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( Phytophthora infestans )


picture of potato blight on leaves
Top and bottom shots of affected leaves, note the whitish bloom of spores on some.

This is a serious plant disease which has been here for over 150 years. It caused the Great Irish Famine of 1845 to 1849 during which over a million people died and a similar number emigrated to escape the devastation. A variety called Lumper, due to it looking like a bag of wallnuts, had been introduced in 1810 and was universally grown because it required little manure and grew in poor soils, but it was highly susceptible to the disease which had spread from mainland Europe.
Although it has been classified as a fungus this is no longer the case due to the make-up of the cell walls which are made from cellulose and fungal cell walls contain a structural polysaccharide called chitin also found in the exoskeletons of insects. P. infestans affects the potato and tomato, both members of the Solonaceae Family; other members of the genera cause Phytophthora Root Rot and Sudden Oak Death in shrubs and trees.

The first symptoms of blight are a dark brown patch and yellowing of the leaves, which turn black, then a white bloom develops on the underside as the foliage dies. The spores produced by this bloom are washed down to the tubers causing dark spots on the skin and reddish-brown stains through the flesh. It survives the winter as mycelium and spores in tubers left behind at harvest. The fungus grows on shoots from these tubers the following year, and produces asexual spores that are carried by the wind to infect the new crop during warm moist conditions. Early crops are usually harvested before serious attacks.

At the first signs of infection the topgrowth or haulms, can be cut off and destroyed to prevent the spores being washed down to the tubers - lift all leaf debris as well. The crop can be left for a few weeks before lifting, it will not develop any more - if possible , lift immediately. After harvesting the crop should be washed and carefully examined for signs of discolouring before storing and any suspect tubers set aside for immediate use - cut away any affected flesh and they can be eaten. (Commercial growers use a spray of Sulphuric acid or paraquat to kill the topgrowth 2 to 3 weeks before harvesting to reduce the risk of the haulms becoming infected and transferring spores to the crop as it is lifted.)

picture of potato tubers infected with Late Blight
These are infected tubers. On the left the damage is slight so it can be pared away, the remaining part is not tainted and can be eaten.

Unfortunately this disease is continually producing new strains which can attack resistant potato varieties. A new "mating type" of the it was recorded in 1978 which can introduce new pathogenic variants at a faster rate, so the battle to overcome the disease has become more intense.

Genetically Modified potatoes have been developed by including genes from a wild Mexican species, Solanum bulbocastanum, which has resistance to late blight. It was announced on 1st December 2006 that permission has been granted for the chemical company BASF to grow trial crops in Derbyshire and Cambridgeshire, England - they will begin in spring 2007.

picture of blighted potato crop
Mild, wet summer weather and distraction from a rabbit attack elsewhere, meant this infection
progressed further than it should. The tops and leaf debris were immediately removed to protect
the tubers from falling spores.


Prevention is better than cure so there are a number of measures which can be taken to avoid infection:-
Use only certified seed which are produced in areas where infection is not prevalent. Rotate crops, aim to repeat planting a plot after a 3 year gap at least. Try to harvest every tuber and destroy any infected ones. If shoots appear from missed previous crops they should be removed immediately with the tuber if possible, as any carried-over infection will arise from the leaves. Avoid overhead watering which would wash spores down to the soil, and earth up well to protect the tubers. Planting early or second early varieties may mean that spraying can be avoided as they will be mature enough to harvest before infection risk is high or if infection is detected the topgrowth can be removed and the crop is mature enough to be harvested. King Edwards are more resistant, but Maris Piper and Desiree are very susceptible. Some new varieties have been specially bred to be resistant. One successful introduction in the last decade was Sarpo Mira, also the white maincrop variety Orla, which is said to have even greater resistance to foliar blight, but as new strains of the disease are constantly developing it has recently succumbed to one called Blue 13. Be vigilant, checking the crop at least daily when the risk is high during July and August, as the disease can appear literally overnight. For chemical treatment:- A protectant such as Zineb, Mancozeb (Dithane) or Bordeaux Mixture* can be used, these damage the spores and prevent their development. The acylalanine fungicides Metalaxyl and Furalaxyl are systemic fungicides, acting inside the plant as a curative and should be applied with a protectant to reduce the development of resistance. When spraying ensure that the all of foliage is covered by moving the nozzle through it and spraying upwards as well.

The Sustainable Use of Pesticides Directive with restrictions and banning of products developed by the European Parliament's Environment committee was passed on 13th January 2009 and included are the triazole fungicides commonly used against blight, eg. Mancozeb. Now restricted to use only by commercial potato growers.

*Bordeaux Mixture is allowable under organic rules but too much is toxic to the plants. However approval for use in gardens in the UK will be withdrawn at the end of 2013 to comply with the implementation of the provisions of Commission Directive 2009/37/EC following the inclusion of copper sulphate in Annex I to Council Directive 91/414/EEC and as of November 2014 Bordeaux Mixture was withdrawn from sale - any stored product can be used until the end of 2015:-
Copper Sulphate 1 lb (250g)
Hydrated Lime 1¼ lb (315g)
Water 10 gal (25 litres)

Predicting Blight

To reduce the need to spray, forecasting the risk of attack has become very important. In Britain the "temperature-humidity rule" was devised by Beaumont in 1947. When 2 consecutive days of humidity over 75% and the temperature is above 10°C conditions are favourable for the release of spores from carrier plants, so blight tends to develop 15 to 22 days later. A date is set for the high risk periods and it varies for different regions; these are called "Beaumont periods".
In the 1950s the Smith Period, with more defined criteria, was introduced. It is defined as 2 consecutive days with temperatures of at least 10°C and 11 hours of relative humidity of 90% per day. Further research at the James Hutton Institute has narrowed the parameters that indicate when there is a risk of blight. So the Hutton Criteria are when two consecutive days with a minimum temperature of 10C, and at least six hours of relative humidity of 90% occur. This was shown to improve the consistency of the alerts.
More information about Hutton Criteria can be found at the Blightwatch site, where there is also a reporting service and which warns when there is a risk of Blight. You can also sign up for free e-mail or SMS warnings when there is an alert for your postcode.

A testing kit called Pocket Check is now available which can detect the presence of the disease when it is only starting. A sample of leaf is used where small traces of browning is noticed at the edges. However vigilance and an awareness of the risk are still needed, so the test may just confirm what is already obvious. Spraying or careful searching for, and removal of affected leaves are necessary to halt the progress of the disease.

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