The humble spud, we are told, is the best package of nutrition in the world, being rich in calories, minerals, vitamins and protein and virtually free of fat. It is a member, my informants also tell me, of the Solanaceae family, which includes tomatoes, aubergines and peppers. There are about 150 species in the wild, but only one, Solanum tuberosum, is grown outside the Andes. Within this species, however, more than 600 different varieties are known in Europe. The potato first made its appearance in Europe about 1570, having been brought from South America by the Spaniards. Traditional wisdom has it that Sir Walter Raleigh introduced the crop to Ireland about 1585. As a staple diet for the native population, it proved to be ideal.
The only problem with potatoes is their susceptibility to disease, the worst of which, Phytophtora infestans, is known more commonly as blight. Before 1842, potato blight was only known in Mexico, where it began in the Toluca Valley on the central plateau. That year, however, it turned up in New Hampshire and Vermont and, three years later it appeared in Belgium.
By mid-August 1845, it had spread to northern France and southern England; it arrived in Ireland in September, with demographic consequences which have shaped our history ever since. Potato blight is a fungus, and its appearance on the crop is often a legacy transmitted by infected tubers which have survived from the previous season. Once established, the spread of the disease is highly weather dependent.
The rate of growth of the fungus depends on temperature; infection from plant to plant requires a film of moisture on the leaf - the longer this film persists, the greater the opportunity for infection.
It has been found that the ideal conditions for the spread of the disease are a relative humidity greater than about 90 per cent, and temperature in excess of about 10 C, both occurring simultaneously over an extended period. Nowadays, potato blight can be controlled - but it has not, as some city-dwellers might suspect, become extinct. On the contrary, 1997 so far has been a year in which it thrived. Major infestations were reported from Donegal to Cork and the number of days when weather condition favoured its advance was more than double the climatic norm.
The earliest observed occurrence was on May 17th in Co Wexford, and the worst period was during the first two weeks of August, when the severe flooding in the south was followed by a spell of the warm, thundery, humid weather that makes Phytophtora so vigorously infestans.