Why the Bottom of Cherry Tomatoes Turns Black and How to Prevent It

Haveany of you who grow cherry tomatoesever noticed that a fruit you finally managed to set suddenly turns black at the bottom, goes soft, and even rots into a hole? This is no coincidence - nine times out of ten it is"blossom-end rot"at work - a disease that singles out the blossom end of the fruit, and it strikes especially often during fruiting. Don't panic - today we'll walk through the causes, the solutions and the prevention tips.

1. The warning signs of blossom-end rot in cherry tomatoesSignals

At first, a tiny water-soaked spot the size of a pinprick appears at the blossom end, pale yellow or pale green, and slightly soft to the touch; the spot then slowly expands and turns dark brown or black, with a sunken center; in severe cases the entire area around the blossom end rots and shrivels, and may even grow mold, leaving the fruit completely worthless.

Note: blossom-end rot only affects the fruit; the leaves and stems are usually fine, so if a fruit looks off, check the blossom end first and you can basically confirm the diagnosis.

2. Causes of blossom-end rot in cherry tomatoes

1. Calcium deficiency
 
Tomato fruits especially need calcium during development; it strengthens the toughness of the skin cells, and once calcium is lacking, water and pathogens slip in and rot follows.

There are 2 critical periods for calcium deficiency: first, the seedling stage, when the roots do not take up enough calcium and later reserves are affected; second, the rapid fruit-swelling stage, when demand for calcium surges and supply cannot keep up, making the disease likely.

Solution: when transplanting, mix bone meal, superphosphate or chelated calcium fertilizer into the potting soil so that calcium is evenly distributed around the roots; during fruiting, as a stopgap use a 0.3%calcium chloride solution or calcium nitrate solution as a foliar spray, focusing on the fruits and nearby leaves, once a week for 1application, for 2-3applications in a row, delivering calcium to the fruit through the leaves.

 
2. Excess nitrogen and potassium 

When nitrogen fertilizer is excessive, the plant goes wild growing leaves and stems but is slow to flower and fruit. Such a plant takes up nitrogen preferentially, which blocks the calcium uptake pathway.

When potassium fertilizer is excessive, the rate at which the roots take up calcium drops.

Solution: if the plant only grows leaves and no fruit, stop applying high-nitrogen fertilizer at once and switch to a low-nitrogen, high-phosphorus-and-potassium fertilizer, diluted 1000-fold and watered onto the roots, once every 10 days1 time, forcing nutrients to shift toward the fruit; read the instructions before every feeding to avoid an excess of any single element, and for container growing at home it is best to follow the principle of "weak but frequent feeding", with each dose not exceeding 1%of the potting soil.

3. Overwatering or underwatering

Too much water suffocates the roots: if the potting soil stays waterlogged for more than 24 hours, the roots are starved of oxygen and rot, and their uptake function shuts down completely.

Drought and lack of water: if the soil stays dry for a long time, the roots shrink from the lack of water and cannot properly take up calcium and other nutrients, so the blossom end is the first to suffer from "a shortage of water and nutrients".

Solution: water by the "dry"principle - how dry? When the potting soil is completely dry 2-3 cm below the surface, and a poke with your finger shows the clod is hard and does not crumble, then water; water slowly and thoroughly around the rim of the pot, avoiding a stream that hits the blossom end directly, and do not use cold water - tap water left to sit in the sun for 1 day is safest; during fruiting, stretch the watering interval a little so the plant gets slightly "thirsty" - this not only reduces blossom-end rot but also lets the fruit build up more sugar, making it sweeter!

Tips: once you find a fruit with a black, rotting blossom end, cut it off at once with sterilized scissors - don't let the rotten fruit keep draining nutrients, and don't just toss it into the pot, as it may breed mold. After removing it, loosen the potting soil around the wound and sprinkle a little carbendazim powder to prevent the pathogen from spreading.

3. Everyday prevention of blossom-end rot in cherry tomatoes

1.Some varieties are naturally resistant, such as "Qianxi" and "Shengnüguo", which are relatively tolerant of blossom-end rot, so beginners can choose these first.

2. Besides calcium fertilizer, add 10% composted coconut coir or peat moss to your soil mix - it both holds water and stabilizes the release of calcium, preventing calcium loss.

3. Spread a layer of dry straw or plastic mulch over the surface of the potting soil; this reduces soil compaction when you water and also keeps the fruit from touching damp soil directly, lowering the risk of disease.