Advice from the Offshoots project - how to deal with potato or tomato blight

04 August 2014

Offshoots’ top tips to take home your harvest of organic potatoes and tomatoes.

So, it’s a warm beautiful day, there was heavy rain last night and now you’re wandering in the garden with a cup of tea, looking at all the wonders of creation (rain = no watering = bliss) when...

Potato Blight

“Oh no! Is that... a brown spot I see on my potatoes?”

Your stomach lurches and you feel a strange affinity with 1840’s Ireland.................It’s blight time!

For the past four years at Offshoots we’ve had blight - first the potatoes and then the tomatoes.  So if you’re thinking “is it just me?” then this blog is to tell you that you are not alone.  Almost everyone gets blight and we want to encourage you with some top tips we’ve learned over the years.  

What is blight? How to recognise the tell-tale signs.

There are two types of blight: early blight and late blight. Early blight is commonly found in North America, but less so in the UK. If you think you have early blight, double check the symptoms as it could be magnesium deficiency. The major threat to UK crops is from late blight.

Late blight is a fungal disease caused by Phytophthora Infestans. This fungus-like organism is spread in wet, warm, humid conditions called the ‘Smith Period’, so you may well spot it whilst having that morning brew after a hot showery night! It will look initially like small brown rotting spot patches on the plant’s leaves – often on the margins.

These will enlarge and have a light green halo effect around them and a white downy underside. These are the spores which are wind-blown from infected plants and deposited in drops of water on the plant’s leaves. If a plant is infected, it will rapidly spread to other leaves and the stem will turn brown before the plant shrivels and dies. In potatoes, spores will wash down into the soil and cause brown colouring on the tubers. These may rot in the soil or rot in storage - even if they look fine on first inspection. On tomatoes, the fruit will develop brown sunken spots. On both plants the spread of blight can be catastrophic to your crop.

Prevention is better than cure.

Gosh, that's all a bit depressing isn’t it? Well fear not, there are things you can do to try and nip that naughty blight in the bud!

1.  Grow the right varieties.  At Offshoots we grow mainly first and second early varieties of potato to try and get them in and out before blight time, which is typically mid-summer. If you grow maincrop try blight-resistant varieties.  At Offshoots this year we have a trial Oca bed. Oca is a South American vegetable grown and used just like potatoes, but with a lemon taste, pretty flowers and edible leaves  (although you shouldn't eat too much of them as they contain oxalic acid).  Best of all, it’s blight free! See Real Seeds for more information.  

2.  Practise good plant hygiene:

  • Always rotate your vegetable beds.
  • As soon as you see blight, take the infected leaves off your tomatoes or cut your potatoes right down. You can dig up the potatoes after a couple of weeks. DO NOT compost any plant with blight, as you may reinfect next year’s crop. If blight comes back on your tomatoes or they are gonners, then you can always make green tomato curry or green chutney straight away, or take a chance that it hasn’t spread to the fruit and try and ripen them inside.
  • Clean your tools thoroughly if you’ve been dealing with a diseased plant so you don’t pass it on to a healthy one.
  • Dig out all potatoes from the previous year. Potatoes left in the bed act as hosts for next year’s blight.
  • Be careful! Don’t water the leaves of your plants or splash soil onto the leaves as it will speed up the spread of blight.
  • Always buy certified disease-free potatoes.
  • Earth up potatoes so spores don’t reach the tubers.

3.  Grow inside.  If you can, grow some tomatoes in a greenhouse, or inside, as they will be more protected - and buy some more disease-resistant varieties.


4.  Track the spread of blight in the UK each year with Blightwatch so you know if it’s in your area.

5.  Know your enemy! There are good photos and advice to help you identify the blighter on the following website:

If you’re still unsure, use this great tip from July’s edition of Grow Your Own magazine. Put an affected leaf in a clear plastic bag in a warm place and, if it’s blight, little white spores will develop within a day.

Thankfully this year we’ve had a good early potato harvest by whipping them out really quickly when we noticed spots on the leaves.

So here’s hoping for a green-tomato-curry-free August (although I might sneak one in!).  

If you’ve any recipes for how to use up a glut of green tomatoes, please write in and we’ll share them on our blog. 


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