Blossom end rot in tomatoes: identification, prevention & treatment

Are your lovely tomatoes turning black on the bottom? Blossom end rot in tomatoes causes the fruits to develop large, leathery, circular, brown or black blotches on the ends. While these fruits are unfortunately lost, there are ways to save the rest of your crop. Find out everything you need to know about blossom end rot and how to prevent and treat it.

Blossom end rot in tomatoes (Solanum lycopersicum), also known as tomato bottom rot, can lead to a serious loss of yield for your plants. The leathery brown to black lesions that develop at the blossom end of the fruit are not due to a pathogen, but rather result from a nutrient deficiency. But fear not – this condition is correctable! Read on to find out how to treat blossom end rot and prevent it in the future.

Blossom end rot in tomatoes: Symptoms

Blossom end rot in tomatoes is caused by a calcium deficiency in the developing fruit tissue, not by a pest or disease. This condition can occur in many plants that produce large fruit, from aubergines and capsicum to watermelons and courgettes. The main symptom associated with blossom end rot is the dark spots that appear at the end of the fruit which eventually cave in and dry out. Severe blossom end rot makes it look like your tomatoes are rotting on the vine. Fruit that is only slightly affected by blossom end rot is still edible; just cut off the bad spot and consume the rest. However, we advise caution as the lesions that result from blossom end rot could allow for fungal or bacterial infections in the fruit. In which case, it may not be safe to consume.

Blossom end rot in tomatoes: Causes

So, what causes blossom end rot? As previously mentioned, blossom end rot in tomatoes is caused by a calcium deficiency in the affected tissues. Calcium is a mobile nutrient that is transported within the plant from the roots to the leaves and fruit. While calcium is a micronutrient, and therefore not required in large amounts, it is required by all plant tissues for cell wall formation. Calcium deficiencies can be down to the soil and can also occur locally within tissues of the plant, even if the soil has sufficient calcium.

At the soil level, pH plays a large part in calcium availability. Acidic soils can bind calcium, thereby making it unavailable for the plant to uptake through its roots.

Dry or irregularly-watered soil can cause localised deficiencies in plant tissues, as plants circulate nutrients from the roots to the leaves or fruit through a process known as transpiration. Therefore, if there is little or no water available in the soil, this can restrict the movement of nutrients throughout the plant.

The fruit, being furthest from the roots, will suffer the most. Also, rapidly-growing plants with prolific leaf development can utilise all available calcium in the plant, not leaving enough for fruit development.

In greenhouses, high humidity coupled with stagnant air can exacerbate localised calcium and other nutrient deficiencies. This is due to reduced transpiration and reduced nutrient cycling. Large fruiting tomato varieties are more susceptible to blossom end rot than small fruiting cherry tomatoes, as the large fruit develops quickly and requires more available calcium.

Blossom end rot in tomatoes: Treatment and prevention

The best treatment for blossom end rot in tomatoes is prevention. To prevent calcium deficiencies from ever becoming a problem, proper soil preparation, adequate and regular watering, and good air circulation are key. However, if it is too late and you do run into blossom end rot, simply remove the affected fruit and correct the deficiency.

Water your plants frequently, keeping the soil moist, but not waterlogged. Ensure proper air circulation by pruning your plants to increase air flow. If you are growing tomatoes in a greenhouse, install fans and open windows to allow more air to circulate through the structure. If your soil is lacking in nutrients, use an organic liquid fertiliser with micronutrients, or a liquid calcium supplement. It is important to use liquid solutions as their nutrients are usually more readily available for uptake by your plants.

When preparing your soil before planting tomatoes, add calcium and correct issues of low pH by adding lime products to your soils. It is also a good idea to apply a high-quality, long-lasting, organic, granular fertiliser, such as Plantura Tomato Food. This will provide a slow release of nutrients to your plants throughout the growing season. Some places suggest treating blossom end rot tomatoes using powdered milk. However, we suggest avoiding these home remedies and opting for lime supplements like gypsum, calcium carbonate, and limestone instead as these provide much more readily available calcium to the soil and, therefore, the plants.

Want to learn more about how to properly feed your tomato plants? Look no further than our in-depth article on fertilising tomatoes.