Special Series in the rootzone #9: Avoiding blossom end rot

April 26, 2012
Written by Andrew Lee
During the warmer summer months, greenhouse plants are transpiring the most and the crop can be stressed due to high light and 24-hour temperatures. To avoid common fruit quality issues, such as blossom end rot (BER) in tomato and pepper crops, it is important to have EC under control in time and to maintain its stability.

Using a water content meter, look at how the EC develops over time, especially in relation to radiation. It should be stable from day to day. In this respect, it is also very important to have your EC refreshed in time during the day.

Look critically at the recorded time of first drain on a climate computer and compare this to the time EC decreased in the slab indicated by the water content meter, they should match. In many circumstances, drain will be recorded but EC will not be controlled (Figure 1). If left unchecked, this can have devastating consequences for the crop.

graph1
 
(Figure 1) Unstable EC: Time of first drain recorded by the climate computer is about 09:30 hours. Dark blue EC is not controlled until 12:00 hrs. The result: EC rises over two days. If left unchecked, this can result in serious quality issues, such as BER. The issue, in this case, was that cycle length was reduced too early.
 
graph2  
(Figure 2, below at left) Unstable EC: This illustrates how cycle length can affect the EC refreshment of the slab. Start time is OK, time of first drain and EC reduction this time is in line but the cycle length is too small, resulting in drain too late. The result EC is unstable and rising – here the reaction was to give a night session; however, the correct course of action would have been to increase cycle length. If left unchecked this could have resulted in aborted fruit.

 

The time of first drain is therefore very important in respect to EC refreshment. Within the irrigation strategy, focus on the start time and the volume of irrigation that is given, to account for the decrease in WC and water uptake by the plant. This can easily be 4.0-6.0 ml/J for the first four to five irrigation cycles (Figure 3).

graph3  
Figure 3, above: Drain and EC refreshment on time (10:00 hours or 600 W/m2) is achieved by applying large volumes in the morning 4.0-6.0 ml/J. Thereafter, smaller more frequent irrigations in the relation 3.0-3.3ml/J keep EC controlled during the peak solar hours and limit stress on the crop so that production and fruit quality is maximized.

 

In this phase of the crop (maximum production), fruit are most susceptible to quality disorders of various kinds as the 24-hour temperature can be high (>23ºC). At these times, stresses on the crop and fruit development are at their highest. It is therefore important that the plants are well-balanced at this time of year and that climate and water management strategies are aligned to minimize stress, yet are able to react to possible changes in the weather.

An often overlooked parameter at this stage in the season is the nutrient balance in the rootzone. This can have a huge influence over quality, especially when recycling a higher amount of drain solution.

Many companies are now using proportionally more recycled nutrient solution, i.e., the EC pre-setting in many pepper companies is still 1.0 mS and up to 1.5 mS for tomato. Under these circumstances it is very important to ensure that the correct balance of nutrients is maintained in the rootzone, especially when things are changing rapidly. Therefore, a weekly analysis of slab and/or drain solution is advised.

 4645 Schematic 06  
 Crop stages.
 

FRUIT GROWTH STAGES
■ Pepper and tomato crops have three distinct stages of development:
  • Slow growth for two to three weeks when the gain in fruit weight is less than 10 per cent of final fruit size.
  • Period of rapid growth for three to five weeks.
  • Slow growth for two weeks where there is little gain in weight but huge metabolic changes, i.e., ripening.
The initial slow growth for two to three weeks is due to cell division, whereas growth in stage two is due to cell enlargement alone. When the fruit starts to change colour, the import of assimilates ceases (stage 3).

BLOSSOM END ROT
■ Blossom end rot (BER) is probably the most common of all fruit quality disorders in tomato and pepper. The external symptoms are characterized by blackening at the end of the fruit (Pictures 1 and 2).

Tomato  
Picture 1: Blossom end rot in tomato
 
Pepper  
Picture 2: Blossom end rot in pepper.
 
BER can occur suddenly and extensively, and often with disastrous financial consequences.

The symptoms are caused by local calcium (Ca2+) deficiency in the fruit tissue, which leads to a breakdown in the structure of the plant cell wall. As Ca2+ is used in the development of new cell walls, the fruits are at the greatest risk during stage one of their growth or 10 to 15 days after flowering.

Once incorporated, Ca2+ is immobile within the plant and it is the only element transported in the xylem (water flow). Fruit possess a limited amount of xylem vessels, so the supply and demand relationship of fruits for Ca2+ will determine the crop’s susceptibility for BER.

The supply of Ca2+ in the drip solution in 99.9 per cent of situations where BER is evident in the crop is more than adequate for plant and fruit growth, provided it has been prepared, mixed and is dosing correctly. Lack of Ca2+ in the feed is therefore unlikely to be the primary cause of BER.

To minimize the incidence of BER, it is important to control the greenhouse environments, aerial and rootzone, creating a balanced plant and balanced rate of fruit growth to ensure the demand from fruits for Ca2+ is matched by supply from the rootzone.

Despite large advances in greenhouse crop production, BER still remains a tiresome quality problem for tomato and pepper growers in all corners of the world. Maintaining the right nutrient balance, EC and pH in the rootzone can also help minimize its occurrence.

EC MANAGEMENT
■ A high substrate EC in the summer is the most common cause of BER. This is because at high EC levels the xylem vessels in the fruit become more constricted, limiting Ca2+ deposition. The fruit load is high in this phase of crop growth so the dripping EC must remain stable. A good irrigation strategy is therefore paramount to keep EC stable from day to day.

The ideal substrate EC is influenced in some respects by the variety grown. For example, cherry types are usually grown at higher EC to attain minimum brix values. For most cluster types, the slab EC in this phase should be stabilized in the region of 3.5-4.5 mS, depending on the prevailing weather, with a fluctuation in the slab during 24 hours of 0.5-0.8 mS.

For pepper slabs, EC should be stabilized in the region of 3.0 mS-3.8 mS with a fluctuation in the slab during 24 hours of 0.3-0.6 mS.

NUTRIENT BALANCE AND BER
■  EC is the measure of total salts in the nutrient solution. However, it is also important to know the balance of individual nutrients present, as high sodium (Na+) and potassium (K+) levels can inhibit the plants’ ability to access Ca2+. Limiting the amount of Na+ in the drain solution and maintaining the correct K+:Ca2+ ratio is therefore extremely important as these elements use the same “pathways” to gain entrance to plant cells.

Under summer conditions, the correct K:Ca ratio in the substrate should be between 1:1–1:1.3.

We must also consider the pH and its influence on the availability of individual elements. The optimal range for nutrient uptake is between pH 5.5 and 6.2. Strong growth (high uptake of N-NO3) will increase pH and can therefore also increase the risk for BER. The pH should be adjusted using N-NH4, within the ranges 0.7-1.75 mMol l-1 (10-24 ppm, for beef tomato max 1mMol l-1, 14 ppm).

SUMMARIZING THE ACTION TO PREVENT BER
  • Make a weekly nutrient analysis and adjust input solution accordingly.
  • Maintain a stable rootzone EC during the day.
  • Maintain root quality with a stable decrease in WC overnight (eight per cent.)
  • Keep the fruit/leaf balance for better Ca2+ distribution to the fruits, and if required, remove additional lower leaves.

Andrew Lee works for Grodan BV as Business Support Manager for North America and Export Markets.
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