Blossom end rot in tomato is a brown/black scar at the base or blossom end of the fruit. It is often a result of moisture stress, calcium deficiency, excess ammonium or a combination of all three.
A lack of calcium is intrinsically linked to the occurrence of Blossom End Rot. The problem occurs during the period of maximum fruit expansion – a couple of weeks after pollination. At this time, any factors that restrict calcium supply to the fruit will increase the risk of BER.
As a result of anionic ammonium as the main source of nitrogen, significantly increases the incidence of BER. The best way of ensuring good calcium supply is to use calcium nitrate as the calcium source.
Over use of N-ammonium will prevent tomato from uptaking other important nutrients like calcium; in this situation the risk of BER increases. Problems are worse when ammonium is used after fruit set and when crops are under moisture stress.
Preventing Internal Blossom End Rot
Internal Blossom End Rot is where the seed and tomato walls turn gray or black, but the surface of the fruit is unaffected. This is normally due to transient calcium deficiency. A good supply of calcium during the whole life cycle of the crop will help to reduce this disease.
Susceptible tomato varieties (e.g. Spectra and Calypso) are usually faster growing and hence need more calcium over a shorter period.
- Salinity restricts calcium uptake, thereby increasing the risks of BER (see figure below). This as a result of greater competition from other cations and increased use of calcium for leaf and xylem development rather than fruit accumulation.
- One of the most common inducing factors for BER is water stress, either due to a deficiency or excess. Under high humidity conditions, transpiration slows and uptake of calcium – which is solely transported within the transpiration stream – is limited.
- Under waterlogged conditions, soil nitrification is reduced and as a result, ammonium levels rise in the soil. In this situation, ammonium competes for uptake with calcium, resulting in increased BER risks.
- Maintaining calcium above 150ppm in the substrate or 400ppm in greenhouse rockwool.
- Controlling plant vigor and the fruit leaf ratio.
- Increasing daytime humidity so as to reduce transpiration and preferential calcium transport of the leaf rather than the fruit.
- Maintaining 64 - 68oF night root temperatures to enhance calcium uptake and redistribution to the fruits.
- Spraying calcium directly on young fruits
- Avoiding over-use of ammonium – keeping it to less than 10% of total N
- Balancing the K/ca ratio – Excessive potassium (>150 ppm) will restrict calcium uptake
- Avoiding excess use of magnesium (>150 ppm)
- Maintaining a good to air ratio in the substrate