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Blossom end rot

Thomas Heß Thomas Heß

You can usually identify blossom end rot by brown, sunken patches on the fruit. Here’s how to prevent it effectively.

What is blossom end rot?

Blossom end rot tends to occur on ripening tomatoes and can be recognized by brown, rotten areas on the blossom end of the fruit. It is not, however, caused by a disease, but a lack of calcium.

Vegetable plants such as tomatoes have a hard time: It is not only diseases such as blight that can quickly destroy an entire plant or – especially in greenhouses – whiteflies that get stuck into tomatoes, the fruit can also be afflicted by a lack of nutrients. A calcium deficiency is particularly serious and can be clearly seen in the form of blossom end rot. The tomatoes may look like they have been infected by a fungus, but blossom end rot is, so to speak, an error on the gardener’s part. A calcium deficiency is not necessarily caused by too little calcium in the soil, the plants often simply cannot absorb it. Blossom end rot is usually caused by too little or an irregular supply of water and using too much nitrogen-heavy fertilizer – specifically ammonium, magnesium and magnesium.

How to recognize blossom end rot

The underside of the tomatoes – the blossom end of the fruit where you often still see withered petals – develops watery spots that gradually get bigger and turn a gray to brown color. The tissue then sinks in slightly, becoming leathery, hard, and dark brown. This does not necessarily affect all of the tomatoes on the vine, the lowest fruit is often the first to develop symptoms. The young leaves sometimes stay small or are malformed, and the entire growing point dies.

Calcium is important for the stability of cell walls, among other things. If there is a deficiency, the cell walls become unstable, collapse, the tissue dies, and sinks in – the typical damaging effects can be seen.

Blossom end rot should not be confused with the fungal disease Alternaria solani. This plant disease also causes brown sunken spots, but with clear concentric rings and not necessarily at the blossom end. Moreover, Alternaria solani causes typical leaf spots which are restricted within the leaf veins and also have concentric rings.

Blossom end rot on tomatoes

Blossom end rot first appears on the blossom end of the fruit

Which plants are affected?

Blossom end rot predominantly affects tomatoes, especially the fast growing varieties. It sometimes also appears on and certain varieties of pepper. The symptoms are generally the same. However, with some varieties of pepper, the brown spots do not sink in, the skin of the pepper simply becomes thin around the flower base and the tissue underneath is light brown. Blossom end rot is much more common in greenhouses than in the garden.

What causes blossom end rot?

Blossom end rot is either caused by a complete lack of calcium or it not being available to the tomatoes in sufficient quantity. A plant has two different transport systems: The xylem has a continuous flow of sap, transporting the soil water and the nutrients dissolved in it from the roots to all of the branches, leaves, and the ripening fruit. Among other things, they serve as components for assimilates and calcium, in particular, as a building block for the cell membranes. The plant produces energy rich assimilates in its leaves, just like in a factory, which are then transported via sieve cells (phloem) to all parts of the plant where they are needed. Evaporation from the leaves is the engine for sap flow in the xylem – when there is a shortage of water at the top, a continuous column of water and thus nutrients is pulled upwards. However, less water evaporates from the fruit than the leaves and therefore the fruit has a poorer supply. If there is a deficiency, the leaves are always supplied first and then the fruit.

The flow of sap and thus the supply of nutrients to the leaves and growing fruit is always disturbed if either high humidity hinders evaporation and not enough water is sucked up to replace it, or if the soil is too dry due to a lack of water and too little calcium is supplied. Furthermore, over-fertilization with magnesium and potassium specifically blocks the absorption of calcium – even if there is plenty available in the soil. A low pH exacerbates the entire situation. Too much nitrogen also promotes leaf growth and the little calcium that is available first migrates to the leaves – the fruit goes empty-handed. For this reason, fast-growing tomato varieties are more severely affected by blossom end rot than others.

Preventing blossom end rot

Prevention is the be all and end all when it comes to blossom end rot. In acute cases, you can sprinkle around the affected plants and then water them. As a preventive measure, make sure the plants have a steady supply of water and ventilate the greenhouse regularly so that the humidity is not too high. You can slow down very strong growing varieties of tomato a little by cutting off the leaves until you reach the first fruit. Fertilize in a well-balanced manner and distribute organic fertilizer in spring for a basic supply of nutrients, working this well into the soil.

Is the affected fruit still edible?

It is safe to eat tomatoes or peppers with blossom end rot. The fruits are not spoiled and also taste no different, the brown areas just look unappetizing. Simply cut out the affected parts.

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