Advice
RHS Help & Advice
Potato blight (Phytophthora infestans)
Symptoms
These first appear as brown or black patches at the tips and margins of leaflets. Under suitable conditions the patches enlarge and the leaflets curl and wither. In moist weather the causal fungus is sometimes visible as a delicate white growth on the margins of the patches, particularly on the underside of the leaves. Brown patches may also develop on the stems, and the disease can spread rapidly to all aerial parts of the plant, resulting in its collapse.
Infection can also spread to tubers from the leaves and stems. Blighted
tubers have a firm reddish-brown rot under a discoloured skin.
They are often subsequently invaded by secondary organisms (particularly bacteria)
producing a soft, foul-smelling rot.
Cause
The disease is caused by the fungus Phytophthora infestans. Spores are spread by wind and rain and infected plants under favourable conditions of high humidity and a minimum air temperature of 10oC (50oF). Infected tubers from the previous year may be discarded at the edge of the vegetable plot, left in the ground or accidentally re-planted, and a small percentage of these will produce plants known as ‘primary infectors’. Under the conditions described above these plants soon develop severe blight and usually act as the source of the following year’s outbreak.
The same fungus is capable of attacking tomatoes, and occasionally also affects some ornamental and weed hosts in the family Solanaceae.
Non-chemical control
The time of the first outbreaks, and the subsequent progress of blight are dependent on the weather - warm, moist conditions are needed for the disease to spread. A hot, dry spell may slow disease spread, but the fungus is likely to survive on infected plants, and the progress of the disease will continue when weather conditions are again more favourable.
Tuber infection can be limited by earthing up or drawing the soil up around the stem. Prompt removal of infected foliage before the disease spreads to the tubers may save the crop. Ensure that the tops of plants are removed and destroyed before lifting the tubers. Most importantly, ensure that any diseased tubers are destroyed - do not leave them in the soil or at the edge of the plot. Remove any diseased tubers before planting.
Potato cultivars vary in their resistance to the disease. Among the most susceptible are ‘King Edward’, ‘Arran Comet’, ‘Arran Pilot’, ‘Epicure’, ‘Foremost’, ‘Golden Wonder’, ‘Home Guard’, ‘Kerr’s Pink’, ‘Majestic’, ‘Sharpe’s Express’, and ‘Ulster Chieftain’.
Cultivars showing some resistance include ‘Cara’, ‘Kondor’, ‘Orla’, ‘Markies’, ‘Valor’, and ‘Remarka’.
A recent development has been the arrival in the UK of the Sarpo potato cultivars such as ‘Mira’ and ‘Axona’. Bred in Hungary, these cultivars are claimed to have extremely good and durable resistance to blight and, although they are still being evaluated under UK conditions, they have so far performed very well.
Chemical control
Apply Bordeaux mixture, copper oxychloride (Murphy Traditional Copper Fungicide) or mancozeb (Dithane) as a routine spray to protect plants before they become affected. Regular applications are required as the fungicides are ineffective once infection has occurred. A suitable spray interval is 10-14 days, but this could be lengthened during prolonged dry spells.