Late Blight, and The Great Hunger in Ireland

Phytophthora infestans is a water mould, that causes the serious disease of the potato, late blight or potato blight.

The early stages of blight are easily missed, and not all plants are affected at once. Symptoms include the appearance of dark blotches on leaf tips and plant stems. White mould will appear under the leaves in humid conditions and the whole plant may quickly collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Seemingly healthy tubers may rot later when in store.

"Discovery of the Potato Blight" by Daniel MacDonald (1821-53)

The potato blight was a major cause of the Irish Potato Famine, also known as The Great Hunger, between 1845-1851. Over one million people died from starvation and related diseases. Another one and a half million emigrated to North America and Australia.

Potato Late Blight cycle

The spores of this water mould overwinter on infected tubers, particularly those that are left in the ground after the previous year's harvest, and are spread rapidly in warm wet conditions when blight can have devastating effects, destroying entire crops.

Spores develop on the leaves, spreading through the crop when temperatures are above 10°C and humidity is over 75% for 2 days or more. Rain can wash spores into the soil where they infect young tubers, or else spores can be blown in from distances of up to miles by the wind.


Growth sequence of the potato with regimen of blight spray applications


Spraying potatoes for blight in Ecuador creates health risks for workers

Bordeaux mixture is a combination of copper sulphate and hydrated lime, invented in the vineyards of the Bordeaux region of France, and used mainly to control garden, vineyard, nursery and farm infestations of fungus. This fungicide has a history of over a century, and is still used, although the copper can leach out and pollute streams.


Spraying by plane.

Killing potato foliage with flame spreader

Red Dragon Potato Vine Flamersoffer producers a totally organic way to desiccate potato vines to stop the growth and set the skins of potatoes. Flamers burn clean, efficient propane, so there is no residue, run-off or contamination to worry about due to chemical or acid use.

This flaming process uses our patented liquid spray torches which are specially designed to spray liquid propane into the vines where combustion takes place. The canopy of vines and foliage help hold the heat helping to make more efficient use of the fuel. The intense heat thermally shocks the green vines and destroys cell tissues in the leaves, destroying the plant’s ability to conduct photosynthesis.

Flaming is also a very effective weapon for blight control. Chemical treatments are expensive, and not an option for organic growers. Flaming to desiccate or scorching the ground right before harvest will help control blight spores.


Blight resistant potato varieties under consideration by USDA

Up until the 1970s, there was only one type of blight (A1) in the UK, and this was unable to produce resistant spores that could survive the winter. There are now two types (A1 and A2) which can mate and after that produce resistant spores, although the indications so far are that this rarely, if ever, happens in the UK. Mating can occur only between moulds of different mating-types and is required for the production of resistant spores.

Prevention and control of potato blight can be achieved by planting only good quality seeds obtained from certified suppliers. Do not save your own seed for replanting, and try to ensure that no ‘volunteer’ tubers are left in the soil after harvest. Potato varieties vary in their susceptibility to blight. Most early varieties are very prone; so that the crop matures before blight starts (usually in July) plant them early. Maincrop varieties which are very slow to develop blight include Cara, Stirling, Teena, Torridon, Remarka and Romano. Some so-called resistant varieties can resist some strains of the blight and not others, so their performance may vary depending on which are around.

Symptom of late blight on the potato leaf.Growing potatoes should be earthed up regularly in order to minimise the risks of spores being washed down into the soil reaching the tubers. If blight symptoms appear, remove all affected leaves immediately. Cut off and burn all foliage in bad cases to help prevent spread to the tubers. Don't harvest the crop for at least 3 weeks. By then, tubers will have thicker skins and blight spores on the surface will have died. A hot compost heap should destroy the spores on the leaves, but it is probably not worth the risk. Any infected tubers should definitely be burned.

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Learn more here:

Irish Potato Famine introduction

Views of the potato famine

More about the famine

Potato Late Blight

U.S. Potatoes Could Get Disease Resistance from Their Mexican Cousins

Red Dragon Agricultural Flamers

It’s Never Too Soon To Prepare For Potato Harvest
Chemical and/or mechanical vine desiccation practices

Tubers naturally mature as the plant senesces. With the improved production methods, potato vines remain healthy and green longer into the season. It is argued that for proper tuber maturity at harvest, vine desiccation is necessary.

Vine killing benefits tuber appearance, limits tuber size, improves tuber release from the vine at harvest, reduces tuber skinning and can lower crop susceptibility to shatter.

Some additional benefits include harvest timing, tuber ripening, tuber size management, disease management, and improved storage life.


Spraying potatoes: health risk for potato farmers

Walking through poison

High in the Andes, BBC reporter Euan McIlwraith meets a man spraying his potatoes. He has no protective equipment. His hands and shoulders are wet from the cocktail of pesticides he carries in his back-pack sprayer.

These products are categorised by their toxicity. The most dangerous carry a red warning label. In this region of Ecuador, 90% of all the pesticides bought are red label products.


Many die

The doctor at the local hospital says he sees the results of acute pesticide poisoning every day. People are admitted with stomach cramps, blinding headaches, serious skin diseases, and tunnel vision. The most serious cases have kidney failure, or have taken an overdose of pesticide. Many die.

The statistics do not reveal the thousands of cases of dizziness, nausea or memory loss which are not referred for medical help - nor the cases of cancer, depression or fertility problems which have been linked to certain chemicals.

Researchers in this area of Ecuador say farmers, their wives and children are all at risk, and six out of ten will have suffered nerve damage due to pesticide exposure.


Pesticide salesman

One of the biggest problems is lack of knowledge about pesticides and how they should be used. In a small corner store, a shopkeeper offers to sell Euan one of the most common red label products used in Ecuador - liquid carbofuran. He says he can supply it to anyone, including children.

When he is pressed to say why he does not sell less hazardous products, he explains it's all a matter of demand. Farmers want the strongest pesticides possible, and they do not buy the alternative products.

Most farmers and pesticide suppliers will have had no formal training about these highly hazardous substances. Researchers and campaigners say the challenge is to get information down to the grassroots as quickly as possible.