It is now known that blight is caused by a fungus named Phytophthora infestans* It was not a sickness of the plants themselves which turned the potato fields of Ireland black almost overnight. Invasion by a microscopic living organism took place, an organism able to reproduce itself with lightning speed and an addition to the known flora of Europe and a part of the creation which had never been catalogued before.

Blight is with us still. Every year since 1845, in potato fields throughout the northern hemisphere, the blight fungus has been present, waiting only for the right weather conditions to multiply with fearful rapidity, as again happened, with exceptional severity, in 1958.

Up to 1939 blight is estimated to have cost the United Kingdom an average of five million pounds a year. In a bad year—1879 for instance—potatoes worth six million pounds were destroyed in Ireland alone. In the United States during the severe attack of 1928 a single state, the State of New York, lost thirteen million bushels.


Where the potato blight originated and how it came to Europe is a mystery. Early botanists and natural historians do not mention any disease resembling blight, and potatoes had been grown in most European countries for nearly two hundred years before blight appeared. A potato disease identical with blight was found in North Germany near Hanover about 1830, but the first fully-recorded outbreak took place in the New World in 1842, when potatoes along the Atlantic coast of North America, from Nova Scotia to Boston, were destroyed. This attack was followed, in Europe, by the serious outbreak of 1845 and the total loss of 1846.

It has been proved that the organism of the blight fungus is so sensitive to heat and drought that its spread, for any considerable distance, by air currents is impossible, and the blight fungus almost certainly reached Europe in a diseased tuber, carried in a ship from

* For the material in this chapter I am indebted to the kindness and patience of Mr. Geoffrey Samuel, late of the Agricultural Research Council, and for the historical aspects to Mr. E. C. Large of the Plant Pathology Laboratory, who has generously allowed me to make a free use of the facts in his remarkable book The Advance of the Fungi. I should also like to thank Dr. N. Robertson of the University of Hull for his valuable suggestions.

North America. When this took place is not known, but the description of blight in Germany about 1830 disposes of a pleasing theory, that blight had to wait for the coming of steam to cross the Atlantic. It had been argued that potatoes stored in the hold of a sailing-ship became so warm during the slow passage through the Doldrums that the fungus was killed, whereas the shorter passage, by steam, allowed it to survive. But the first crossing of the Atlantic by a steamship was not accomplished until 1838, eight years after blight was observed in Germany; and early steamships, owing to their extravagant consumption of fuel, were not used to any extent as cargo vessels for more than fifteen years after the initial crossing.

Blight is now treated by spraying with copper compounds, such as Bordeaux mixture, the compound of copper sulphate and quicklime first used in the vineyards of France against Peronospora, the deadly fungus of the vines. Potato crops attacked, or threatened by blight are nowadays sprayed on a large scale, frequently from aeroplanes and helicopters, and though blight remains the most serious plant pestilence in the northern hemisphere complete destruction of a crop no longer takes place.


In 1846, however, there was no notion of treating or protecting potato plants, nor any comprehension of the nature of blight. More than fifteen years were to pass before blight was acknowledged to be the work of a fungus and nearly forty before, in 1885, Bordeaux mixture was first used.

Yet almost as soon as blight appeared the truth was discerned; what was lacking was proof.


In the summer of 1845 the Reverend M. J. Berkeley, a country clergyman, perpetual curate of the parishes of Wood Newton and Apethorpe, in Northamptonshire, observed that whenever the mysterious new disease attacked the potato planes in his parish a tiny growth, a minute fungus, was invariably to be found on the blighted parts of leaves and tubers. Mr. Berkeley was no ordinary country parson. A gentleman eminent in his knowledge of the habits of fungi, he had done valuable work on molasses, seaweeds and algae when a curate at Margate, and been responsible for the volume on fungi in Smith's famous English Flora, published in 1836.


Mr. Berkeley was in the habit of corresponding with a-French botanist of eminence, Dr. Montagne, originally a surgeon in the Napoleonic army who had become an authority on mosses and lichens. When blight appeared in France, Dr. Montagne also observed the tiny growth, and communicated with Mr. Berkeley; drawings and descriptions were exchanged, and the growth in England and the growth in France proved identical. On August 30,1845, at a meeting of the Societe Philomatique, in Paris, Dr. Montagne described the growth and claimed the discovery of a new species of fungus. The claim was recognized and accepted.


