By Sean Henahan, Access Excellence

DUBLIN - One-hundred and fifty years ago a fungus called Phytophthora infestans began a destructive process that would change the course of Irish and, indirectly, world history. The country is now commemorating the 150th anniversary of the potato famine that would lead to the deaths of millions of citizens and the emigration of many millions more.

A hundred and fifty years ago in September, 1845, the Dublin Evening Post reported a "disease in the potato crop". This turned out to be potato blight which destroyed 40% of the crop that year and almost 100% the following year. The ensuing devastation was known as "The Great Hunger" and resulted in widespread starvation and mass emigration to Britain and the Americas. As a result the population declined from 8,295,000 to less than six million in a few short years.

Towards the end of the 18th century and well into the 19th there was a rapid rise in the Irish population. More and more land was used for potato growing to the neglect of other crops. Some of the older, better quality potatoes were abandoned in favor of the high yielding but poor quality "Lumper". By 1800, 90% of the population were wholly dependent on the potato and in large parts of the country many depended on a single variety, the Lumper.

In 1845 a fungal disease, called late blight, appeared in the Irish potato crop with devastating results. The disease, called late blight, manifests itself in any part of the plant. Discoloration of the leaves accompanied by downy mildew is sometimes the first sign. The tubers may also be infected in the field or in storage, with discoloration of the skin, with a reddish brown dry rot extending into the tuber. Later a slimy, foul smelling rot may destroy the tuber.

It is now known that the blight is caused by the fungus Phytophthora infestans an Oomycete (class) which belongs to the family Pythiaceae. This species is characterized by coenocytic mycelium and the production of biflagellate, motile zoospores. The fungus is heterothallic and can reproduce sexually in the presence of an opposite mating type. Sexual reproduction follows fertilization of an oogonium by an antheridium resulting in the production of an oospore. Overwintering can be in the form of resting mycelium (most usual) or oospores. After germination both will result in the production of sporangia which can germinate directly by means of a germ tube or indirectly by means of the motile zoospores. Germination is followed by encystment and appresorium formation.

Penetration of the leaf surface is by means of an infection peg followed by intercellular invasion with penetration of the cells by means of an haustorium. Invasion of he leaf is rapid, eventually resulting in necrosis and death of the leaf tissue. Under favorable conditions, the sporangiophores emerge through the stomata with a fresh crop of sporangia within 4 to 5 days of the initial infection. This multicycle, airborne pathogen can therefore spread rapidly within crops and from one crop to another.

About 40% of the potato crop was lost to the blight in 1845,and the entire crop was lost by early August, 1846. The loss of that year's crop had its worst effects in the early part of "Black '47", the year when mass emigration started,. Apart from starvation, the lack of food led to other diseases such as scurvy, dysentery, typhus and cholera. Although the 1847 crop was unaffected by blight, famine conditions intensified because there was a shortage of seed potatoes and not enough potatoes had been planted .

In 1845 the population of Ireland was estimated at 8,295,000. When in 1851 the famine was officially over, the population had been reduced by some two and a half million. One and a half million people had died of starvation and disease and a further million had emigrated.

Phytohphthora infestans also affected potato crops in the northeastern United States and Southern Canada as well as much of western Europe in addition to Ireland in 1845, but the effects were nowhere near as devastating as in Ireland because in those places the diet was much more diversified and the people were not so dependent on the potato.

In 1882, Millardet, a French botanist, noticed that grapes in a vineyard which had been sprayed with a mixture of copper sulfate and hydrated lime (in order to prevent theft) had remained free from attacks of downy mildew. Millardet went on to demonstrate the effectiveness of this "Bordeaux Mixture" in preventing potato blight. By the beginning of the twentieth century, spraying of potatoes with this fungicide was in general use in Ireland and remained so until the introduction of organic fungicides in 1934, and more recently, the newer systemics.

Potato breeders are attempting to develop "field resistance" or durable resistance to the fungus. The Lumper, the potato of the famine times is no longer grown. About 25 varieties of potatoes are grown commercially in Ireland today. The main variety grown is Kerr's Pink which has low resistance to blight. Three other popular varieties, Record, Golden Wonder and Rooster have very good resistance to tuber blight and moderate resistance to foliage blight. Another popular variety, Cara, has excellent tuber and foliage resistance. The gene R2 has been found to delay the onset of an epidemic by 10 to 14 days and attempts to incorporate this gene into varieties with a high level of durable resistance are underway.

A complex strategy of blight control has been developed involving protectant, systemic and translaminar fungicides, at times combined with adjuvants to enhance their effectiveness. This strategy is designed to control the blight and avoid the problem of fungus resistance.

Although it happened 150 years ago, the Irish have not forgotten "The Great Hunger". Because of the influence of weather on the development and spread of late blight, the Irish Meteorological Service continues to broadcast warnings of weather favoring the spread of potato blight. Moreover, the Irish people have gained a reputation for contributing personnel and resources in areas of the world currently facing famine, well out of proportion to the size and economy of their small island.

Anyone planning on visiting Ireland may want to visit the recently opened Famine Museum at Strokestown House, Co. Roscommon.