Tomato Blight: Identification and Treatment

Tomato blight is a disease produced by fungus that spreads via tomato leaves, especially during rainy conditions. The two most common types of tomato blight are early blight, caused by Alternaria solani or Alternaria tomatophila fungal spores, and late blight, produced by Phytophthora infestans water mould spores.

About Tomatoes

The tomato is a popular fruit and vegetable crop in many parts of the world. It is grown in a variety of ways, including as an annual or biennial plant, as a vine or as a perennial herb. The species is believed to have originated in South America. It was first cultivated in Mexico and Central America before spreading to Spain during the 16th century. It quickly spread throughout Europe and was later introduced to North America by Spanish colonists.

When growing tomatoes, it is important to ensure that they are planted at the right time of year for their area of origin and climate conditions (i.e., early spring or late summer). Tomato plants need warm soil temperatures between 20°C and 30°C (68°F and 86°F) with at least 6 hours of sunlight a day for optimal growth rates throughout their lifecycle (Flannigan & Sloane 1998). In general, tomatoes prefer acidic soil with low levels of nitrogen (N) fertilizer applied during vegetative growth stages; however, some varieties may prefer higher levels than others (Gralinski et al. 2003).

Tomatoes are susceptible to stress due to diseases such as blight caused by fungi, especially when growing under warm conditions during the summer months. Blight on tomato plants causes them to die back near the base of the plant, resulting in dead leaves at ground level, which can be seen easily. Early blight, as the name suggests, usually strikes during early tomato season, while the late blight strikes in the mid-to-late season.

Although these diseases are made up of different microorganisms, the result is the same. For tomato farmers, the destruction caused by the blight can be devastating making for a long growing season.

Tomato plant leaves afflicted with  Alternaria solani disease
Alternaria solani effect on leaves.
Credit Jerzy Opioła, CC BY-SA 4.0via Wikimedia Commons

Early Blight


Early blight can develop as a result of Alternaria tomatophila or Alternaria solani fungus infection. A. tomatophila is far more dangerous than A. solani. In locations where it exists, A. tomatophila is the primary cause of tomato blight. A. solani produces tomato blight in areas where A. tomatophila is not present. Early blight on tomato plants is distinguished by lesions that resemble bull’s-eyes. The blight appears initially in the oldest leaves of tomatoes as little brown-black, dry, papery patches with concentric rings up to half an inch in diameter. Lesions on the foliage may become yellow. Mature lesions may develop velvety patches of fungal spores, whereas fruit spots are black, leathery, and have elevated concentric rings.


Early blight can cause collar rot, which renders stems dark, dry, and depressed and causes seedling mortality. The blight spreads throughout the fruit via insect attachment points or wounds, contaminating the entire fruit. The dark brown lesions on the leaves will swiftly get larger in warm and humid conditions, causing chlorosis and considerable defoliation, leading to sunscald. Infected fruits might fall off the stem. Black, blistering blotches appear on infected fruits.

Late Blight


Phytophthora infestans, which means “plant destroyer” in Latin, is late blight. Late blight affects tomato plants and also potatoes. The illness is distinguished by tiny, necrotic, water-soaked light to dark green patches that are frequently encircled by yellowish rings. These lesions do not have leaf veins and can develop across them. The dots expand into big black and purple patches as they mature. A white, fluffy growth surrounds the rotting region on the underside of the leaves during wet weather. Infested tomatoes emit a nasty, decaying stench. Stem lesions are dark brown in colour and have well-defined edges.


Late blight is the most serious disease that affects potatoes and tomatoes. It is famous for creating the Irish Potato Famine of the 1840s, which killed a million people and resulted in the huge migration of over a million and a half Irish nationals. The illness causes the tomato skin to develop a dark purple tint. The dark, granular rot with vague boundaries may be seen extending into the pulp when the fruit is sliced. Late blight can swiftly damage leaves and spread down the petiole into the stem, eventually killing the plant. The illness will spread if tomatoes are stored in warm, humid conditions.

Tomato Blight Cultural Prevention


Moisture creates ideal circumstances for spores to grow. Watering your tomato plants from above should be avoided. Water the soil around the plant’s base to keep the leaves dry. Watering should also be done in the morning so that the plant may dry out during the day.


Rotate tomato crops to different parts of your vegetable garden every year and do not plant near potatoes that are affected by blight.

Spacing and Pruning

Allow lots of space for tomatoes to grow and stake them rather than caging them. If the plants are sufficiently widely, the wind will keep them dry and make it difficult for the rot to spread. Pruning tomato plants promotes improved ventilation and more vigorous growth.


Mulching tomato plants helps to prevent the spread of spores that cause early and late blight. If the plant becomes diseased, the surrounding mulch may harbour spores and must be removed and burnt.

Organic Fungicides

Copper fungicides are also useful in fighting off early and late blight. Read the instruction label carefully and wear gloves to avoid getting fungicides on your hands.

Chemical Control

Chemical fungicides are very useful in fighting tomato blight. However, it is important to rotate between different chemicals to avoid developing resistance in pathogens for certain fungicides. Some active ingredients to control tomato blight include:

  • penthiopyrad
  • boscalid
  • pyraclostrobin
  • fenamidone
  • azoxystrobin
  • cymoxanil
  • famoxadone
  • fluxapyroxad
  • pyraclostrobin
  • pyrimethanil
  • difenoconazole
  • cyprodinil
  • mancozeb
  • zoxamide
  • mandipropamid
  • fludioxonil

Make applications of chemical fungicides when environmental conditions are favorable and repeat as necessary.

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