Identifying, preventing & treating tomato blight

Tomato blight is actually two separate and devastating diseases; both are highly infectious and can spread quickly through your plants. But do not worry, we will explain how to prevent tomato blight in your garden in this article.

Tomato blight can be one of two different diseases, late tomato blight and early tomato blight. Late tomato blight is the more severe of the two, and if left unchecked, it can result in a total loss of yield and even spread to other crops. Early blight, while less severe, will still cause yield loss and can also spread throughout your tomato crop. Read on to learn everything you need to know about these devastating diseases and how to prevent them.

Different types of tomato blight

Late tomato blight, also known as tomato late blight or simply late blight, infects plants in the Solanaceae family, especially crops like potatoes (Solanum tuberosum), aubergines (Solanum melongena), and tomatoes (Solanum lycopersicum). It was the disease responsible for the potato famines of the mid-19^th century. Early tomato blight also infects members of the Solanaceae family. While not as severe, if not monitored and treated or removed, like late blight, it can spread throughout your tomato plants, and even infect other Solanaceae species growing nearby.

Tomato late blight: causes and symptoms

Late blight on tomatoes is caused by the fungus-like Oomycete protist species, Phytophthora infestans. It is transmitted by the spread of airborne spores, which are spread by the wind, and thrives in wet, hot, humid conditions. Because the spores carried by the wind settle on moist, damp leaves of the upper canopy, symptoms typically first appear at the top of the plant. Spores that fall on the soil, however, can also be splashed back up onto plants by rain or irrigation. Early symptoms begin with curled, misshaped leaves, which rapidly turn into dark brown lesions with lighter rings around them. In severe cases, where moisture levels are high, white growth, similar to fungal growth, may appear on the underside of the leaves. Dark lesions also appear on the stems and eventually the fruit. On the fruit, the symptoms start out as yellowish, slightly wet looking patches, but quickly turn dark and collapse upon themselves. If conditions are favourable, the entire plant will succumb to the disease in a matter of days.

Tomato early blight: cause and symptoms

Early tomato blight is caused by the fungus Alternaria solani. The fungus is first transmitted by spores splashed from the soil onto the plant by rain or irrigation, or by contact with infected tissues of other plants. The early stages look quite similar to late blight, with the only difference being where the symptoms appear. Early tomato blight symptoms start at the lower levels in the older leaves nearest to the soil. As the disease progresses, typical symptoms appear as brown lesions with concentric circles moving outwards to a yellow perimeter on leaves, fruits, and stems.

How to prevent tomato blight?

Prevention and identification are the best tools for combating both types of tomato blight in your garden. There is little that can be done once symptoms of tomato late blight appear on a plant; the plant must be culled. With early tomato blight, symptomatic tissue can be removed to slow the onset of the disease. However, the best cure is prevention. Keep reading for our advice on how to prevent tomato blight from taking hold in your garden.

Choosing blight-resistant tomato varieties

To begin with, healthy plants offer the most protection against tomato blight. The healthier your plants, the better the chance they have at keeping these and other diseases at bay. That being said, while there are no 100% tomato blight resistant varieties, there are certain varieties of tomatoes that naturally offer, or are bred for, some resistance to these diseases. One such is the Humboldt tomato (Solanum humboldtianum), a wild tomato variety that is becoming popular as a cultivated tomato today, more than 200 years after its discovery in Columbia. The currant tomato or pimp (Solanum pimpinellifolium), is another wild tomato from South America that offers slight resistance to tomato blight. These wild tomatoes are used in breeding programmes with cultivated varieties to increase their disease resistance. One of the most noteworthy blight-resistant tomato varieties is S. lycopersicum ‘Crimson Crush F1’ tomato. Since it is an F1 hybrid, the ‘Crimson Crush F1’ seeds may not stay true to form. Other resistant varieties are S. lycopersicum ‘Legend’, S. lycopersicum ‘Old Brooks’, and S. lycopersicum ‘Fantasio F1’.

Other prevention methods

Proper location and crop rotation are crucial to preventing tomato blight. The spores can live in the soil as well as being airborne, so if you experience tomato blight, wait three to four years before replanting tomatoes or other plants in the nightshade family (Solanaceae) in the same location. Also avoid planting tomatoes directly after other nightshades, even without blight outbreaks. Using covers or growing tomatoes in greenhouses can help a little bit in the prevention of tomato blight, as they can help to keep the plants dry during rainstorms and help to reduce the number of spores that settle on the plants. Be careful, though, as high humidity levels and high temperatures are advantageous to the development of tomato blight. Therefore, ventilation plays a key role in preventing the onset of blight. While more airflow can mean more spore transmission, the increased ventilation reduces humidity, making conditions less favourable for spore development. Lower planting density and proper tomato plant pruning will both increase airflow around the plants. Proper watering techniques also help to prevent the onset of both types of tomato blight. Watering low to the ground and mulching will reduce splashing the spores from the soil onto the plants and help to keep the leaves dry.

Tip: If your plants do succumb to tomato late blight or early tomato blight, make sure to dispose of them properly. Cut or dig your plants out of the soil and bury them deep somewhere outside the garden or dispose of them in your household waste; do not compost them. Clean and disinfect your secateurs or tools as well as your hands to reduce the spread of spores. If you are growing tomatoes in pots, dispose of the soil, disinfect your pots and use new soil for your next plantings.

Tomato blight treatment

There is currently no tomato blight treatment that is 100% effective. There are many chemical products on the market that claim to be effective in treating tomato blights. Unfortunately, many are either dangerous and toxic for the environment or only slow down the onset of the diseases. Most of these products are chemical fungicides. While they are effective on Alternaria solani, the fungus causing early tomato blight, the late blight causing Phytophthora infestans is a fungus-like protist and, therefore, is not entirely susceptible to these products. Several copper-based solutions on the market have proved effective in helping to slow down spore germination and growth, but as was mentioned earlier, the best cure is prevention.

The best prevention is proper care, keeping an eye out for symptoms, and healthy plants. In order to keep your plants growing healthy and strong, feed them with a long-lasting organic fertiliser like our Plantura Tomato Food. Our fertiliser provides a slow release of nutrients, including potassium, which is needed to build strong cell walls, making it more difficult for pathogens like Phytophthora infestans and Alternaria solani to infect healthy plants.

Tomato Food, 1.5kg

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Want to learn more about the nutrient requirements of tomatoes? Read our in-depth article on fertilising tomatoes.