Potato & Tomato BlightPotato & Tomato Blight

Potato and tomato blight is a disease caused by a fungus-like organism which spreads rapidly in the foliage and tubers or fruit of potatoes and tomatoes in wet weather, causing collapse and decay.  Once you have it, it is pretty much game over for the infected plant, so how do we limit its effect.....

If any plot has blight infections, these plants must be removed and either burnt or taken home immediately to try to reduce the impact on neighbouring plots growing crops, failure to do so will mean the infected crops will be removed for you and you may be stopped from growing that crop next year!

What to Look for:

You may see the following symptoms:

In Potatoes

In Tomatoes

Once you have Blight on your potatoes and tomatoes there is really nothing you can do but remove the infected plants immediately to try to reduce the risk of the disease spreading, it is mandatory to remove the debris from site or burn it if the wind is blowing the right way, NEVER compost the debris as Blight can last up to 4 years in a dormant phase so increase the risk of infection in following years.

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  Recognising Blight
  Controlling Blight
  Minimising Blight Risk
Recognising Blight

Recognising BlightRecognising Blight

What to Look for:

You may see the following symptoms:

In Potatoes

  • 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

In Tomatoes

  • The symptoms on tomato leaves and stems are similar to those on potatoes
  • Brown patches may appear on green fruit, while more mature fruits will decay rapidly

Once you have Blight on your potatoes and tomatoes there is really nothing you can do but remove the infected plants immediately to try to reduce the risk of the disease spreading, it is mandatory to remove the debris from site or burn it if the wind is blowing the right way, NEVER compost the debris as Blight can last up to 4 years in a dormant phase so increase the risk of infection in following years.

Controlling Blight

Recognising BlightRecognising Blight

Controlling Blight

Non-chemical control
  • Infected material should be deeply buried (more than 45cm deep), consigned to the green waste collection or, ideally, burned never composted composted.
  • Earthing up potatoes provides some protection to tubers.
  • Early-harvested potatoes are more likely to escape infection.
  • When infection levels reach about 25 percent of leaves affected or marks appear on stems cut off 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 are dug up and stored. To prevent slug damage avoid leaving tubers in soil after this time.
  • Operate a rotation to reduce the risk of infection, ideally of at least four years.
  • Destroy all potatoes left in the soil and waste from storage before the following spring.

The genetic population of the fungus is ever changing and new findings have shown that one dominant new strain seems to have overcome major gene resistance. In the past some potato varieties had shown some resistance, these included ‘Cara’, ‘Kondor’, ‘Orla’, ‘Markies’ and ‘Valor’, but this is not currently effective. The ‘Sarpo’ range exhibit more effective resistance than other cultivars and can be grown satisfactorily without fungicide protection.

Some old favourites are very susceptible, eg ‘Arran Pilot’, ‘King Edward’, ‘Majestic’, ‘Sharpe’s Express’. Varieties that were previously rated resistant have been retested against this new dominant strain and the results have been published.

Tomatoes are generally very susceptible, but the varieties ‘Ferline’, ‘Legend’ and ‘Fantasio’ are claimed to show some resistance, but will eventually succumb in wet, warm weather. It is probably best not to rely on host resistance for blight control in tomatoes.

Chemical control

Because infection is so dependent on certain combinations of temperature and rainfall that periods of high risk (blight infection periods or Smith Periods) can be predicted accurately. Advisory services issue warnings for commercial potato growers on which they can base their spray programmes.

Gardeners are able to access these warnings (visit the Fight Against Blight website), but must rely on a more restricted range of protectant fungicides containing copper (Bordeaux Mixture or Fruit and Vegetable Disease Control), since the more effective systemic products are not approved for home gardener use. A fine spray covering all the foliage will give the best protection.

When wet weather is forecast from June onwards, protectant sprays are advisable, especially for outdoor tomatoes. However, in wet periods the fungicides sold to gardeners will only slow the spread, and not prevent infection. In dry seasons good control can be achieved.

Minimising Blight Risk

Recognising BlightRecognising Blight

How do I avoid potato blight?

There are a number of things you can do to reduce the instance of potato blight from the type and variety of potato you grow to good garden hygiene and finally damage limitation when it does hit. 

Non-chemical control

Growing early varieties

Blight is most common in July and August so growing early varieties which are harvested before blight arrives allow you to avoid the disease. You will need to grow early potatoes for immediate use rather than maincrop potatoes for storage.

Allotment Hygiene

  • Make sure all potatoes are removed from the soil at the end of the season, this can be difficult as some very small tubers can be difficult to find.
  • Avoid dumping infected potatoes near your plot.
  • Potatoes from the previous season growing on a compost heap are a common source of infection in many allotments so try to keep a potato free heap.
  • Blight is spread by the wind, so removal of infected plants and debris is essential, burn it or remove from site.

Chemical Sprays:

  • All of the sprays available to the amateur grower are really only preventative measures.
  • Bordeaux Mixture
  • Baking Soda mixed with vegetable oil with water
  • Bayer Garden Fruit and Vegetable Disease Control
  • Recent studies have indicated that spraying tomato plants with a diluted asprin solution tricks the tomatoes defence mechanism into toughning the leaves and stems making them 47% less likely to be effected by blight.  This also managed to make fruit sweeter and higher in vitamins too.