Tomatoes: Getting to grips with late blight

Question: Suddenly my few patio pot tomatoes have been infected with black spots on the stems and leaves, which are also curling yellow. I have removed them but fortunately still have some green leaves left. Is there anything else I can do? (R. Walpole)


The most likely cause is potato and tomato blight, properly called late blight, a disease of the foliage and fruit or tubers of tomatoes and potatoes, causing rotting. It is most common in wet weather. Potato and tomato blight is a disease caused by a fungus-like organism which spreads rapidly in the foliage and tubers or fruit of potatoes and tomatoes in wet weather, causing collapse and decay. It is a serious disease for potato and outdoor tomatoes, but not as common on tomatoes grown in greenhouses. Blight is specific to tomatoes and potatoes, and some ornamental relatives of these two crops are also susceptible. Cases have been recorded on some ornamental Solanum species (e.g. S. laciniatum), and also on Petunia.

The initial symptom of blight on potatoes is a rapidly spreading, watery rot of leaves which soon collapse, shrivel and turn brown. During humid conditions, a fine white fungal growth may be seen around the edge of the lesions on the underside of the leaves. Brown lesions may develop on the stems. If allowed to spread unchecked, the disease will reach the tubers. Affected tubers have a reddish-brown decay below the skin, firm at first but soon developing into a soft rot as the tissues are invaded by bacteria. Early attacks of blight may not be visible on tubers, but any infected tubers will rot in store.

The symptoms on tomato leaves and stems are similar to those on potatoes. Brown patches may appear on green fruit, while more mature fruits will decay rapidly.

The late blight pathogen is a microscopic, fungus-like organism whose sporangia (spore-bearing structures) easily break away from infected foliage and may be wind-blown for long distances. The actual infective spores are released from the sporangia into water and need to swim in a water film before settling on the plant surface and penetrating into leaf tissues; this is why the disease is so serious in wet summers. The pathogen then spreads rapidly, killing the cells. Under humid conditions, stalks bearing sporangia grow from freshly killed tissues and the disease can spread rapidly through the crop.

The pathogen overwinters in rotten potatoes left in the ground or by the sides of fields. However, the great majority of infections in gardens arise from wind-blown sporangia originating in other gardens, allotments and commercial crops. In the UK, outbreaks may occur from June onwards, usually earliest in the South West. The fungus can also produce resting spores (oospores) in the plant tissues that can contaminate the soil. Little is known about their survival and their potential as a source of the disease. The investigations into oospores are continuing and more information may be available in a few years.

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Late attacks of blight defoliate potato crops, but if the disease arrives after the tubers are set and they are harvested before they become infected, little is lost. However early attacks can also be devastating and blight is the most important commercial disease of potatoes. Outdoor tomatoes are at high risk of infection if the weather is suitable. The disease is less of a problem under glass as the spores have to find their way into the glasshouse through doors and vents. If, however, blight establishes in a glasshouse the high humidity inside usually leads to very rapid development of symptoms.

Infected material should be deeply buried (more than 45cm deep), consigned to the green waste collection or, ideally, burned rather than composted

Operate a rotation to reduce the risk of infection, ideally of at least four years

Ensure all potatoes left in the soil, or as waste from storage are destroyed 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.

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.

Because infection is so dependent on specific 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 potato crop review website), but must rely on a more restricted range of protecting fungicides containing copper (Bordeaux mixture or Fruit and Vegetable Disease Control), since the more effective systemic products are not approved for amateur use. A fine spray covering all the foliage will give the best protection.

When wet weather is forecast from June onwards, protecting 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.