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HOW TO SPOT AND DEAL WITH LATE-BLIGHT ON POTATOES AND TOMATOES
WILD WAY DIARY: 11/08/23
Late-blight is a “fungus-like” organism called Phytophthora infestans that will quickly kill potato and tomato plants because they are closely related in the Solanum plant family. You can see how closely the plants are related by looking at the structure of their flowers. This summer, with non-stop rain, is the perfect, wet, humid and warm conditions for the blight to spread quickly and sure enough, our potatoes and outdoor tomatoes are showing the signs.
In the first photo of tomatoes and the above photo of potatoes you can see the dark patches where the leaves have begun dying back. The blight usually shows up first on the leaves before also affecting the tomatoes and potatoes themselves.
Unfortunately, when you spot the patches it is too late, it spreads quickly. You could try removing leaves if it’s not everywhere but the spores, which you can’t see, will likely be all over the plants.
You can compost the foliage, in the UK at the moment blight generally only survives on living plant material, such as potato tubers left in the ground by accident. There is some evidence that blight has changed to rest in soil but currently this is low-risk, so keep an eye on the situation and I’ll update you if it changes.
Thankfully, if you are quick with potatoes, you can cut off the above ground parts of the plant to save the tubers being affected. This will obviously impact growth of the tubers. Personally. I will just dig our main crop potatoes up because the ‘Pink Fir Apple’ are ready and the ‘Sarpo Mira’ aren’t far off. So that’s my task for today.
Tomatoes aren’t so lucky, all you can do is harvest what you have and remove the plant. The best way of dealing with late-blight on tomatoes is prevention and actually, I’m really excited we have late-blight because my outdoor tomatoes are all blight resistant, now I can test how resistant they really are and report back to you in a future newsletter!
Another way of protecting tomatoes from blight is by growing them undercover in a polytunnel or greenhouse. This helps shield them a bit from the spores, which float around in the air, and keeps the foliage dry, which is what causes the blight to grow. I’m growing our main crop of tomatoes in our polytunnel.
You may remember that earlier in the year, during June’s intense heat, I reported distortion of leaves on our polytunnel tomatoes. The fear was some kind of weedkiller contamination in the compost or the tubs I was using to make homemade comfrey fertiliser. It turned out to be heat stress and they eventually recovered, though it did slow down their growth and they are only now starting to crop.
That said, I have noticed some dieback on a few leaves of tomatoes in the polytunnel which I fear may be blight. I’ve removed some of the leaves and will keep a close eye on remaining patches.
How to prevent late-blight
There is no way of stopping blight completely and you cannot treat it, it’s a reality of growing potatoes and tomatoes. Some things you can do to help prevent it include:
Only grow cultivars with proven blight resistance - my outdoor blight resistant tomato trial is now being put to the test allowing me to make recommendations to you
Grow early and new potatoes - they will likely crop before late-blight is a risk
Grow tomatoes undercover - this shields them slightly and importantly, keeps their foliage dry
Double check and remove all potatoes from the ground - this prevents a plant with blight growing back next spring
Rotate crops - to prevent any resting spores in the ground infecting plants in subsequent years (I do a 5 year crop rotation)
Has late-blight reached you, if so please share below when it struck and roughly where you are to give a sense of its spread.
p.s. the other week we went on holiday to Pembrokshire in Wales via East Sussex and I wrote about the highlights here