Mr. Berkeley now went further. In January, 1846, after the first failure, he published an article in the Journal of the Horticultural Society of London, entitled 'Observations, Botanical and Physiological, on the Potato Murrain', in which, after describing the new fungus, he asserted that it, and it alone, was the cause of the recent potato pestilence. The disease known as blight, he declared, was caused by the growth of the fungus, as a parasite, on the potato plant, and by nothing else.


Vehement controversy followed. Mr. Berkeley's theory, the 'fungal theory', as it was called, contradicted the doctrines generally held at that time, and many scientists, including Dr. Lindley, the well-known editor of the Gardeners' Chronicle, disagreed. A prolonged altercation followed, and Dr. Lindley and Mr. Berkeley argued hotly, week after week, in the columns of the Horticultural Journal and the Gardeners' Chronicle.

It was generally believed then that fungi were the consequence, not the cause, of decay. Because they were usually to be found on rotting matter it was argued that fungi appeared as a result of the heat and fermentation which accompany the processes of decomposition; and there was also a lingering belief that the fermentation and heat of decomposition could somehow generate life, that overripe cheese could generate mites and bad meat blowflies. An earlier generation had believed that old rags and stale cheese, shut up together in a box, could produce mice, and though scientists had discarded these fables more than a century ago, they still believed that such rudimentary forms of life as fungi could be produced by the processes of decomposition.

Therefore, though Dr. Lindley, too, had observed the invariable presence of the tiny fungus on blighted plants, he had passed it over as being a normal consequence of decay, or of the 'wet putrefaction' and 'dropsy' which, in his opinion, were the cause of blight.

Moreover, Mr. Berkeley was now asked some very awkward questions. Mr. Berkeley's theory, the 'fungal theory', must depend on the order in which the fungus and the blight appeared, and if the fungus caused blight, it must come first; could Mr. Berkeley prove this? Could he demonstrate that healthy plants were attacked by the fungus and then developed blight? It was admitted that the fungus was invariably to be found on the blighted parts of leaves and tubers, but that fact proved nothing, except its close association with decay, which was already known.

There was another important-question. If a fungus was responsible, how was it that potatoes not yet dug and still in the ground were found to be blighted? True, certain species of fungi had airborne spores, and might spread from leaf to leaf of the potato plants, but how could tubers, which were underground, be affected?

To these and other questions Mr. Berkeley could give no satisfactory answer; though with a flash of genius he had divined the truth, he had little evidence to support his theory, and the 'Observations', now regarded as a landmark in botanical history, which he published in the Journal of the Horticultural Society were almost universally rejected.


Unfortunately Mr. Berkeley never did produce proof, and the truth of the fungal theory was established very slowly, over more than three-quarters of a century. While mycology, the science of the fungi, made notable advances in the fifty years following 1845, and the life cycles of Rust, the fungus of wheat, and Oidum, the fungus of vines, were traced and a remedy brought within sight; the life cycle of the potato fungus remained a mystery. It was not until well into the twentieth century that, after 'one of the longest games of hide-and-seek in natural history', the enigma was solved and the habits, the method of functioning and the manner in which Phytophthora infestans survives and propagates itself became known.

By a stroke of poetic justice it was in Ireland that much of the final research was carried out, by Professor Paul Murphy, a Kilkenny man, at the Albert Agricultural College, Glasnevin, Dublin.

Phytophthora infestans first makes its appearance as a minute, whitish growth, resembling a fringe, just visible to the naked eye, surrounding the blighted and decaying parts on the leaves of infested potato plants. Under the microscope, this 'down' is seen to be made up of countless long, slender, branching filaments, each carrying at its tip a minute pear-shaped swelling. The filaments are, in fact, fungus-tubes, and the pear-shaped swelling each carries is a container, like a capsule, which contains the spore of the fungus. The blight fungus consists of these fungus-tubes; they form a vegetable organism of great destructive power; without roots, without flowers, without any differentiation between stem and leaves, which grows and develops within the plant, and, by means of the spore container, is able to propagate itself with frightening rapidity. The spores formed on a single potato plant which has been invaded by the blight fungus can, if weather conditions are favourable, infect many thousands of other plants in a few days.

