What is tomato and potato blight?

Potato and tomato blight (late blight) is a disease caused by a fungus-like organism that 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 potatoes and outdoor tomatoes, but not as common on tomatoes grown in greenhouses.

Blight is specific to tomatoes, potatoes and some ornamental relatives of these two crops. Cases have been recorded on ornamental Solanum species (e.g. S. laciniatum), and also on Petunia.

What is early blight of potatoes?

Early blight is a different disease that is found widely in North America, and is commonly reported on the internet. This fungal disease of potatoes is caused by Alternaria solani and A. alternata. It is not a common problem in British gardens, and is often confused with the symptoms caused by the much more common magnesium deficiency.

Symptoms

You may see the following symptoms with late blight:

Potatoes

• 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

Tomatoes

• 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

Control

Non-chemical control

• Infected material should be deeply buried (below the depth of cultivation), consigned to the local council green waste collection or burned, rather than composted (see 'Biology' section, below)
• Earthing up potatoes provides some protection to tubers
• Early-harvested potatoes are more likely to escape infection
• Gardeners are able to access forecasts of when blight will be active in their region, check if there have been previous instances of favourable weather for the disease, or see if there have been confirmed cases - visit the BlightSpy website, developed for professional growers but providing useful information for gardeners
• Picking off  leaves or leaflets when just a few are affected may slow down the progress of the disease very slightly, but will not eradicate the problem
• When infection levels reach about 25 percent of leaves affected or marks appear on stems cut off the 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 can be dug up. To prevent slug damage avoid leaving tubers in the soil after this time
• Use the tubers from blighted crops as soon as possible, checking any stored tubers regularly for decay
• Operate a rotation to reduce the risk of infection, ideally of at least four years
• Destroy all potatoes left in the soil, and any waste from storage, before the following spring

The genetic population of the fungus is ever-changing and recent research has shown that new strains seem to have overcome the resistance previously exhibited by some cultivars. In the past some potato cultivars had shown limited resistance, these included ‘Cara’, ‘Kondor’, ‘Orla’, ‘Markies’ and ‘Valor’, but this is not currently effective. There are, however, cultivars currently thought to show good resistance to the disease, including Athlete, Alouette, Carolus and the Sarpo cultivars (e.g. Sarpo Axona, Sarpo Mira). Some old favourites are very susceptible, eg ‘Arran Pilot’, ‘King Edward’, ‘Majestic’, ‘Sharpe’s Express’. Visit the British Potato Variety Database for more information. The latest situation with regard to the dominant strains of blight can be found on the Euroblight website.

Most tomato cultivars are very susceptible, but a number of cultivars now claim resistance (see seed catalogues for details). Even these cultivars, however, will probably succumb to the disease to at least some extent in wet, warm weather.

Chemical control

There are currently no fungicides available for use by gardeners against blight on potatoes or tomatoes.

Biology

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 (zoospores) 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 presence of new blight strains in the UK means that the pathogen now has the potential to produce resting spores (oospores) in the affected plant tissues. The oospores are released from the rotting tissues to contaminate the soil. These resting spores have yet to be found in the UK, but analysis of the recent variations occurring in blight strains in some parts of the UK suggests that they could be being produced. Little is currently known about their survival and their potential as a source of the disease, but investigations are continuing and more information is likely to become available over the next few years. However, because oospores are resilient structures, if they are produced in infected foliage it is quite possible that they will survive many home garden composting systems. This is why it is preferable to dispose of waste from blighted crops in other ways. Municipal and commercial composting systems reach the very high temperatures necessary to kill oospores and other resilient pathogen propagules.

Attacks of blight occurring in late summer 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 be devastating and blight is the most important disease of potatoes, both for gardeners and commercial growers. 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.