Late Blight
Stem blight, tuber blight & leaves developing late blight (Photo S. Harvey)
Pathogen
Fungus
Hosts
Potatoes
Symptoms
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.
Development
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.
Importance
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:
Shorter latent period of just four days.
Earlier sporulation at lower temperatures.
Bigger lesions and higher sporulation rates.
Greater aggressiveness on tubers. Using state-of-the-art DNA genetic fingerprinting, the British Potato Council sponsors the monitoring and changing development of these genotypes dating back to 2003 and these are published on the EuroBlight web site (www.euroblight.net)
Control
Varietal
There are no resistant varieties, but less susceptible varieties should be selected for both late blight and tuber blight as documented in the National List
Avoid liming before planting
Ensure rows are well earthed-up
Destroy tops at least 2 weeks before lifting
Remove infected tubers after lifting and prevent growth on dumps
Tubers must be stored in dry, cool conditions
Decision support systems are widely available to predict the onset of disease and help with the timing and frequency of foliar fungicides
Fungicide treatment should be started after the first 'Smith Period' has been recorded, or before haulms meet in the rows, whichever occurs sooner
With the introduction of new more aggressive strains of blight, growers are regularly treating crops at a 7 day interval until haulm destruction
