What causes tomatoes brown or black rot on the bottom ?

Answer 1

If they're not rotting from touching the ground, then it's "Blossom-End Rot" a calcium deficiency due to soil moisture problems that prevent calcium from making its way to the fruits. It can occur even when there is abundant calcium in the soil. Poor drainage, root damage and soil pH that is too high or too far below the optimum 6.5 , or too much nitrogen, magnesium, potassium, sodium, ammonium salts, or a deficiency of soluble calcium salts, cause a decrease in calcium & can be contributing factors.

Inconsistent watering is the most common cause of the inability of the plant to delivery calcium to the developing tomato.

A small darkened or water-soaked area around the blossom end of the fruit, appears about the time the fruit begins to ripen. "Blossom-end rot usually causes the fruit to ripen prematurely and to be inedible. Quite commonly, the affected fruit areas become infected with secondary pathogens, which appear as black, felt-like growth on the fruit."
http://pubs.caes.uga.edu/caespubs/horticulture/blossom-rot.html

Remove affected fruits to improve the development of other healthy fruit on the plant. You can correct the problem by improving drainage, watering plants consistently as needed, & mulching plants to conserve moisture & to provide a more uniform water supply. To provide calcium, eggshells can be crushed and added to the soil. Foliar sprays of calcium won't correct blossom-end rot once it has occurred on the fruit, but it can help prevent the condition from occurring on developing fruit. Soil-applied treatments and prevention by cultural practices are generally preferred over sprays. Chelated calcium solutions also provide an excellent source of calcium.

Gypsum & compost improves drainage. Water early in the day, & give plants room so they don't stay wet. Fluctuations in soil moisture during periods of rapid plant growth create moisture stress and limits calcium distribution to the fruit. Make sure the plants are getting at least and inch of water per week. If fertilizer is needed, reduce nitrogen levels by using 5-10-10 fertilizer in place of 10-10-10. Windy conditions in the spring coupled with low relative humidity can cause high transpiration rates that can induce blossom-end rot, as does planting too early while the soil is still cold or undergoing severe hardening off.

Fortunately, by the time a second set of fruit begins developing, your plants will have expanded and developed a root system capable of gathering and delivering calcium to the tomatoes.
Good luck! Hope this helps.

Answer 2

Probably blossom end rot, which is mostly a calcium deficiency, also related to water stress (It has been hot this year). Apply some dolomite (finely ground, white). You can use other types of lime if dolomite is not available. Lime raises the ph of the soil (makes it less acid), and adds calcium to the soil. Dolomitic limestone also gives you a little magnesium for the same money. Organic amendments to the soil, some fertilizers, and just plant growth over time tend to acidify the soil. Plan to till some lime into the garden every 2 or 3 years.

Answer 3

Black Rot Tomatoes