FACT- FILE [Potato Pests 01]

Potato Blight Fact-Sheet

The Webmaster of the Aeron Vale Allotments Trust web-site, (charity trustee and plot cultivator) has been appointed an official "Blight Scout" by The Potato Council.  The area covered is the Aeron Vale Allotments Trust allotments site and all other allotment and gardening groups in the SA46 and SY23  postal areas.

If you suspect that blight has attacked any crops on your site or nearby, then please contact me (by clicking on the "Request Analysis" button below). A sample will be taken of the affected crop. The sample will then be forwarded to the Potato Council for lab analysis. Positive results for potato blight will be entered on the UK register and map as a confirmed outbreak in the area.

You can get free registration to blight alerts and Smith periods from

"Fight against Blight"

Further details available at:



How to Identify Potato Blight

The first signs of potato blight are dark brown markings on the edges of the potato plant leaves.

The next stage is that the leaves curl and the whole potato plant wilts and eventually falls over. This happens extremely quickly and the whole process can take only a matter of days.


The picture on the left shows the first signs of potato blight. If you see this on a couple of your potato plants the only course of action is to cut the plant down to ground level immediately.

This will prevent the disease reaching the potato tubers under the ground.

(Click on picture for enlarged view)

Burn the foliage immediately because this is a highly infectious disease. The spores will live in the soil for a couple of years if they are not destroyed. The same fungus also causes Tomato Blight.

If the foliage is destroyed immediately Potato Blight is noticed, there is a very good chance that the potato tubers under ground will be unaffected. Growth of the potato tubers underground will be slow when the foliage has been destroyed but they will be edible and will store for some time in the ground. See the picture below to identify if the potato tubers have been affected.

Treatment of Potato Blight
Once you have it there is no treatment. To help avoid potato blight there are two main courses of action:

1. Destroy all infected plants by burning them. Do not plant potatoes (or tomatoes) on the same patch of land without leaving an interval of three years.

2. Spray potato and tomato plants with Bordeaux Mixture (available at most garden centres) in May and June to help prevent infection.

As far as we are aware, eating potatoes infected with potato blight will not cause serious medical problems.

However the picture on the right shows the effects on the tuber if the disease is left to take its natural course. Its not as pretty sight I am sure you will agree,

Crop Rotation
The fungus which causes potato blight goes under the name of Phytophthora infestans. It is spread by either wind or rain in temperatures above 10oC (50oF)

Avoid it occurring in subsequent years by good crop rotation and burning all infected plants as soon as the disease is noticed.

The Potato Council have distributed fact sheets on Potato Blight one of which has been reproduced below for you.

Potato Council on Potato Blight



 Watch Out for Potato Blight

 Gardeners and allotment owners are being urged to be on the look out for potato blight this summer, the disease that caused the Irish potato famines in the 1840s. Experts say that more and more outbreaks of blight are originating from allotments and gardens. These can spread to other gardens and allotments, but also affect commercially grown potatoes.

 Gary Collins, Technical Executive with the Potato Council, said: “In the current climate more people are growing their own potatoes. As potato blight is a major concern to the industry, we keep a check on where outbreaks are coming from. Between 2007 and 2008 the number of outbreaks from allotments and gardens more than doubled. Because potato blight can spread very easily over large distances, everybody needs to keep a close eye on their potatoes.  Not only can it reduce the quality of your home-grown crop, but also your neighbour’s crop, and the local farmer’s crop too. A little knowledge and effort can go a long way to reducing the impact of this disease”

 Blight is most likely to appear between July and September, particularly when the weather is warm and humid. Gardeners should look out for dark brown or blackish patches or lesions on the top of the leaflets [1], often surrounded by a pale yellow halo. These lesions may also be found on the stems [2]. Under humid conditions, there may be a downy white coating that looks like fine cotton wool developing on the underside of the leaflets, or again on the stems. Blight spores released from this white coating is how the disease spreads from the foliage to other plants, or down on to the soil, where they can infect the tubers, causing sunken areas on the potato surface, and a chestnut-coloured rot under the skin.

