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how to stop blight

How to stop blight

What is late blight?

Late blight is a serious and widespread disease of the Solanaceae family. It is often called potato blight or tomato blight as it particularly affects these crops, and can destroy a tomato or potato crop in as little as 10 days. However the causal pathogen is the same. This destructive fungal disease is caused by spores of Phytophthora infestans which are spread on the wind and may also contaminate potato tubers in the soil. The severity of this disease was seen in the 1840s when a succession of potato blight epidemics led to the Irish Potato Famine.

When does late blight occur?

Late blight is particularly prevalent during warm humid weather and can be especially problematic in late summer during wet weather. In the UK, tomato and potato blight may occur as early as June in the south. This disease is most damaging to outdoor crops, but can also affect greenhouse crops of tomatoes if conditions are humid.


What does potato blight look like?

Dark brown blotches appear on the leaves, particularly towards the leaf tips and edges. White fungal spores develop around these lesions on the undersides of the leaves, and further lesions develop on the stems. Leaves and stems rapidly blacken and rot causing plant collapse.

The spores are released on the wind and quickly spread to infect neighbouring plants. Spores may also be washed down into the soil where they can infect potato tubers causing a red-brown rot directly beneath the skin which slowly spreads towards the centre of the tuber. In tomatoes, similar symptoms of blight occur to the foliage and damage to the fruits is identifiable as they turn brown, and then shrivel and decay.

How does late blight spread?

The fungal spores of late blight overwinter in old potato tubers that remain in the ground or on the compost heap. These tubers may grow the following year to produce infected shoots. Fresh fungal spores of blight are then released on the wind to infect new crops. Spores may also be washed into the ground by heavy rainfall to infect tubers growing there.

Top tips for tomato and potato blight control

Prevention of late blight is important as there is little that can be done to save an infected crop, so it is well worth taking these precautions:

  1. Always plant healthy disease free seed potatoes from a reputable supplier.
  2. Choose blight resistant potato varieties such as the Hungarian Sarpo range developed specifically for their superb resistance to late blight and viruses. Blight resistant varieties allow the gardener to produce reliable disease free crops without the need for constant spraying.

    potato carolus

  3. Always choose an open planting site with good airflow and leave sufficient space between plants. Better airflow will allow the foliage to dry quickly after rainfalls and therefore slows the spread of blight between plants.
  4. Crop rotation will help to prevent a build up of disease, and will avoid infected plants springing up undetected from potato tubers that were missed during last year’s harvest.
  5. Spray potato crops with a protective fungicide even before signs of blight become apparent. Begin spraying this potato blight treatment from about June, particularly when periods of wet weather are forecast and spray again after a few weeks to protect any new growth.
  6. If plants become infected they should be removed and destroyed (not added to the compost heap). However, where potato crops have already developed tubers then these can be saved by cutting away the foliage and stems. Leave the soil undisturbed for 2/3 weeks to kill off any lingering spores so that they don’t infect the crop when it is lifted.
  7. Given that old potato tubers can harbour blight spores over winter, it is important to destroy any unwanted or diseased tubers. Also remove any plants that spring up the following season from old tubers that were left in the ground over winter.

Sue Sanderson

Written by: Sue Sanderson

Plants and gardens have always been a big part of my life. I can remember helping my Dad to prick out seedlings, even before I could see over the top of the potting bench. As an adult, I trained at Writtle College where I received my degree, BSc. (Hons) Horticulture. After working in a specialist plantsman's nursery, and later, as a consulting arboriculturalist, I joined Thompson & Morgan in 2008. Initially looking after the grounds and coordinating the plant trials, I now support the web team offering horticultural advice online.