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I observe a phenomenon for many years now. I observe it on a wide diversity of tomatoes produced all over Europe and Northern Africa that are sold at my local supermarket.

When I keep my tomatoes at ambient temperature around 20 to 25 degrees celsius, they deteriorate after a variable time, usually starting a five to fifteen days after I bought them. Nothing abnormal so far.

However, I observed that the mold pattern of virtually all these tomatoes is the same. It almost always starts a few millimeters from the junction where the tomato was attached to its branch, forming a ring on the top of the fruit.

I have thought why this could happen and my main hypothesis is that when they are grown the sun hits the top of the tomatoes (except where it is attached to the branch, obviously) and some ultraviolet radiation might weaken the tomato resistance in some way. The other reasonable hypothesis, although I cannot think of a definite mechanism, is that gravity would favor this polarized degradation. I have other hypotheses but they are too far a stretch, I believe.

So what is really happening?

I am a biologist, just not into botany or agriculture, so feel free to go technical and not just oversimplify.

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Mold spores float around everywhere in the air, in your home and on farms. They are also likely found on the surfaces of farm equipment used for harvesting (shears, knives, etc.). Grey mold can infect tomato leaves and stems where there are lesions, such as those introduced when harvesting fruit, when cutting the fruit off at the stem. This could happen wherever such spores can find ingress between the farm, produce distributors, the market, and your home.

Cite: https://content.ces.ncsu.edu/botrytis-gray-mold-of-tomato

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  • $\begingroup$ But there is no apparent lesion there, and the flesh is as firm there as the rest of the fruit until a week passed, then it gets softer, darker, and cracks open, allowing mold to take place. Are you saying that harvesting damages the tomatoes around the stem location although it is invisible? $\endgroup$ – Exocytosis Aug 18 at 8:25
  • $\begingroup$ When you cut fruit off the vine, that cut is a lesion. In handling the cut tomato, the stem can be pushed and separated to allow spores entry. $\endgroup$ – Alex Reynolds Aug 18 at 15:35

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