Potato and tomato blight, properly called late blight, is 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 ornamentalSolanum species (e.g. S. laciniatum), and also on Petunia.
What is early blight of potatoes?
Early blight is widely found in North America, and is commonly reported on the internet. This fungus disease of potatoes is caused by Alternaria solani and A. alternata. It is not a common problem in British gardens and is frequently confused with magnesium deficiency.
Fungicides used to control late blight will have some effect on early blight.
You may see the following symptoms:
- 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
- Infected material should be deeply buried (more than 45cm deep), consigned to the green waste collection or, ideally, burned rather than 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 damageavoid leaving tubers in soil after this time
- 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. 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.
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 protectant 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, 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.
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.
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.
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