Potato Late Blight as a Case Study

The Irish potato famine began in 1845 when the potato late blight disease decimated crops there. One hundred and fifty years later, in 1995, the famine was commemorated by many events in Ireland and elsewhere. It seems ironic that after a general subsidence of the disease, due to increasingly effective integrated pest management programs, the disease is once again causing concern in potato growing regions around the world. The first appearance had terrible consequences for Ireland and birthed the science of plant pathology. Fortunately the current resurgence is not as devastating as the first occurrence; nonetheless it increases human misery. This disease remains the world's most devastating agricultural disease.

The United States and Canada were initially spared the effects of the most recent outbreaks. However, the situation changed dramatically in the early 1990s. Migrations of the oomycete pathogen Phytophthora infestans initiated the calamity in the 1840s and migrations are once again causing current problems—thus illustrating the point that pathogens of plants as well as of humans are an increasing concern in a world of rapid mass transport.27

During the 1980s and 1990s, problems due to late blight began to worsen worldwide. Late blight is still the potato crop's most devastating disease. More chemicals are applied annually to potatoes worldwide than to any other food crop. It now costs the world $1.8 billion US a year to control potato late blight. By regions, the former Soviet Union tops the expense list at $620 million, followed by Europe at $479 million, Asia at $461 million, Africa at $78 million, North and Central America at $74 million, South America at $91 million, and Oceania at $5 million. Potato harvests worldwide are now severely affected, and the fungus is responsible for losses of 15%, costing about $3.25 billion in lost yields.28'29

The center of origin of the late blight pathogen is in central Mexico, where a highly diverse sexual population with both the A1 and A2 mating types exist. The A1 genotype was the only one that initially spread worldwide. It was not until 1984 that A2 mating types were reported in western Europe. Since then, they have appeared in an increasing number of countries in Asia, North and South America and in Africa. Current scientific information on population dynamics of the fungus and its genetic structure indicates that the frequently reported A2 mating types belong to a new sample of the sexual population that carries both A1 And A2 and which escaped from central Mexico in the late 1970s. This new migration is rapidly displacing the old population. It seems to be more diverse, more aggressive, and is more fit than the old one that is represented mainly by a few clonal lineages of A1. Reports from several countries indicate that the disease has become more severe. It appears earlier in the season, and despite heavy sprays, control is difficult.27,30

The potential threat of this renewed disease may lie in:

1. The spread of new strains with increased fitness, aggressiveness, capacity to produce inoculum (sexual and asexual), and potential to initiate disease earlier;

2. The shortage of chemicals and effective integrated control measures; and

3. Lack of widely accepted, resistant commercial potato varieties.

Given this scenario, a rapid commitment to support research in this area, particularly by industrialized countries, is the most positive option to prevent a potential catastrophe.

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