Late blight of potato
The most devastating disease of potatoes
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What is it?
This is the most devastating disease of potatoes worldwide, and can result in complete destruction of crops. It was responsible for the Irish potato famine of the 1840's that led to the deaths of one million people. Caused by the fungus-like organism Phytophthora infestans, late blight spreads rapidly in warm, wet or very humid weather, destroying plants quickly if undetected. Infected tubers can decay to a foul smelling mush, caused by secondary bacterial rots.
What does it look like?
Brown patches at the tips and margins of leaflets. In high humidity and temperatures of 10oC or above, the patches enlarge and the leaflets curl and wither. In moist weather, the fungus is often visible as a delicate white growth on the underside of the leaf, at the margins of the brown patches. Brown patches may also develop on stems, and the entire plant may be killed in a matter of days if the weather is favourable. Blighted tubers have a reddish-brown rot under a discoloured skin.
How is it spread?
Spores are spread by wind and rain. Infected tubers from the previous year may be discarded at the edge of the vegetable plot, left in the ground or accidentally replanted. A small percentage of them will produce ‘primary infector’ plants, which under favourable conditions soon develop severe blight and usually act as the source of the following year’s outbreak.
How can I control it?
Picking off and destroying infected leaves may occasionally check the progress of the disease enough to allow tubers to reach a harvestable size, if the disease is spotted early. Fungicides can be used, but must be applied regularly, particularly during ‘blighty’ weather. Forecasts are available that show when conditions are suitable for infection – farmers uses these to time their fungicide sprays – and these may be available on the internet. Gardeners who do not wish to use fungicides should grow varieties with good resistance to the disease, e.g. Cara, Kondor, Remarka, Sarpo Mira, Sarpo Axona. Tuber infection can be limited by drawing the soil up around the stem (‘earthing up’). Do not lift tubers under wet conditions, and do not attempt long-term storage of tubers from a blight-affected crop.
When do I test?
Symptoms of late blight are likely to be observed from mid-summer onwards (although they could appear earlier if spring and early summer are very unsettled). Testing should be carried out as soon as symptoms are detected and monitoring for symptoms should continue until cropping is complete.
What do I test?
Where possible, select areas of leaf or stem which show symptoms of disease. For best results include material where healthy and diseased zones meet. Do not use completely dead plant material. As a general guide, leaf material should be approximately 25mm2 and stems should be approximately 25mm. The test also works on blighted tubers, although secondary bacterial rots and their breakdown products might cause problems with the test. For best results, use a firm area of the tuber flesh with blight symptoms, from just below the skin.