Blossom End-Rot: Not All Tomatoes Were Created Equal

One obvious observation in my garden this season is the susceptibility of 'San Marzano' tomato to blossom-end-rot.  All the fruits harvested so far have the distinguishing ugly black spot on the far end.

Blossom-end-rot is caused by lack of available soil moisture.  It is a common knowledge that blossom end-rot is a calcium deficiency symptom.  But I say that in this area (Zone 9), it is more of water stress damage.  I have noticed that symptoms always show up few days after plants experienced water stress.  The obvious indicators of water stress is wilting.  Such wilting is often unavoidable during periods of hot days, as in 100-108 (F) days that we had last week, when the rate of transpiration is greater than the rate of water uptake.  As the temperatures soar above a certain level, C3 plants like tomatoes will automatically close their stomata to avoid further water loss.  This phenomenon halts transpiration which also halts water absorption by the roots.  Imagine a micro-siphon that starts from the tips of the roots and ending at the stomata on the leave surface - that is the xylem that carries water and nutrients (including calcium) to the different parts of the plant. Nutrient-carrying water passes through this siphon constantly except when there is no water available within the root zone that result in the closing of the outlet (stomata).  In other words, when the plant is wilted, the passage of water has been closed.  No matter how much calcium is in the soil, when  water is not running through the xylem, calcium will be limited in the plant. Calcium is needed in the formation of cell membranes. When the plant is deficient in calcium, the membranes on the furthest end of the fruits break down. Hence, the blackened spot.

Because different varieties respond differently to water stress, they also differ in their susceptibility to blossom end rot.  This season I grew two varieties of tomatoes side by side in the same growing conditions.  While the fruits of 'Pink Brandywine'were all beautiful and blemish-free, the fruits of San Marzano showed blossom-end-rot.  So I might just plant 'Pink Brandywine' in the future.  If I were to grow San Marzano again, I would have to change some of the growing techniques in order to get better fruits:

1.  Mulch.  Not just mulch but thick mulch and compost around the base of the plants.  Refresh mulch in June as the temperature begins to get really hot.

2.  Automatically irrigate.  One day of skipped watering during hot days will cause damage on the fruits.  Increase irrigation time during the month of June all the way to July.

3.  Apply fertilizer only in low doses.  The salts in fertilizers compete with the plants for water.

4.  Choose an area that gets morning sun and afternoon shade for tomatoes.

When choosing tomato varieties, consider their resistance to blossom-end-rot.