The past few weeks have been hot and humid. This means not just uncomfortable sleeping but it is perfect for fungal diseases. I have never had such bad blackspot on the roses – though I think the lack of foliage is partly down to the awful winds too – and soft fruit is going mouldy on the plants. But, of course, wet and warm weather means potato blight too.
This disease plays a big part in Irish history, partly because the Irish took to the newly introduced potato in such a big way. In fact it was probably grown in Ireland before it was introduced to England. Sir Walter Raleigh grew it on his estate at Youghall, Co. Cork, before it was taken to England,m probably in 1597. And while it was eaten as a novelty in England it became a staple food in Ireland. All went well until successive failures in the crop, from 1845-49, led to widespread starvation. It is not the place here to go into the details of the tragedy.
Blight is an airborne disease and the spores germinate on the foliage and create a problem when there are Smith periods. A Smith period is (roughly) when the minimum temperature is 10°C and the relative humidity is more than 90% for 48 hours. So you can see that the recent warm, wet, thundery weather has been perfect for blight. Blight tends to affect the heavy-cropping maincrop potatoes becaue they are still in full growth in August when these conditions are most likely. Early potatoes, which are usually harvested in July, often escape blight.
At first, black patches appear on the leaves, then, in warm weather, a white mould can be seen on the leaves and then the plants collapse. If the weather stays wet the spores drip onto the tubers and they can rot in the ground. If you are quick you can harvest the tubers and try to store them, if they are dry, but you need to watch them at all times because if one starts to rot it will rapidly spread.
There are no sprays that amateurs can use to prevent this (in the EU). This disease also affects tomatoes. But if tomatoes are grown under glass or plastic so the foliage is kept dry, they should not be affected.
I know that everyone prefers one potato or another but I strongly feel, in this circumstance, that, if you must grow your own potatoes, and they take up a lot of space, you try one of the 100% resistant types. I think the best of these are the Sarpo varieties. They crop heavily and I think they taste fine. The only problem with them is that the tops are very vigorous and they make huge plants.
Just a note about potato fruits. The seed potatoes that we plant are not truly seeds but just small tubers. But the flowers will produce fruits that contain seeds and if these are sown they will produce plants but these will not be the same as the potato variety from which they are taken – the way new varieties are produced. Don’t eat the fruits!