In the catastrophic Irish potato famine of 1845-46 a million people died and another million emigrated to avoid starvation. The famine was brought about by late blight disease, a fungus ailment of potato plants, which horticulturists say is still the No. 1 potato pest around the world.
Experts also say that new varieties of the fungus are resistant to control by pesticides.
"It's a pretty nasty disease that's been controlled in the United States through the use of pesticides, hygiene and quarantine," says Ray Bressan, professor of horticulture at Purdue University in West Lafayette, Ind. "The fungus has to have the precise weather conditions to develop, but when those are present, a farmer can go into the field later and the plants are all down on the ground, wiped out completely."
Bressan is a member of a team of Purdue researchers that has successfully genetically engineered the first potatoes to resist late blight disease. The researchers say their work should result in plants that can naturally fight off a number of fungal infestations, reducing the amount of pesticides needed to grow these crops.
The researchers already are working to transfer this natural resistance to corn, soybeans, peanuts and other plants susceptible to fungal diseases.
Although the fungus (Phytophthora infestans) has been controlled in the U.S. for the last 140 years, in 1991 a new variant of the fungus, known as A2, came into the country from Mexico or Central America, presumably on tomatoes.
"Now there's a lot more concern about this disease, especially since the two types can mate and recombine," says Bressan.
Dennis Corsini, a U.S. Agricultural Research Service plant pathologist stationed at the University of Idaho, says the significance of genetic resistance to late blight disease in potatoes can't be overstated.
"There are resistant varieties of potatoes out there, but so far none of them has been acceptable to the potato industry, so we're left with using chemicals to fight the disease," says Corsini. "The problem is that strains of Phytophthora have developed resistance to the most effective and commonly used fungicide in several major producing areas."
The researchers say their work, which was published in the March issue of the Proceedings of the National Academy of Science, eventually will lead to a reduction in the use of pesticides. Bressan points out this is of special interest because fungicides are among the most toxic pesticides.
To develop the naturally resistant potato variety, the researchers first identified a gene in tobacco that codes for a protein called osmotin. This protein, which is produced by many plants under stress, was known to kill the fungus in a petri dish.
"We've been working on this gene for 10 years," says Bressan. "Its role as an anti-fungal gene was proven in 1991.
"We'd had suspicions of its anti-fungal characteristics because the DNA sequence of that gene is similar to that of other anti-fungal genes, but other groups developed the tests and showed it did kill the fungus."
The researchers removed the gene from tobacco, combined it with a promoter gene sequence that would increase the amount of protein produced, and moved the genetic material into potato plants. The additional osmotin in the genetically engineered potato plants enables them to fight off the late blight fungal infestations for one or two days.
"Weather conditions have to be just right for the onset of the disease, and a delay of a couple of days could have a large effect on decreasing the disease," Bressan says.