Early blight in tomatoes is one of the most common fungal diseases found in all the tomato growing parts of the world. It is highly dangerous and damages almost all parts of the tomato plants including leaves, stems and fruits. The plant does not experience death but the disease can severely decrease the yield of the plants when susceptible varieties of the plants are used and the weather is also favorable for multiplication of the fungus. Early blight generally attacks older plants but can also attack the young seedlings in some cases. Early blight is also a very alarming problem for potato plants.
Description of the pathogen responsible for the disease.
Scientific name – Alternaria solani
Kingdom – Fungi
It is a fungus belonging to the division of Ascomycetes responsible for this deadly disease. The fungus can come from a variety of sources. It can already be present in the soil or it can arrive with the purchased seeds or seedlings. The spores of this fungus can remain viable in soil or in the seeds for at least one year. The blight can occur at any time of the year, but damp and humid conditions of the rainy season typically accelerate the process of multiplication of the fungus.
Identification of Alternaria fungus.
Since nearly all the fungus species are microscopic in nature, Alternaria is also not an exception. As a result, we cannot see the fungus and identify normally by means of our naked eyes. However, early blight can be easily identified by carefully studying the symptoms of the disease, such as –
· Initially, small and dark spots appear on the foliage near the ground.
· Leaf spots start to become round and brown and become larger in diameter giving a mosaic appearance.
· Larger spots have concentric rings, the part of the leaf surrounding the spot turns yellow.
· Severely infected leaves turn completely brown and fall off. However, some leaves might get dried and dead but still cling on to the stem.
· Seedling stems are infected just above the soil line. The stem turns brown, sunken and dry.
· If the infection completely girdles the stem, the vascular tissue connections get cut off and thus the whole seedling wilts and dies.
· In older plants, the stems show irregular patches and rings dark brown in colour with concentric rings.
· Fruits can be infected at any stage of maturity.
· Fruit spots are leathery and black with raised concentric ridges. The spots generally occur in fruits which are near to the stem.
· Infected fruits drop from the plant and the yield gets completely wasted.
Stages of growth susceptible to infection.
The early blight in tomatoes can appear in all stages of the life cycle namely flowering stage, fruiting stage, post-harvest stage, pre-emergence stage, seedling stage and vegetative growth stage.
Damages caused by Alternaria blight.
Early blight in tomatoes is one of the most deadly and fearful diseases that cause irreparable damage for the farmers. When the fruits are affected, the yield becomes highly limited and thus the income generated for the farmers decreases drastically. Also, the spreading capability of this fungus is very high which results in difficulty in controlling the spread of the infection once it is initiated. Affected leaves and stems are also responsible for less yield from the plants.
Prevention & Control.
The disease is deadly, but it can be prevented by means of certain steps such as –
1. Use of resistant varieties or cultivars
Nowadays with the gradual advancement of science, there are a number of genetically engineered disease-resistant tomato cultivars available in the market. Biotechnology has been able to incorporate disease-resistant genes into some improved and varieties of tomato. However, resistant varieties are also not completely immune to the early blight, moderate levels of resistance are present to prevent leaf infection, stem infection and fruit infection. A few common varieties with early blight resistance are Iron Lady, Mountain Supreme, Mountain Magic, Jasper, Julie, Verona, etc.
2. Use of cultural practices
This includes –
Use of pathogen-free seeds and collection of seeds from disease-free plants.
Practice of crop rotation.
Controlling susceptible weeds such as black nightshade which serve as the breeding ground for fungal spores.
Maintenance of proper fertilization for optimum plant growth. Overfertilization and under fertilization both are harmful.
Plants should be kept as it is in extremely humid conditions.
Drip irrigation should be used instead of traditional irrigation for keeping the foliage dry.
Plastic or organic mulching should be applied for reducing humidity and providing a barrier between contaminated soil and the plants.
3. Chemical means of control –
Application of fungicides can also be an effective way of controlling the early blight of tomatoes. They should be applied when the environmental conditions are highly favourable for the growth of the disease. However, it is important to alternate between different chemical families to avoid the development of pathogen insensitivity. Some important fungicide chemicals that can be used for controlling include cymoxanil, azoxystrobin, fenamidone, boscalid, penthiopyrad and so on. It should always be kept in mind that fungicides should be used strictly within proper limits and by properly following the instructions written on the label.