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Brown spots on tomatoes are typical symptoms of blossom end rot infestation. By the time the devastating damage occurs, affected home gardeners have already invested a lot of time and effort in their tomato plants. The question is rightly raised as to whether damaged fruit is edible beyond the point of rot. This guide explains the influence of blossom end rot on the edibility of tomatoes in a practical and comprehensible manner.
Brown spots on tomatoes
The good news first: Blossom end rot is not a disease but a physiological disorder caused by an acute lack of calcium. What appears at first glance to be a fungal infection actually has nothing to do with an infestation by pathogens. The usual suspects in the form of spores, viruses or bacteria can be ruled out as the cause of brown spots on tomatoes. Rather, affected tomato plants lack calcium, one of the main nutrients and an important building block for healthy cell walls. If there is a deficit within the plant physiology, the cell walls collapse and the tissue gradually dies.
The causes of blossom end rot have not yet been fully researched. Nevertheless, there is no doubt as to which symptoms should be used to diagnose the deficiency. The following indicators signal that a tomato plant is suffering from a calcium deficiency:
- Early stage: grey-brown to dark brown, watery spots at the base of the flowers
- Advanced stage: sunken, hardened, dark necroses
- Side effects: noticeably thin skin, deformed leaves
The fact that the tomato plants themselves make a vital and healthy impression causes perplexity. Furthermore, by no means all the fruits of a panicle show the symptoms mentioned. Quite a few panicles of a tomato plant are often completely spared fruit with rotten spots, while brown-black damaged spots on panicles in the immediate vicinity spoil the anticipation of delicious tomatoes.
tip: Brown spots on the leaves of a tomato plant signal infestation with late blight, a dreaded fungal infection in tomato and potato cultivation. Fruits of affected plants are not edible. The triggering pathogen Phytophthora infestans releases toxins which, in the worst case, are carcinogenic.
In the search for the causes of calcium deficiency and the resulting blossom end rot, there are several starting points in focus. Either tomato plants cannot absorb the nutrient in sufficient quantities or it is not even present in the soil.
The availability of the important nutrient is closely related to the local water supply. Calcium is always transported from the soil to leaves and fruits via the pathways of a tomato plant. This flow of sap is regulated by the degree of evaporation via the above-ground parts of the plant. Drought stress or excessive humidity throw the sap flow out of balance. The result is an undersupply of calcium, whether or not there is sufficient calcium in the soil. To make matters worse, tomato fruits evaporate significantly less water than leaves and shoots. The already small amount of calcium primarily reaches the leaves via the pathways and is only transported to the fruit secondarily by the juice stream.
It is exacerbated when tomato growers apply nitrogen-rich fertilizers, such as blueseed and similar mineral fertilizers. The resulting overgrowth of the leaves is at the expense of the fruits and their supply of calcium. For this reason, brownish to black necrosis on tomatoes is particularly common on fast-growing tomato varieties.
A complete blockage of calcium occurs when tomato growers cause an oversupply of potassium and magnesium. Over-fertilization with these two nutrients locks calcium in the soil. In combination with a pH value that is too low, calcium transport comes to a complete standstill and brown spots on tomato fruits are inevitable.Tomato diseases, blossom end rot
Edible despite blossom end rot?
Brown or calloused spots on tomatoes are usually confined to the epidermis. The inner pulp remains intact and is edible without any problems. Taste impairments are also not to be feared. After harvesting, simply cut out the rotten spots.
tip: Changeable weather with dry-hot and wet-cold phases promotes blossom end rot on tomatoes. By growing the plants under a rain shelter or tomato canopy, you mitigate the effects of irregular water supply and subsequent calcium deficiency.
Foliar fertilization as an emergency aid
If the first watery rotten spots appear on tomato fruits, contain the further spread with targeted foliar fertilization. Because in view of the first symptoms, it is too late to compensate for the obvious lack of calcium with soil fertilization. By applying liquid calcium fertilizer directly to the leaves, the nutrient goes straight into the pathways and finds its way to the starving fruit. Various special fertilizers to compensate for calcium deficiency are available in well-stocked specialist shops, such as Wuxal-Calcium or Purital C15 from Green24. How to administer a foliar fertilizer correctly:
- Important: Wear safety goggles to prevent eye irritation from splashes
- Best time is early morning or after sunset
- Mix foliar fertilizer with soft water according to the manufacturer's instructions
- Add a squirt or two of dish soap to improve adhesion to the foliage
- Pour into a handheld or pressure sprayer
- Apply evenly to the top and bottom of the leaves
- Avoid spraying the fruit if possible
Only prepare the nutrient solution immediately before use. Prolonged storage significantly reduces its effectiveness. Undoubtedly, completely excluding fruit from foliar application is a difficult endeavor. Tomatoes that have been wetted with liquid calcium fertilizer are safe to eat provided the fruit is rinsed thoroughly with clean water before eating.
If you no longer want to bother with brown rot on the fruit of your tomato plants in the future, we recommend these preventive measures:
- Fertilize tomato plants in a balanced way with compost and horn shavings
- Alternatively, apply an organic long-acting tomato fertilizer
- In addition, spoil the heavy eaters with liquid, organic tomato fertilizer
- Water regularly for continuous soil moisture without drought stress or
A pH test provides more information to rule out too acidic soil as the cause of the calcium blockage. Garden centers and hardware stores offer easy-to-use test sets at a reasonable price. If the result signals a pH value below 5.5, please lime the soil. This succeeds in harmony with nature with the help of lava rock powder or bio-activated lime. Sprinkle the granular material on the root disk of your tomato plants, rake it in superficially and then water again.