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Why are tomatoes turning black (dark) from the bottom up when they begin to ripen. They are also dark on the inside.

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A

The condition that you are seeing is called blossom end rot.  Blossom-end rot is a physiological disorder caused by a calcium deficiency in the young fruit. Utah soils have plenty of calcium so even though technically it is the problem, the real issue is fluctuation in the amount of water available to the plant that makes the plant unable to transport the calcium as needed. Tomatoes, squash, peppers, eggplants and melons are all vulnerable to this problem. The good news is that normally only the first few fruits to ripen are damaged. 

There are a couple of different things that you can do to help prevent this condition. Overwatering your plants can aggravate this condition, especially if your plants are in a heavier clay soil. Keep a close eye on how much water you apply and how much moisture is in the soil.  Mulching around your plants can also help to reduce the amount of water lost to evaporation and can help keep soil moisture levels more consistent. In addition, over fertilizing your plants can also contribute to this condition.  I have included a link below to our tomatoes fact sheet.   
 

http://extension.usu.edu/files/publications/factsheet/HG_2004-05.pdf

Posted on 10 Aug 2009

Heidi Wayman
Horticulture Intern, Salt Lake County

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