Tomato Blight (Phytophthora infestans)

Also known as late blight, tomato blight is a highly contagious and destructive disease caused by Phytophthora infestans, a fungus-like organism (Oomycete) within the Kingdom of protists. Affected plants include potatoes, tomatoes and to a lesser extent eggplant and petunias.  Tomato blight was responsible for the Irish Potato Famine in the 1800s which killed 1 million people and is one of the most significant plant diseases, with a worldwide economic cost of US $5 billion annually.

There are three types of blight in tomatoes. Septoria leaf spot, early blight and late blight. This article refers specifically to late blight.

P. infestans has a worldwide distribution, thriving in cool areas with high rainfall.  There are two types of tomato blight, A1 and A2. Each type is able to asexually reproduce and sexually reproduce, resulting in the exchange of genes and leading to new strains that are resistant to fungicides and resistant cultivars.

The name Phytophthora is derived from the Greek word phyto = plant, and phthora = destroyer.

How is tomato blight transmitted?

Transmission occurs when fungus-like water mould spores known as sporangia are dispersed by wind, wind-driven rain, or splashing raindrops from an infected plant.  P. infestans is a necrotrophic parasite, spores penetrate the foliage, growing into a dark lesion, which in turn contains thousands of new spores and spreads to new plants within the Solanaceae family. All above-ground parts of the plant are affected including the leaves, stems and fruit.


Sporangiophores are stalks with a flattened disk at the tip that release thousands of sporangia (asexual spores). Sporangia are carried by the wind to their host (plant). When they land on plant tissue (which must be covered with a film of water to aid in the movement of sporangia to the site of penetration), they penetrate the plant via the stoma (natural openings) or form a penetration peg to penetrate the epidermis. Mycelium (root-like structures)secrete proteins and peptides known as effectors, which target the extracellular space and subcellular compartments which suppress the plant’s natural defences and destroy plant cell walls,  resulting in characteristic lesions.

What does tomato blight look like?

Tomato blight on a leaf
Tomato leaf infected with Phytophthora infestans

Tomato blight affects the leaves, fruit and tubers of both tomato and potato plants and occurs late in the growing season (around August in the Northern Hemisphere) in temperatures above 10 degrees centigrade, and in high humidity (almost 100%). Visible signs appear approximately three days after infection and kill the plant within days.

  • Areas of rapidly enlarging irregularly shaped lesions on the leaves, especially after periods of wet weather. These may appear water soaked in wet weather, in dry weather, the centre will dry out. The lower leaves are usually affected first before lesions rapidly spread to the stems and higher leaves.
  • Lesions appear on the fruit as dark, greasy spots, secondary organisms can then invade the fruit, leading to soft rot.
  • As the infection progresses, the leaf, stem, fruit and tubers develop a white fuzzy appearance when the organism reproduces, and eventually collapses.

Tomato blight

How to treat tomato blight

Fungicides can be used to prevent blight, but it is not possible to treat tomato blight in a plant already infected.

Fruit may be removed from the plant, the plant should then be carefully destroyed. Infected plants should be placed in a plastic bag & placed in the bin. Do not put infected plants in the compost.

Fruit may be edible, leave it for a few days to see if it develops lesions. If it doesn’t, then it is fine to eat. Discard any fruit that develops signs of tomato blight.

Killing tomato blight in pots and soil

Phytophthora infestans can survive on plant debris including potato tubers, fruit, leaves, and stems of infected plants. Oospores can survive in soil and any remaining plant matter left behind.

Infected tomato plants in containers should be carefully disposed of, along with the potting mix. If the pot is to be reused, it will need to be sterilised.

Sterilising pots

  • Clean any debris remaining on the pot with soapy water, this stem is important as bleach is deactivated in the presence of organic matter
  • Allow the pots to air dry
  • Make a solution of 1 part bleach to 10 parts cold water and soak the pot for a minimum of ten minutes
  • Remove pots from the bleach solution and thoroughly rinse in water

Killing tomato blight in soil

Solarisation can be used to kill tomato blight in bedding soil. This chemical-free method utilises high temperatures from sunlight to kill soil-borne pathogens and is most effective when carried out during the summer months.

Solarisation method

  • Clear the area of rocks, debris and plants
  • Turn over the soil so that it is crumbly and then rake until the soil is smooth
  • Soak the area with water making sure to not leave any uneven layers in the soil
  • Measure out the area to be solarised
  • Place thick clear plastic over the entire area, being careful not to leave any air pockets to ensure the heat is retained
  • Bury the edges of the plastic to prevent them from being dislodged in windy conditions.
  • Remove the plastic after 6 – 8 weeks

What is the difference between early blight and late blight?

Early blight is caused by the fungus Alternaria solani and affects plants in warm and humid climates. Affected plants develop a characteristic bullseye on the foliage as well as lesions on the stems, fruit and tubers. Late blight is caused by the oomycete Phytophthora infestans and infects plants in damp and cool conditions. Both A. solani and P. infestans infect members of the Solanaceae family.

How to prevent tomato blight

Drainage Plant tomatoes in locations with good drainage to help
prevent excess moisture.
Mulch This not only reduces the need to water as often but also helps prevent splash-up from rain and watering
Space plants apart Ensure there is plenty of space between tomato plants to
allow for good airflow and reduce the risk of tomato blight spreading to other plants.
Be careful when bringing
tomato plants/cuttings etc., into your garden
Make sure you choose healthy-looking plants with no signs of disease.
Choose resistant cultivars Tomato plants that carry Ph-2 and/or Ph-3
genes have been found to be resistant to tomato blight. Research from Cornell University found the following species of tomato the most resistant to tomato blight; Mountain Magic, Jasper, Regal Plum, Legend, Juliet, Pruden’s Purple, Aunt Ginny’s Purple, Black Plum, Brandywine, Tigerella, and Matt’s Wild Cherry.
Select early season cultivars Most tomatoes mature 70 – 80 days to reach maturity, while early season varieties only take 55-65 days. Providing adequate nutrition can also speed up the growth of seedling tomatoes.
Grow in greenhouses It is easier to protect tomatoes from rain in greenhouses. It is important to ensure is well-ventilated to keep humidity down.
Prune Trim the foliage and side shoots of tomatoes to increase airflow.
Provide adequate support A tomato cage or stake should be used to support the
tomato plant and keep its leaves off the wet ground.
Fungicides Fungicides containing chlorothalonil or copper can be used on plants to prevent tomato blight, but are unable to treat the disease.
Check for signs of tomato blight Regularly check all tomato plants for signs of tomato blight and immediately remove and carefully dispose of any plants showing signs of infection.