Where does blight come from? Well, it blows in the air as minute spores when the weather is humid.
When two consecutive night temperatures remain at 10C or above and the humidity is 90 per cent or above, blight strikes.
Ten days ago the temperature was over 10C at night but the humidity was 78 per cent. The following night it went down to 72 per cent – we were very lucky.
So what can we do to prevent blight?
Even if you don’t see leaves on the outdoor tomatoes and potatoes curling, or if there are no signs of brown and grey patches on the leaves, still spray the entire crop.
I would recommend that you use Bordeaux Mixture or Dithane 945 as soon as you can.
This will cover the leaves with the fungicide and, when the spores land on the leaves, the fungicide will prevent them entering.
Hopefully it won’t rain afterwards, otherwise you will have to repeat the spray.
Even if you have never had to spray before, please do it because you’ll be really lucky if you don’t get affected by blight. If the season becomes wet and humid, it will thrive.
If your potato haulms are already covered with brown patches, it’s still wise to spray.
But the best thing to do is to cut off all the haulms with secateurs, just leaving four inches of stem.
Carefully pull out the weeds too and put them into a big bucket straight away, making sure you don’t throw the haulms around. The less movement the better.
Remember, if you throw the haulms around, the disease can blow on to a neighbour’s potatoes.
Bag everything up, including the weeds which will also be coverd in spores, seal the bag and take it to the recycling centre.
If you don’t do this with care, the spores will continue to affect foliage which has not been contaminated.
Once this has been done, the potatoes can be lifted, but dry them off before bagging them up.
Instead of the normal practice of filling a potato bag, only half fill it because it will be essential to check once a week to ensure the tubers aren’t going rotten.
Once a tuber becomes soft, it spreads to all the others.
I advise doing the same for tomatoes. Spray them because you might get blight if you don’t.
Having said that, it isn’t often found in the greenhouse unless you were throwing the potato haulms around or going in there with clothing smothered with spores!
Are the spores visible? No – it takes 100 to be the size of a pin prick.
As always, it’s best to prevent problems rather than trying to cure them later.
That’s particularly true in this case, as I’m afraid there is no cure for blight.