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Why your tomatoes are rotting at the bottom

Blossom-end rot is a common garden problem caused by a lack of calcium and/or uneven watering.

Farmers always get concerned when they notice a dry, sunken decay on the blossom end of their tomato crop. This is common on the first tomato crop grown after a dry period. The disorder is known as blossom-end rot.

The disorder first appears as water-soaked spots on the blossom-end, or bottom, of the tomato. The affected tissue breaks down rapidly and the area becomes sunken, dark brown or black, and leathery. This can happen at any time as the tomatoes mature, and most often on the first tomatoes of the season.

Causes

Blossom-end rot is caused by insufficient calcium in the tissue of the tomato. Calcium is taken up into the plant through the roots, however, in this case it settles in one part of the plant. This means the rot can occur even when there is sufficient supply of calcium in soil, stems or leaves. Actively growing parts of the plant such as developing tomatoes must have a continuous supply of calcium to prevent these spots from developing.

The conditions that cause blossom-end rot are closely linked to inconsistent soil moisture throughout the growing season. Since calcium is only moved into the plant with sufficient moisture supply, when drought occurs the fruit continues to develop but will be affected by a calcium deficiency.

Rapid early growth of the plants can cause the rot because calcium is needed by the tomatoes when they are actively growing and the plants may not be able to take up sufficient calcium quickly enough through the roots. Although it is difficult to wait for those first ripe tomatoes from the home garden, do not force the plants to grow too quickly to avoid this rot.

Root damage can also lead to decreased moisture intake. Cultivating too close to plants or burning them with fertiliser can reduce nutrient and water uptake through the roots. Waterlogged soils also interfere with the root’s ability to take up nutrients. Soils may also have low levels of calcium and this can lead to blossom-end rot. Soil testing helps to identify deficiencies and correct them in good time by adding lime. Do not add lime before testing the soil first.

Control

Unfortunately, the rot will not go away once a fruit is already affected. The solution is to pluck off the affected fruit to prevent others from contamination. Avoid watering your plants too much or too little. Instead, water consistently and evenly. If you forget to water, do not overwater. If it’s rainy, ensure plants have good drainage and soil dries out. Watering with a soaker hose is preferable to overhead irrigation.

Have your soil tested periodically to determine if there is sufficient calcium. If it lacks the mineral, add an organic source of calcium such as lime, bone meal, or finely-crushed eggshells. Check the soil pH on a regular basis, particularly if you use lime as a calcium source. A pH of about 6.5 (slightly acidic) is ideal for growing most vegetables, as it allows for the best nutrient uptake.  

Because blossom-end rot is caused by a calcium deficiency, you might be tempted to just fertilise your plants more. However, in most cases, there is already sufficient calcium in the soil; the plants just cannot absorb enough of it. Over fertilisation is also harmful, and having too much nitrogen, magnesium, and potassium in the soil can make blossom-end rot worse. 


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