is the number one source of vegetable oil for the world; fully one third of all vegetable oil produced worldwide is from the palm oil tree (palm fruit oil and palm kernel oil). That is a vast amount of dietary fat and calories that would be lost if some disease were to affect that type of plant. And sooner or later, it will happen. No plant is immune to every plant disease or pest.
So the problem of modern agriculture, that we rely very heavily on relatively few crops, and on just a few varieties of each type of crop, is an agricultural and food disaster waiting to happen.
 Fred Powledge, AboutBiodiversity.org, The American corn blight
 FAO.org, Wheat rust tracking site launched, FAO launches Rust SPORE to step up global surveillance of Ug99 strain, 2 June 2010
 Bananas do not grow on trees, but on a type of plant with an unusually thick and tall stem.
 Wikipedia, Cavendish bananas
 Washington Post, Bye, bye, bananas