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Late Blight

Stem blight, tuber blight & leaves developing late blight (Photo S. Harvey)






Leaves have dark brown spots surrounded by a pale green margin. White mould-bearing spores develop around these spots during humid weather conditions. Tubers have irregular brown/purple sunken areas on their skin and reddish brown granular markings in their flesh. 


The disease is carried over from year to year by infected seed, infected ground keepers and infected plants on old potato dumps. The fungus passes up from the tuber, breaks out on the leaves and produces a mass of spores, which are spread to neighbouring plants by wind and rain. Healthy tubers are then infected after spores have been washed off the leaves and into the soil by rain or irrigation. Alternatively, tubers may be infected by contact with blighted foliage during lifting.

Favourable Factors

Warm humid weather and crops grown in alkaline soils, sheltered positions such as river valleys and wooded areas, or areas subject to sea mists. There is a high risk of incidence after a 'Smith Period', which is defind as 2 days (ending at 0900 hours) with temperatures not less than 10C and relative humidity above 90% for at least 11 hours each day. However there are concerns that these criteria should be changed to account for the current pathogen population. Hot dry conditions, however, may temporarily stop the spread of the disease.


The most important disease of potatoes and cause of the Irish potato famine in the late 1840`s. Tops are killed prematurely causing yield losses, although the extent of loss depends on the stage at which growth is stopped. Further yield losses may result from blighted tubers rotting during storage. In recent years more aggressive strains of blight have established in the UK. These require shorter infection windows, can infect at lower temperatures (at least 6C) and move through their life-cycles more rapidly which make them difficult to manage. Recent years have seen 36_A2 and 37_A2 become common blight strains in the UK. Other major strains are 6_A1 and 13_A2 which have the following characteristics:


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