What turns the healthy green leaves and white flowers of the potato plant black and how do we get rid of it? Over 150 years ago, our ancestors pleaded with God and botanists to provide the answer as they pulled rotten potatoes from the earth.
Unfortunately, the answers came long after more than a million people perished and one million emigrated during the Great Irish Famine.
Today, the fungus that originally travelled from Mexico and brought widespread starvation to Ireland from 1845-1849, phytophthora infestans (type A1), has mated and created strains of a chemical-resistant "super-blight" that is spreading around the world.
"It is the major disease problem affecting potatoes in Ireland particularly and worldwide as well," says Dr Leslie Dowley of Teagasc, the agriculture and food development authority. The potato is a valuable Irish crop grown for use in domestic and export markets but each year Ireland loses approximately 64,000 tonnes to blight.
"The value of the crop in the Republic is £100 million. Seed exports would be in the order of £4 million per annum. Every year there is an £8 million loss, including seeds," says Dr Dowley.
The more virulent and aggressive strains of the fungus seen today are thought to be caused by the introduction of type A2 to older populations of A1. When different mating types exist in nature, it permits sexual recombination, allowing the fungus to mutate and adapt to environmental changes.
"Prior to the 1980s, it was assumed that only the A1 mating type existed outside Mexico and therefore no sexual recombination was possible in nature. In 1991, the A2 mating type was reported in field populations of p. infestans in Ireland," according to a 1991 article in the Irish Journal of Agricultural Research.
Ireland closely monitored the situation and observed a reduction in overall frequency of A2 strains from 35 per cent in 1991 to just 3 per cent by 1994. "It has been between zero and 5 percent for the past few years and we don't expect that to change," Dr Dowley says.
In the early 1990s, the fungus arrived in New York state, destroying crops and spreading to 23 states and into Canada. Parts of Europe have also been affected by phytophthora infestans. "It's mainly a northern European problem and more prevalent in areas with high rainfalls, like Ireland, the west coast of Wales, Cornwall and Scotland. Not so much in Scandinavia, but Spain has also had problems," Dr Dowley explains.
Debate as to how type A2 migrated to Europe is still raging in the scientific community. "My personal belief is that we don't believe there are old and new populations. We believe there is continuous change in the fungus. It is extremely adaptable. This thing is in a perpetual state of flux. It doesn't stay in the same form every year."
There are other characteristics that make the fungus difficult to control. "There is a very small section of the total community that is hermaphrodite. They can crossfertilise themselves. The fungus can also form these resting spores which can go into the ground and stay there for years and affect a potato crop before it even emerges," he says.
One thing is certain: the fungus is a survivor and has become very good at staying one step ahead of attempts to control it. Spraying the potato crop with fungicides is still considered the most effective method of control.
"Once into the tubers there's nearly always a 100 per cent loss on that particular growth," he says. Frequent spraying is therefore essential and must now be repeated 15 to 16 times a year, up from four or five annual treatments in the 1940s and 1950s.
AN eventual cure for blight may ultimately be found in the potato itself. Scientists in Peru, Mexico and Europe are attempting to genetically modify the potato to be more resistant to late blight.
Teagasc's Research Centre in Oak Park, Carlow, is involved in the search for a blight-resistant potato variety using facilities in Latin America.
"Mexico is a unique place and it boasts nearly all of the physiological [blight] races and mating types to be found and therefore it is an ideal place to test new varieties. There are a few varieties from Oak Park there at the moment, and they are doing well," Dr Dowley says.
Until a permanent control is found, the blight that deeply scarred Ireland's people will continue on its destructive path.