We planted potatoes in containers and they have succumbed! Blight!
The leaves are browning and I don’t expect anything from them, they simply need throwing away!
This is down to a fungus and the situation is very difficult to control.
The fungus Phytophthora infestans causes the devastating disease potato blight. It’s a problem in all members of the solanaceae, which includes tomatoes – you may have noticed that potato fruits (which are poisonous and must not be eaten) resemble unripe tomatoes.
Blight gains a hold of crops in warm moist conditions – typically on a hot day following a shower of rain, and once established it’s very difficult to eradicate, partly because of the speed of infection. If you ignore blight for a day you may well lose your whole crop.
The first signs are black or brown spots on a few leaves, and this leads to the fairly rapid blackening of the whole plant and spoiling of the potato. On the leading edge of the black patches you can sometimes see white furry growths, which are the fruiting bodies of the fungus.
Crops that have been harvested with the smallest quantity of fungal spores on them, turn into a mushy unpalatable mess almost overnight. Just when you think your crop has been gathered in it is ruined!
The fungus spreads in humid, warm conditions and when the soil and vines are wet from rain the conditions are just right for its spread. Commercial growers can telephone for a ‘blight alert,’ which is a prediction as to the likelihood of blight occurring, based on the prevailing conditions and the incidence of the disease upwind.
Sometimes a hot dry spell can slow down the growth of the fungus, but if it’s been preceded by rain, and if blight had already started its growth, it will appear like a flood once the humidity increases. This is because, once started, the spores will settle on the plant and burst into growth once the conditions are right.
Firstly, make sure there are no potatoes left in the ground from last year. Plant new stock, preferably varieties with high disease resistance. Plant them thinly, and make sure there’s a lot of ventilation space around each vine, so that the microclimate for each plant doesn’t remain hot and moist.
Secondly, avoid watering the plants over the leaves, water only at soil level, this will help control humidity, and stop spores washing from the leaves to the soil.
Thirdly, as soon as you see diseased leaves, remove them well down the vine. Never compost potato vines – burn them and add the ash to your compost.
Always remove the vines some time before you dig up the potatoes. Even if they appear healthy, cut them at soil level, take them away and burn them – adding the ash to the heap.
Always disinfect your hands and tools before going from the potato patch to the greenhouse to prune the tomatoes and visa versa. Remember, it’s more humid in the greenhouse and therefore blight is more easily spread.
Research has shown that copper-based fungicides have no impact on this disease, so washing with things such as Bordeaux mixture is of little value.