Today is the sixth anniversary of the death of Dr Austin Bourke who was for many years the world's foremost expert on Phytophtora infestans, the organism that causes potato blight.
From 1964 until 1978 Austin Bourke was Director of the Meteorological Service, now Met Eireann. Some years previously he formulated what came to be known as the Irish rules for identifying conditions conducive to the spread of the disease.
Phytophtora infestans is a fungus, and its appearance on the potato crop is often a legacy transmitted by infected tubers that have survived from the previous season. Once established, the spread of the disease is weather dependent.
The rate of growth of the fungus depends on temperature, and infection from plant to plant requires a film of moisture on the leaf. Bourke's Irish rules assert that for spores to infect the plants three conditions must coincide over an extended period: the leaves must be wet, the relative humidity must exceed about 90 per cent, and the temperature must be greater than 10 degrees.
In practice, whenever the latter two conditions are met, it is rarely that the first will not follow automatically. Rain is often present, during periods of high humidity.
These principles were tried experimentally by the Meteorological Service in the early 1950s. They were formally adopted as the basis for an Irish blight forecasting system in 1952, and no modification has been necessary for nearly 50 years.
Indeed they have been adopted by other countries, with slight modifications to suit local conditions, for operational use in forecasting potato blight. Weather of this kind was a frequent visitor to northern Europe in the early summer of 1845. The blight first appeared in Belgium in late June and by mid-August had spread to northern France and southern England.
It appeared in Ireland in September, but only caused a bit of concern. There was no panic as three-quarters of the potato crop was harvested successfully.
But the Horsemen of the Apocalypse lay dormant for the winter, and resumed their work from late July 1846 onwards. As the all-important potato plants were reaching maturity, it was abnormally wet and warm in Ireland.
The blight thrived, and by early August the fields were black with rotting potato plants. The season's crop was an almost total loss, and thousands of people, as we know, died in the resulting Famine.