Potato Blight

So at this year’s AGM, we took action and introduced a new rule that states that any plotter who spots blight on their potato crop needs to immediately take action.   Potato blight on the allotments is becoming a big problem and although blight itself  is driven by the weather, because there are so many potatoes getting grown in the allotments, once one crop is infected, it rapidly spreads to others.

The pictures below show the before and after effects of blight in the space of only 7 days this year.  First photo was taken on 30th July – the second only 7 days later the following weekend.   In the space of one week you go from having a perfectly healthy crop and the next week your crop is decimated.

So this is all very well, but a lot of our plotters who are just starting out growing veg have no idea what blight is and what to look out for.    Here is a quick guide for all you plotters out there.


So what is potato blight?   Here is a close up of the potato leaves and infected tubers.  It is easy to spot and identify – a brown spot that first appears on the leaves and then spreads to the stalks.

The initial symptom of blight on potatoes is a rapidly spreading, watery rot of leaves which soon collapse, shrivel and turn brown. During humid conditions, a fine white fungal growth may be seen around the edge of the lesions on the underside of the leaves.

  • Brown lesions may develop on the stems
  • If allowed to spread unchecked, the disease will reach the tubers
  • Affected tubers have a reddish-brown decay below the skin, firm at first but soon developing into a soft rot as the tissues are invaded by bacteria
  • Early attacks of blight may not be visible on tubers, but any infected tubers will rot in store


Here is what the Royal Horticultural Society say about blight and how is is transmitted.

The late blight pathogen is a microscopic, fungus-like organism whose sporangia (spore-bearing structures) easily break away from infected foliage and may be wind-blown for long distances. The actual infective spores are released from the sporangia into water and need to swim in a water film before settling on the plant surface and penetrating into leaf tissues; this is why the disease is so serious in wet summers. The pathogen then spreads rapidly, killing the cells. Under humid conditions, stalks bearing sporangia grow from freshly killed tissues and the disease can spread rapidly through the crop.

The pathogen overwinters in rotten potatoes left in the ground or by the sides of fields. However, the great majority of infections in gardens arise from wind-blown sporangia originating in other gardens, allotments and commercial crops. In the UK, outbreaks may occur from June onwards, usually earliest in the South West.

The presence of new strains in the UK means that the pathogen now has the potential to produce resting spores (oospores) in the affected plant tissues. The oospores are released from the rotting tissues to contaminate the soil. These resting spores have yet to be found in the UK, however, and little is currently known about their survival and their potential as a source of the disease. The investigations into oospores are continuing and more information may be available in a few years.

Late attacks of blight defoliate potato crops, but if the disease arrives after the tubers are set and they are harvested before they become infected, little is lost. However early attacks can also be devastating and blight is the most important commercial disease of potatoes. Outdoor tomatoes are at high risk of infection if the weather is suitable. The disease is less of a problem under glass as the spores have to find their way into the glasshouse through doors and vents. If, however, blight establishes in a glasshouse the high humidity inside usually leads to very rapid development of symptoms.

Preventative Measures

So that’s the official line on the biology of blight and you can see why it is important to follow some simple guidelines to minimise the chances of it re-occurring every year or infecting your neighbour’s plot.    Currently, there is no fungicide available to gardeners for spraying onto potato plants like what they use on farms and the potato fields.

Here is what you must do if you see early signs of blight:

  • Keep vigilant – inspect your crops at least once a week during the growing season – you can see what happens in a week
  • Gardeners are able to access forecast warnings of when blight is active, or when there have been confirmed cases, in their region.  Sign up to the free Blightwatch alert service – https://www.blightwatch.co.uk
  • Infected material should be gathered and bagged and disposed of off site or, ideally, burned rather than composted.   Never compost blighted material – the spores will lie dormant in the ground and infect your crop next year
  • Earthing up potatoes provides some protection to tubers
  • Early-harvested potatoes are more likely to escape infection
  • Picking off leaves or leaflets when just a few are affected may slow down the progress of the disease very slightly, but will not eradicate the problem
  • When infection levels reach about 25 percent of leaves affected or marks appear on stems cut off the foliage (haulm), severing the stalks near soil level and raking up debris.
  • When the skin on tubers has hardened, after about two weeks, the tubers can be dug up. To prevent slug damage avoid leaving tubers in the soil after this time
  • Use the tubers from blighted crops as soon as possible, checking any stored tubers regularly for decay
  • Operate a rotation to reduce the risk of infection, ideally of at least four years
  • Destroy all potatoes left in the soil, and any waste from storage, before the following spring

Hopefully this helps us all control an ever increasing and frustrating problem that results in potatoes having to lifted too early and as a result, not being very big.

Hopefully you now understand blight a bit better and you are armed with the information for next year to get on top of blight early.

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