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Recognising Blight

Controlling Blight

Non-chemical control

• Infected material should be deeply buried (more than 45cm deep), consigned to the green waste collection or, ideally, burned never composted composted.
• Earthing up potatoes provides some protection to tubers.
• Early-harvested potatoes are more likely to escape infection.
• When infection levels reach about 25 percent of leaves affected or marks appear on stems cut off foliage (haulm) severing the stalks near soil level and raking up debris. When the skin on tubers has hardened, after about two weeks, the tubers are dug up and stored. To prevent slug damage avoid leaving tubers in soil after this time.
• Operate a rotation to reduce the risk of infection, ideally of at least four years.
• Destroy all potatoes left in the soil and waste from storage before the following spring.

The genetic population of the fungus is ever changing and new findings have shown that one dominant new strain seems to have overcome major gene resistance. In the past some potato varieties had shown some resistance, these included ‘Cara’, ‘Kondor’, ‘Orla’, ‘Markies’ and ‘Valor’, but this is not currently effective. The ‘Sarpo’ range exhibit more effective resistance than other cultivars and can be grown satisfactorily without fungicide protection.

Some old favourites are very susceptible, eg ‘Arran Pilot’, ‘King Edward’, ‘Majestic’, ‘Sharpe’s Express’. Varieties that were previously rated resistant have been retested against this new dominant strain and the results have been published.

Tomatoes are generally very susceptible, but the varieties ‘Ferline’, ‘Legend’ and ‘Fantasio’ are claimed to show some resistance, but will eventually succumb in wet, warm weather. It is probably best not to rely on host resistance for blight control in tomatoes.

Chemical control

Because infection is so dependent on certain combinations of temperature and rainfall that periods of high risk (blight infection periods or Smith Periods) can be predicted accurately. Advisory services issue warnings for commercial potato growers on which they can base their spray programmes.

Gardeners are able to access these warnings (visit the Fight Against Blight website), but must rely on a more restricted range of protectant fungicides containing copper (Bordeaux Mixture or Fruit and Vegetable Disease Control), since the more effective systemic products are not approved for home gardener use. A fine spray covering all the foliage will give the best protection.

When wet weather is forecast from June onwards, protectant sprays are advisable, especially for outdoor tomatoes. However, in wet periods the fungicides sold to gardeners will only slow the spread, and not prevent infection. In dry seasons good control can be achieved.