They actually grew well, produced numerous flowers and bore fruit and now the bad surprise: the tomatoes are turning black and rotting at the bottom! This is due to a potassium deficiency, which causes blossom end rot. We explain what you can do to save the harvest.
Tomatoes turn black at the bottom: This creates a potassium deficiency
Potassium is an important nutrient that fulfills many functions in plant metabolism. On the one hand, it supports the process of photosynthesis and the transport of important carbohydrates, and on the other hand, it regulates the tomatoes' water balance and makes them more drought-resistant. Potassium, which is only present in small amounts in the garden soil, promotes flower and fruit formation and shortens the ripening time.
Heavy eaters like tomatoes in particular need more potassium to grow healthily and bear lots of fruit. However, this turns out to be problematic because:
- The soil volume in the greenhouse is limited and potassium is often completely washed out after several waterings.
- For potted plants, the potting soil does not contain enough potassium.
- Outdoor plants on clay soil cannot properly absorb water and nutrients.
The end result is that the plants need additional fertilizer. Otherwise you will stop growing and bear less fruit, which is often attacked by fungi or bacteria. It is precisely these pathogens that can make plants sick. As a result, the fruit begins to rot.
A potassium deficiency also occurs if tomatoes are fertilized incorrectly. Nitrogen fertilizers support growth, but are unfavorable for the ripening period. Too much magnesium in the fertilizer can also prevent the absorption of potassium.
The potassium deficiency occurs in tomato plants in late June to early July and can be remedied by adding fertilizers containing potassium.
Tomatoes have black spots: blossom end rot
Every amateur gardener who has tomatoes in their garden has had to deal with blossom end rot at least once. The problem is probably known to everyone: After the tomatoes form their flowers, they need an extra portion of calcium for fruit formation. If the heavy eaters don't get them, the fruits remain small, ripen slowly and develop black spots. TheCalcium deficiency has a typical symptom: it first affects the fruits at the bottom through sunken tissue and brown-black spots on the underside of the tomatoes.
The reasons for a calcium deficiency are very different. Most of the time, however, it is not due to a lack of nutrients in the soil, but rather the ability of the plants to absorb them through the underwater. The causes include:
- Clay soil that sticks together and the tomato roots cannot absorb the water. In this case, you will need to periodically loosen the soil or add some sand to improve drainage.
- Irregular watering or long dry periods can make it more difficult for nutrients to be absorbed. Plants respond to heat waves by providing water first to the shoots, then to the leaves, and then to the fruits. However, because fruits slowly evaporate water, they are almost always malnourished in hot weather. But too much water can also cause the fruit to rot. You should therefore dose the water very carefully.
- Fertilizing with nitrogen fertilizer can also promote calcium deficiency. You should switch to calcium fertilizer during the fruiting phase or at least choose a suitable NPK fertilizer.
- The calcium deficiency usually occurs in mid-July to the end of July and can be remedied with suitable fertilizers.
Black spots on tomatoes: brown rot as a possible cause
Brown rot is a fungal disease that often affects potatoes and tomatoes. Plants are particularly vulnerable after heavy rain and often fall victim to pathogens.
In contrast to blossom end rot, the first symptoms are first found on the leaves. The disease then spreads to all parts of the plant and also affects the fruits.
As a preventative measure, you can cut off the tomato leaves below so that the fungal spores from the ground cannot reach the leaves. In addition, drainage for potted plants and soil permeability for outdoor plants should be improved.
Effective microorganisms can help prevent the fungal disease. These are strains of bacteria that are added to the soil and can improve plant structure. They make tomatoes more resistant to various diseases and strengthen them.
Prevent black spots on tomatoes
There are various reasons why tomatoes can develop black spots. These include, among other things, fungal diseases and nutrient deficiencies. However, to ensure a bountiful harvest, you should take some important preventive measures.
These include, among others:
1. Adjust the fertilizer composition to the respective development phase. Add a little more potassium during flowering and calcium during fruit formation.
2. Protect the soil from waterlogging, water the tomato plants from below and protect them from rain. Keep the humidity in the greenhouse below 70%.