The spore containers grow at the ends of the fungus-tubes, like fruits on a branch, until they are mature, when they become separated. The lightest breeze detaches them; the gentlest rain or dew washes them off. Countless thousands then fall to the ground; other myriads become airborne, and drift.

When an airborne spore container drifts on to the leaf of a potato plant it settles and, given one necessary condition, germinates at once. The necessary condition is moisture. The spore of the blight fungus is water-borne; when it moves it swims and, therefore, to germinate effectively it needs a drop of moisture. The scientists of 1846 who attributed blight to the wetness of the summer were very nearly right. Though rain and damp are not the cause of blight, without them the fungus does not multiply rapidly. Consequently, in a dry summer there is little blight, and the fungus, though present, is more or less dormant; while during a damp season blight is at its most vigorous. Violent driving rain does not provide the conditions most favourable to the spread of blight; in gales of rain the down-like fringe, consisting of thousands upon thousands of fungus-tubes, is washed off. It is when the atmosphere is moist and muggy that spore production reaches its height, and the blight fungus spreads with such rapidity that potato fields seem to be ruined overnight. The soft, warm climate of Ireland, particularly in the west, with its perpetual light rains and mild breezes, provides ideal conditions for the spread of the fungus, and has been truly described as a forcing-house for blight.


Given adequate moisture, the container proceeds surprisingly to germinate in two different ways. Sometimes it germinates as one unit, sending a single germ-tube instantly into the potato leaf, sometimes its contents split up within the container and become from six to sixteen smaller spores, which are then released in a swarm. Under the microscope these spores can be seen, at the moment of release, jostling each other, 'much more like little uni-cellular animals let out of a bag than anything one might expect to find in the vegetable kingdom'. The tiny spores are called zoospores, meaning that they are able to move; after liberation they swim away and, settling on a fresh part of the leaf, each sends out a minute germ-tube to invade the leaf, but at six to sixteen points instead of one.

In a short time the leaf is overrun by a system of radiating fungus-tubes, pushing their way through, to emerge in due course, each bearing at its tip the pear-shaped containers which, in a very few hours, will release fresh hordes of spores. 

In this process the potato plant is destroyed. As the fungus-tubes, whether originating from large or small spores, work their way through the leaf, lengthening and branching, they leave ruin behind, the juices of the leaf are drained and the tissues exhausted; a change takes place in the matter of which the leaf is composed, fermentations appear, followed by discoloration and mortification; finally, the entire foliage of the potato plant turns black, withers and dies. Yet this process is not purely destructive; it is from the fermentation and decay of the leaf that the fungus extracts its nourishment, the 'protoplasm', or vital substance, which enables the fungus-tubes to live.

The unfortunate potato plant is now not only being devoured but choked as well. 'If a man,' writes Mr. E. C Large, 'could imagine his own plight, with growths of some weird and colourless seaweed issuing from his mouth and nostrils, from roots which were destroying and choking both his digestive system and his lungs, he would then have a very crude and fabulous, but perhaps an instructive idea of the condition of the potato plant. . ..'

Meanwhile, beneath the ground, the blight fungus is attacking the potatoes themselves. How this happens was for many years one of the major mysteries of blight. It used to be thought that the disease travelled down the stem of the plant to the tubers, and one of the earliest treatments for blight, still occasionally practised today, was to cut off the stems and foliage of infected plants, close to the ground. But this operation by no means invariably prevents infection, and if done too early it may prove as ruinous as blight itself. Once stems and foliage are amputated, none of the food material which the plant derives from the green chlorophyll in the leaves can pass down to the tubers, growth stops, and the result is a useless crop of wizened, dwarfed potatoes the size of walnuts.

