Farm chiefs have a narrowing chance to diversify vital crops at rising threat from drought, flood and pests brought by climate change, food researchers warned on Monday.

The world's nearly seven billion people are massively dependent on a dozen or so crops that, thanks to modern agriculture, are intensively cultivated in a tiny number of strains, they said.

When climate change gets into higher gear, many of these strains could be crippled by hotter and drier -- or conversely wetter -- weather and exposed to insects and microbial pests that advance into new habitats.

"Farmers have always adapted, but the pace of change under climate change is going to be much greater than in the past. There's going to be a real need to move fast," Bruce Campbell, head of a research programme called Climate Change, Agriculture and Food Security (CCAFS), told AFP.

In a series of studies, the experts highlighted the risk for staples such as wheat, corn, bananas and cassava.

They described the example of the potato, whose starch is a vital nutrient to hundreds of millions of people.

Although it is a hardy tuber, the potato is vulnerable to heat stress, which curbs growth and starch formation.

Warming will make potato-growing more hazardous in southern Africa and tropical highlands, and encourage the spread of an insect pest, the potato tuber moth, into more northerly latitudes. On the other hand, "late blight" -- the fungus that unleashed the Irish potato famine of the mid-19th century -- will be less of a threat.

At least $7 billion per year in extra funding will be needed for irrigation investments, agricultural research and rural infrastructure, according to the estimates.

To diversify crops, seed banks and genome libraries will play a key role.

Drawing on knowledge of DNA traits in wild plants will help breeders splice in genes to help cope with harsher conditions.

Genetic engineering, contested by many green groups, is also an option but using it "is a question that is left to society to answer," said Campbell cautiously.

The UN's Intergovernmental Panel for Climate Change (IPCC) estimates Earth's surface will probably warm by between 1.8 and 4.0 degrees Celsius (3.24-7.24 degrees Fahrenheit) over the 21st century.

Campbell said many scientists suspect that climate change is already well on the march, as evidenced by shifts in rainfall patterns and growing seasons in many observed locations.

He cautioned against "waiting 10 years" before the world moves to diversify plant strains.

"There are two sorts of changes that are going to happen. One is a gradual temperature increase, the other is the extremes, extremes of heat and floods, and I think they are already here. In the meteorological records, there are so many extremes that are being beaten, although it's very difficult to pin them to climate change."

The adaptation strategies are being published in a compendium book, Crop Adaptation to Climate Change.

The CCAFS programme is partnered by two NGOs, the Consultative Group on International Agricultural Research (CGIAR) and Earth System Science Partnership (ESSP).

Separately, a paper published on Sunday by British bioscientists identifies a mechanism in plant immune defences, a finding that should help crop breeders to produce new disease-resistant varieties.

The main tactic amongst plants to fight off a bacterium or fungus is to kill off cells surrounding the point of invasion, thus depriving the pathogen of food.

The work unveils the role of an enzyme called NADPH oxidase in unleashing the molecular defenses. It appears in the journal Nature.

In September 2009, the UN's Food and Agricultural Organisation (FAO) warned that food production would have to rise by 70 percent to feed the world's population, then around 6.8 billion, when it reaches an expected 9.1 billion in 2050.