 If you discover what looks like potato blight you should snip off the leaflets, or even whole stems to stop the disease spreading and remove them from the crop. If vigilant, this may slow the spread to a point where the crop is unharmed, and you will have done your bit to help reduce the spread of blight. The potatoes can still be harvested after two to three weeks. Make sure you remove all the potatoes from the ground, and do not leave any in the soil or on garden tips.

 Tips for preventing blight include:

  • Always buy your seed potatoes from a reliable source

  • Choose a variety that is less susceptible to blight

  • Ridge the soil well after planting your potatoes

  • Mulch to reduce the amount of water required

  • Water the soil and not the potato foliage

  • Harvest all your potatoes – even the tiniest ones



 To find out more about potato blight, visit www.potato.org.uk/blight for an in-depth fact sheet. On this site, you can also register free to receive weather warnings that let you know when the conditions are right for the spread of blight - known as a Smith Period, or that there may be blight in your area. These are sent by email and as a text message.

Note to editors

·         Potato Council is part of the Agriculture and Horticulture Development Board www.ahdb.org.uk working on behalf of British potato growers and purchasers to promote potatoes.  Potato Council is funded through a statutory levy on 3,000 potato growers and potato trade purchasers and aims to support the British potato industry 

·         Please below a Fact Sheet with more information on blight.

Potato Blight – The Facts

  • Potato blight is the most important potato disease.

  • Potato blight is a fungal disease spread by spores.

  • Potato blight is most likely to appear between July and September, when the weather is at its warmest.

The signs of potato blight are:

  • dark brown or blackish patches on the leaves, often surrounded by a pale yellow halo.

  • On the underside of the leaves there may be a downy white coating of spores in moist conditions, particularly at night. These blight lesions may also be on the stems.

  • The blight can spread from the foliage to the tubers, causing sunken areas of the potato, and chestnut coloured rot under the skin.

  • Once blight affects a plant, the infection can easily spread given the right weather conditions.

  • If the weather is humid, the spores that spread blight can spread over long distances.

  • Between 2008 and 2009, the percentage of outbreaks of blight originating from allotments or gardens rose from 3.6% to 9.1%.

  • Organisations such as Potato Council monitor conditions for periods when blight infection can spread most rapidly. These are known as ‘Smith Periods’, and occur when there are at least two consecutive days of temperatures of 10°C, and on each day there are 11 hours when the humidity is greater than 90%. You can find more information and register for forecasts at www.potato.org.uk/blight

  • Gardeners who discover potato blight should remove the affected leaves or stems to prevent blight spreading to the potatoes. The earlier this is done the better. This will also stop the spores spreading. This may reduce the number and size of the tubers growing on the stem

  • Tubers should be harvested after two to three weeks, and may be unaffected.

 Those growing their own potatoes can take a number of steps to ensure that there is a reduction in the likelihood of blight occurring:

o   Always buy your seed potatoes from a reliable source. Do not bring seed potatoes from another country, or save your own as they may be infected.

o   Some maincrop varieties show some resistance and are slow to develop blight.

o   Always remove all potential sources of blight. Harvest even the tiniest potatoes, and remove any potatoes that grow by themselves (volunteers).

o   Never abandon old tubers around the garden or allotment, or try to compost them.

o   Remove blighted tubers before storing, and always store your potatoes under dark, cool conditions.

o   Avoid planting in sheltered sites and plant in rows into the wind if possible.

o   If watering is required apply to the base of the plants, soil improvement and mulching will reduce the amount of watering required.

o   Blight spores on foliage are washed down through the soil to infect tubers. Earthing up potatoes, or mulching the soil with hay or straw can reduce levels of infection.

·         Potato blight caused the Irish potato famines in the 1840s and led to widespread starvation.