It has now been established that blight penetrates the soil to the tubers. Moisture is, once more, the deciding factor; if rain is sufficiently heavy and continuous, some of the myriads of spore containers which fall to the ground are washed down, through the soil, on to the potatoes. The process of destruction which took place on the leaf is now repeated: the spore container germinates, each spore, whether entire or the result of splitting up, sends a germ-tube into the tuber and the fungus then works its way from cell to cell. Blackened and decomposing patches appear on the skin of the potato and in its flesh, and eventually the exhausted tissues collapse into pulp.

As a rule, however, blight fungus remains inactive for a considerable period when it has entered the potato; only a discoloration of the skin betrays the presence of the fungus within, and such infected tubers are the means by which blight is most commonly spread. 

If tubers containing the dormant blight fungus are planted either accidentally or because the importance of the partial infection is not realized, as happened in Ireland in 1845-46, a small number will throw up shoots early in the season; these are infected with blight when they appear. A fungus-tube from within the potato has grown up inside the stem of the shoot, and thus, at the beginning of the season, a nucleus of infection is established, ready to develop with lightning rapidity when the weather becomes warm and moist in July, August, or September.

The Ministry of Agriculture in London forecasts the onset of blight each year from a study of the weather records. As soon as conditions favouring blight occur, warnings are issued recommending potato growers to spray their crops.

The blight fungus also infects potatoes after digging, a source of despair and bewilderment in 1845. 

The top and foliage of a plant can be destroyed by blight while the potatoes in the ground beneath may be sound: either the potatoes were too well-covered with earth for the blight spore to reach them or, as was frequently the case in Ireland, rain was light and did not wash the spore containers down through the soil. But, even so, danger of infection is not over; countless thousands of live spore containers are on the leaves of surrounding plants, and as the potatoes are dug they are showered with spores. If the weather is dry no harm is done, but if it is moist the spore containers find the drop of water they must have to germinate, and within a few hours the fungus is active, growing rapidly through the tubers. In a few weeks the potatoes which were sound when dug are a mass of rottenness.

In 1845 much of the infection occurred after the potatoes were dug. In 1846 rain was exceptionally heavy, the spore containers were washed down on to the tubers, which were then devoured by the fungus and became rotten in the ground.


The life of the blight fungus is short. If the air is dry the spore containers carried at the end of each fungus-tube live for only a few hours; if the weather is damp, and the spore germinates, the new germ-tube must penetrate a leaf quickly, or it dies. When cold weather comes, the work of destruction being completed, the fungus dies.


For long it remained a mystery how the fungus survives the winter and starts its work of destruction again the following year. It has never been proved that the spore of the blight fungus can survive the winter in European soil, but it appears that the fungus survives from season to season, lying dormant in the slightly diseased potatoes which are occasionally planted, through ignorance or accident, with healthy tubers. The fungus grows up within the stem, diseased shoots develop, and as soon as conditions of weather and temperature are favourable the fungus begins to form its spores again. 


Once spore production has started the blight fungus can spread with astonishing rapidity. In moist, warm conditions one diseased plant within a day or two releases several million spores, each one of which is capable of dividing within itself and producing a swarm of smaller spores. If a number of slightly diseased seed potatoes have been planted in different places, and diseased shoots appear in any quantity, blight can become general in a few weeks. Countless millions of spore containers germinate hourly; germ-tubes work their way into leaf and tuber, reducing green and healthy plants to decay; fields are seen to turn black, tubers, hastily dug, collapse into stinking masses, and the fearful stench of decomposition hangs over the land.

In Ireland in 1846 conditions favoured the spread of the blight fungus to an extent which has not been recorded before or since. 

There had been an outbreak of blight the previous year, and very many slightly diseased potatoes had been planted in the fields, sending up diseased shoots. The weather of 1846 was wet—'continual rain' yet warm; on June 6 The Times recorded a heat wave. Ignorance was complete; blight was not known to be a fungus; treatment with Bordeaux mixture was not attempted for nearly forty years.

The great Irish failure of 1846 is the classic example of an outbreak of blight, and the people of Ireland, gazing over their blackened fields, despaired. 

They were already exhausted. What resources they possessed had been used up, and death from starvation was not a possible but an immediate fate.

Once more, the question so frequently asked in the past was on every lip—what would the British Government do to save Ireland ?