Tomato blight (also known as late blight) is a disease caused by a fungus-like (Oomycete) organism that spreads rapidly through the foliage and fruit of tomatoes in warm, wet weather, causing collapse and decay.

It can be a very serious disease on outdoor tomatoes. Tomatoes grown in greenhouses are less likely to be affected but can still suffer badly at times.

Blight is specific to tomatoes, potatoes (on which it is known as potato blight) and some ornamental relatives of these two crops. Cases have been recorded on ornamental Solanum species (e.g. S. laciniatum), and also very occasionally on Petunia.

You may see the following symptoms on tomato plants with blight:

• The initial symptom of blight is a rapidly spreading, watery rot of leaves, which soon collapse, shrivel and turn brown. During suitable conditions, when the pathogen is spreading actively through the leaf tissues, the edges of the lesions may appear light green, and a fine white 'fungal' growth may be seen on the underside of the leaves
• Brown lesions may also develop on the leaf stalks (petioles) and stems, again with white growth sometimes visible under wet or very humid conditions. These lesions can lead to collapse and death of leaves, stems or even the entire plant
• Watersoaked patches, turning brown, can appear on green fruit, whilst more mature fruits will decay rapidly

• 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 the 'Biology' section, below)
• Clean any plant supports and other garden equipment that has previously come into contact with blight with a disinfectant such as Jeyes Fluid before re-use, to make absolutely sure that there is no disease transfer. Hard surfaces and the glass in greenhouses can be cleaned in the same way.
• 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. This has been developed for professional growers of potato crops, but can also provide useful information for gardeners growing tomatoes as well as potatoes
• Picking off leaves when just a few are affected may slow down the progress of the disease very slightly, but will not eradicate the problem
• Ensure that the foliage of greenhouse-grown tomatoes is kept as dry as possible
• Use the fruit from affected crops as soon as possible. Once a plant has developed high levels of the disease on the leaves and stems it is likely that many of the fruit will also have been infected, even if they don’t yet show symptoms. Attempting to harvest and ripen green fruit from affected plants indoors may therefore result in a large percentage of the fruit subsequently rotting, so using the unripe fruit immediately (for example to make chutney or sauces) may be a better option
• Due to the potential risk of long-lived resting spores being produced by the blight pathogen (see the ‘Biology’ section, below) it is best not to use the soil or compost from a blight-affected greenhouse crop to grow tomatoes in the following year. Re-using soil or compost will also increase the risk from a range of other diseases that can attack the root system or the vascular system of the plants. The soil or compost could be distributed over an area of the garden not used for growing tomatoes or potatoes.
• Operate a rotation to reduce the risk of potential infection from resting spores in outdoor soil-grown tomato crops, ideally of at least four years (also avoid growing potatoes in the soil during this period)

Many tomato cultivars are very susceptible, but cultivars are now available for most types of tomato that claim resistance to the disease (see seed catalogues or packets for details). Even these cultivars are not usually completely immune, and will probably succumb to the disease to at least some extent if prolonged favourable weather occurs.  However, they may still give a good yield when susceptible cultivars would succumb completely to the disease, and are particularly useful if the cultivar is one that is suitable for outdoor crops. For example, the cultivars 'Crimson Crush' and 'Mountain Magic' performed very well at Wisley in 2021, when blight was extremely widespread and susceptible cultivars were severely affected.

Greenhouse-grown tomato crops are less likely to be affected, simply because the wind-blown blight spores are less likely to reach the plants, but if spores do blow in through the door or vents then the disease can be just as damaging.

The genetic population of the blight pathogen is ever-changing, so it is possible that new genetic strains could arise at any time that may overcome the resistance previously exhibited by some cultivars.

Chemical control

There are currently no fungicides available for use by gardeners against blight.

The late blight pathogen is a microscopic, fungus-like organism whose spores (sporangia) are easily shed from infected foliage and may be wind-blown for long distances. In order for infection to occur prolonged surface wetness (several hours) is required; this is why the disease is so serious in wet summers. The pathogen then spreads rapidly through the plant tissues, killing the cells. Under humid conditions, stalks bearing sporangia grow from freshly killed tissues and the disease can spread rapidly through the crop.

The primary source of the disease each year is usually affected potato plants - the pathogen overwinters within infected potato tubers left in the ground (and also left by the sides of fields in the case of some commercial crops affected the previous year). Whilst it’s possible that tomato or potato material left in an individual garden could act as a source of the disease for the following year, the great majority of infections in gardens arise from wind-blown sporangia originating from other gardens and allotments, and from commercial potato 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 genetic analysis of the recent variations in blight strains occurring in potato crops 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 also 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 usually reach the very high temperatures necessary to kill oospores and other resilient pathogen propagules.

Outdoor tomatoes are at particularly 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.