Blossom end rot is something that all tomato growers prefer not to experience, yet it affects most of us. Blossom end rot on tomato fruit appears at any time during fruit development but is most common on the first tomatoes of the season. Calcium deficiencies most often occur in long-fruited varieties (Roma type) and in large-fruited varieties. Initially, it appears as a light brown water-soaked lesion (speck) located at the end of the fruit – where the flower was attached – hence the name of this physiological disorder. If you don’t react immediately after the first symptoms appear, the damage will continue to grow, turn black, and leathery. The damaged tissue will also be susceptible to fungi and bacteria that break down the fruit tissues.
Blossom end rot is not caused by disease or insects. Blossom end rot is a physiological disorder caused by inadequate levels of calcium in developing fruit and can occur at any stage of fruit development. There are several different reasons why tomato plants may not be getting enough calcium.
Blossom end rot can be caused by:
too little water uptake by the plant due to lack of water in the growing medium
inability to uptake calcium due to under-developed or damaged root systems
insufficient nutrition within the growing medium
excessive concentration of potassium, magnesium, and ammonium nitrogen, and sometimes also sodium in the substrate (substrate salinity)
excessive moisture or fluctuations in the moisture content of the growing medium
low air humidity (below 60%), especially during flowering and fruit set
rapid fluctuations in air and substrate temperature
Most often, a lack of calcium in the soil is not the true cause of blossom end rot. The problem is the plant’s inability to absorb it, which becomes difficult under certain conditions. Calcium uptake by plants depends on active transpiration (leaves loose water to take more water and nutrients from the soil, among other things). Anything that inhibits transpiration can inhibit calcium uptake. In addition, calcium enters the leaves more easily than the fruit.
There are several things that can be done to enable the plant to manage calcium properly, thus preventing the occurrence of blossom end rot. Avoid over-fertilization during early fruiting, mainly with fertilizers that use ammonia as a nitrogen source. Ammonia competes with calcium absorption and promotes the occurrence of blossom end rot.
Avoid frequent, shallow waterings- water less frequently and deeply. It is also important to properly water the plants before anticipated periods of drought and hot weather.
In case your problem is truly a lack of calcium in the soil (which can be confirmed with a soil test), you can add calcium to the soil in many forms- bone meal, gypsum, or dolomitic lime. Remember that calcium additions will not be immediately available to plants.
For immediate use, spray the fruit trusses with 0.5-0.8% solution of calcium nitrate as soon as possible, paying particular attention to even application of liquid to the fruit. Preventatively, such spraying should be carried out once a week. All gardeners who grow tomatoes in containers should pay close attention to moisture levels, container soil dries up much faster than the ground soil.