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Potato Late Blight


Potato Late Blight

Usually appear in

Spring to Summer

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Caused By:-     The Fungus Phytophthora infestans


The early stages of the blight on leaves. Potato Late Blight is one of the worst disease problems for the potato grower and in history it is the cause of the Irish famine from 1845 to 1855. It can wipe out the plants almost overnight and, worse still, it can infect the tubers from the above ground parts and from tubers to tubers causing them to rot in storage. It is caused by the fungus Phytophthora infestans which can also infect other plants in the Solanaceae family like tomatoes, squashes, datura, tobaco and melons. It is believed that this fungus was introduced from the americas with seed potatoes in the 1840's. The fungus is an oomycete or water mold that needs a period of about 48 hours where the relative humidity is around 90% and the minimum temperature is 10C. to germinate and infect plants. Weather forecasters often gives an indication when these conditions will appear so that farmers can apply preventative spraying.
Spores can also travel long distances on the wind and develop on the leaves, spreading through the crop to the tubers. Rain can wash spores into the soil where they infect young tubers.
The variety of potato being grown has great effect on the chances of being infected as many varieties are resistant or partly resistant to this fungus. Research is continuing with various crosses and genetic engineering to further reduce the effects of this and other fungi. See the British Potato Variety Database.
The early stages of potato blight are easily missed, and not all plants are affected at once. Symptoms include the appearance of dark blotches on leaf tips and stems. A white mould will appear under the leaves. The whole plant will quickly collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin. Cutting the potato in half will reveal a brownish rot spreading down from the skin eventually the entire potato will be a mass of soft, foul smelling rot.
Seemingly healthy tubers may rot later when in storage.
The fungus will overwinter in tubers left in the ground and infected plant materials and the spores are wind blown.
Ordinarily the spores do not last long in the soil, but resistant spores are produced when two different strains of the fungi fuse and mate which rarely happens.
Pictures from Wikimedia Commons.


Try to get all the potatoes out from the ground at harvest time.
Water from the base of the plant rather than spraying potatoes.
Removing the foliage early helps prevents the disease getting into the tubers, as long as they are well covered with earth. Leave the crop alone for at least two weeks to let the blight spores on the surface die and the potatoes develop a thicker skin.
Practice crop rotation, removal and burning of infected plant debris, and eradication of weed hosts to reduce the number of spores that can infect the plants.
After harvest, check regularly for signs of blight and remove any suspect tubers at once from your store.
Use only good quality seed potatoes obtained from certified suppliers
Fungicides for the control of potato blight are normally only used in a preventative manner, perhaps in conjunction with disease forecasting. In susceptible varieties, sometimes fungicide applications may be needed weekly. An early spray is most effective. For organic farmers a copper based fungicide is often used.
Plant resistant varieties; Visit The British Potato Variety Database for more information.

P_potblight P_potblight[2]

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