Blossom-end Rot Part II - Causes & Prevention

Calcium Deficiency - Vascular Tissue
A shortage of vascular tissue, particularly while the fruit is growing larger, predisposes the distal sections of the fruit to calcium deficiency. The number of vascular bundles decreases from the stem end to the distal end of the fruit.

Rapid expansion of the fruit, two weeks after anthesis, means that the density of bundles decreases substantially - paralleling a sharp decline of calcium in fruit tissue.

- Stress Related Causes of Blossom-end Rot
Low daytime humidity, highlight, high temperature and water stress are commonly believed to cause BER. One theory is that stress reduces calcium transport to the fruit. Transport of calcium only takes place in water-conducting tissues known as xylem. If water uptake is reduced, calcium uptake is also reduced.

- Daytime Humidity
Because fruit and leaves compete for water, humidity will affect calcium transport. High temperature and low daytime humidity levels will increase transpiration, forcing more calcium to leaves than to the fruit. On the contrary, high humidity decreases transpiration which lowers the transportation of calcium to the leaves and increase the calcium concentration in the fruit.

- Nighttime Humidity
Fruit calcium uptake increases at night with high humidity, however, more calcium is absorbed during the day. Calcium uptake only increases slightly at night due to high humidity compared to how humidity affects calcium uptake during the day. In grow chambers high relative humidity will reduce calcium concentration and increase Blossom-end Rot.

Controlling Blossom-end Rot (BER)
At this point, BER is fairly well understood, however, controlling it is not always successful in practice. One example, is large fluctuation in soil water deficits' increases the chances of BER developing. Mulching may decrease BER by reducing soil moisture losses. According to Elmer and Ferrandino (1991) black plastic mulch increased early season BER, however, seasonal total was not significantly affected - presumably because the plastic mulch reduced penetration of early season rain water to the roots.

source : hydroponicarticle