Blossom end rot is one of the most common complaints for tomato growers, particularly on plants grown in containers. It’s a complex problem that results from calcium not being available to the plants as the ovary at the base of the flower is fertilized and begins to develop into a tomato. It is very typical in plants that are in soil that is alternately dry and then wet. That fluctuation means that even if calcium is present, it is not consistently available to the plant.
Another contributing factor is the use of high nitrogen fertilizers, which cause rapid growth, placing higher demands on the plants.
The result is a single, bruise-like patch that appears (as the name suggests) at the blossom end of the fruit (the end farthest from the stem). The spot spreads as the interior of the fruit begins to rot. It is not a reversible condition, so any affected fruits should be discarded. However, the soil chemistry and watering regimen can be adjusted to reduce blossom end rot. We like to recommend prevention rather than treatment.
This environmental disorder is also seen in the fruits of pepper plants. When it occurs in squash fruits, the cause is more likely to be incomplete pollination.
To prevent blossom end rot in tomatoes, be sure to start with containers that are large enough to accommodate lots of soil and a large root system. Choose quality soil that has good water retention qualities. Tomato plants prefer slightly acidic conditions, so avoid over-applying agricultural lime (which is the typical product growers look to for calcium). Instead, mix Glacial Rock Dust and a balanced organic fertilizer into the soil at transplant time. Even watering is essential, so consider drip irrigation so the soil remains constantly moist, not wet/dry/wet/dry…