Do you remember the long, hot summer of 1976? Day after day for months on end the sun shone from a blue, cloudless sky.
Temperamentally the entire Irish nation shifted 10 degrees of latitude further south, possessed pro tempore of a carefree Latin gaiety and becoming Hispanicis ipsis hispaniensior in their euphoria.
"Bliss it was in that dawn to be alive,/ But to be young was very heaven." Phytophtora infestans has good reason to be grateful for that summer, too. For the troublesome instigator of potato blight that dry summer brought an end to more than a century of enforced celibacy. For the first time since 1842, European Phytophtora found itself provided with a mate, and its ensuing fecundity allowed it to renew its ravages of the world's potato crops with unprecedented vigour.
Before 1842, potato blight was known only in Mexico. That year, however, it turned up in New Hampshire and Vermont, and three years later it appeared in Belgium. By midAugust 1845 it had spread to southern England; it arrived in Ireland in September, with demographic consequences that have shaped our history ever since.
The success of Phytophtora infestans, the fungus which causes blight, is highly weather dependent. Its growth depends on temperature, and 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.
Now, chemical fungicides have been an effective defence against Phytophtora. Their success was enhanced by the fact that the fungus which escaped from Mexico all those years ago comprised only one clone of the organism, referred to as A1. And A1, it seems, can only reproduce sexually with fungi belonging to a second mating type, A2. Without an A2 partner, therefore, the A1 fungus we had in Europe could only reproduce asexually, producing endless clones of itself. This limited its capacity to evolve genetically and, in particular, rendered it unable to adapt successfully to become resistant to the fungicides used in its control.
But all changed in 1976. When drought caused a failure of the potato crop in Europe, imports of potatoes were allowed from Mexico. The imported tubers contained a selection of A2 clones of Phytophtora, allowing the now ecstatic fungus to mate with great success. The adaptive skills of the organism have provided since then a major challenge for both potato growers and the manufacturers of fungicides.