The symptoms of these two means that they are easily confused. In this article, we look at ways of telling which problem you may be dealing with in your potato patch, so that you can either remedy the magnesium deficiency or crack on with harvesting the potatoes and cutting back foliage to slow the spread of potato blight.
Firstly, there are two types of potato blight – early potato blight which is not very common in the UK, and late potato blight which IS common in the UK.
Early potato blight (remember, the rare one) looks more like magnesium deficiency than late potato blight. This is useful to know, because if you see something that looks like magnesium deficiency in potatoes being grown in the UK, it most probably IS magnesium deficiency.
Late potato blight (the common one) looks slightly different and is characterised by brown patches which begin on the leaves and then you may see brown patches on the stems of potato plants, and eventually it will work its way into the potato tubers themselves. The picture on the left here shows potato blight in its early stages, with just the leaves affected. Brown patches on the stems/stalks would be a real giveaway.
Late potato blight tends to take hold in late summer as the weather is warm, but wet/damp. For this reason, it tends not to affect first early and second early potatoes which are normally harvested by mid July. It is more likely to affect maincrop varieties which are not harvested until late summer/early autumn.
Magnesium deficiency in potato plants can start in a similar way, with brown patches. Where it is a little different is that it tends to be more spotty in appearance at first, and the leaves will take on a yellow hue. It’s often seen during dry spells, as the plants aren’t getting enough water in order to draw up the key nutrients from the soil.
Telling the difference:
In truth, it is difficult to tell the difference. It’s quite helpful to look at it like this – if it’s dry and early on in the summer, with brown spots on the leaves, you’re probably looking at magensium deficiency, especially if the soil is dry. If it’s late summer, and it’s war and damp and there are brown patches spreading, it’s probably blight.
Sometimes, it is possible that you’ll be seeing both at the same time – perhaps the plants were already low on magnesium in midsummer and then blight arrived in late summer, for example.
What to do
If you’re in doubt, here’s what we would suggest. Cut off the affected leaves as soon as you see them. If the soil is dry, give the plants a thorough watering early in the morning so that the foliage dries off again quite quickly (in case it is blight, as this will help to prevent it from spreading). Do this a couple of times (you could use a liquid feed) and see if the leaves continue to turn yellow/blotchy or if they start to come through healthily. Hopefully they will.
If the leaves return and the brown blotches spread, you can assume it is blight. Keep removing affected foliage and watch for the brown patches spreading to the stems. If they do, cut the foliage altogether. You can then leave the potatoes in the ground and harvest sporadically over the next 2-3 weeks.