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Potato Blight (often referred to as Late Blight) is caused by the microorganism Phytophthora infestans. It is similar to a fungus but is actually a mold which can exist in moist environments. This microorganism also attacks tomato plants and in this case is often referred to as Tomato Blight.
The lifecycle of Phytophthora infestans is complicated because not only does it reproduce asexually but also sexually. For the amateur gardener, and commercial potato growers for that matter, the life cycle is a bit irrelevant. Understanding it has so far proven irrelevant. Other factors are far more useful to understand.
If you keep your garden clear of all potato debris each year, you can be almost certain that any attack of Potato Blight has arrived by wind. The spores of Phytophthora infestans weigh almost nothing and can easily travel large distances (50 miles plus has been reliably recorded) on the wind.
Potato Blight has existed for centuries and it was seen with devastating effects during the Irish Potato Famine during the mid 1800's. Millions died in parts of Ireland through starvation because their potato crops failed for several years.
There are very specific conditions which allow Potato Blight to spread in the UK and, unfortunately, our climate has those conditions several times each and every year.
There is absolutely nothing the average gardener can do to control these conditions but it is relevant to understand what those conditions are. Forewarned is forearmed.
The conditions involve a combination of air temperature and humidity, these conditions are defined specifically in the paragraphs below.
The highest risk time for Potato Blight occurs after what is defined as a 'Hutton Period' (now replaces the less relevant "Smith Period"). This occurs as two consecutive days where temperatures are 10C or higher AND relative humidity is above 90% for 6 hours or more for both days.
Potato Blight is first visible as brown marks on the potato plant leaves. These brown marks expand in size as the disease progresses. The picture below shows the early brown marks on the leaves.
Early signs of potato blight
The next and final stages of Potato Blight are very mixed and dependent on weather conditions and the severity of the attack. The stems will now also show signs of brown marks on them and may well start rotting.
Eventually the whole potato plant is left with no leaves on it and the stems are significantly showing signs of rotting.
Two actions you can take to avoid blight are to grow resistant varieties and / or grow First Early varieties which are likely to reach maturity before blight can attack your crop.
The best varieties for blight resistant potatoes are those with "Sarpo" in their name. These varieties were bred specifically to have high resistance to potato blight. The most recently developed of them is Sarpo Axona.
Cara and Vitabella have good blight resistance, although not as good as the Sarpo family of potatoes.
One variety, Setanta, was hailed by the distributors as being blight resistant when it was first released but this that been proven not to be the case in practice.
Their is no "treatment" for potato blight however there are actions you can take to minimise the damage. A couple of paragraphs below about how potato blight spreads through a potato plant will help you in treating this disease.
The spores of potato blight are spread by wind and they first land on the leaves. They then infect the leaves resulting in brown spots appearing. The infection will then pass into the plant's stems and then down to the potato tubers themselves.
The speed at which all this happens is dependent on weather conditions and how resistant the infected variety is to potato blight.
Take the potato variety Orla for example. It has average resistance to potato blight of the leaves but has very good resistance to potato blight spreading to the potato tubers themselves.
On the other hand, the ever popular Maris Piper variety will allow the spread of potato blight to pass from the leaves to the potato tubers faster than you can say "I think they are suffering from potato blight".
So, the quicker you spot the first signs of potato blight on the leaves, the better chance you have of stopping it spreading to the potato tubers. A quick glance at your potato plants as often as possible, preferably daily, is the best method.
The action to take, when you determine that your potato plants have the first signs of potato blight, is to immediately cut the stems off at ground level and remove all the stems and leaves. Either burn them completely or take them to the local rubbish dump. That's it, there are no other options.
Personally I would err on the side of cutting the stems away and disposing of them rather than waiting to see what happens next.
If you cut the stems away from a potato plant it will have two effects. If potato blight has not yet infected the tubers, this will stop that happening.
It will however stop the potato tubers from developing further. How much that affects your potato crop depends on how far the tubers have developed when you cut the plants down. The results will vary from almost no effect to a significant reduction in the size of your crop.
The section below allows you to send us any questions you may have about potato blight. It does however not allow you to attach pictures.
If you want to send me a picture of a potentially infected plant then send an email to dave@davemarks.co.uk (copy and paste that into your email client) and attach the picture(s) to the email. I will do my very best to respond as quickly as possible with my best diagnosis.
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