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Grower Solutions Magazine - Lefroy Valley April 2002


What is cracking? Cracking is the splitting of the epidermis around the calyx or stem scar. There are two types of fruit cracking in tomatoes. 
(a) Concentric cracking, which is a splitting of the epidermis in circular patterns around the stem scar. 
(b) Radial cracking which is a splitting of the epidermis from the stem scar towards the blossom end.

Why does cracking occur? Cracking depends on the ability of the epidermis and its cells to stretch. Some varieties have an epidermis that stretches well and will have very little or no circular cracking. Other varieties can have the opposite situation where they do not stretch well and have a lot of cracking.

When Does racking occur? Cracking occurs as the tomato nears maturity. More susceptible varieties crack in the mature green stage and more tolerant varieties at later stages. The earlier the cracking then the deeper and longer the crack becomes. The problem is usually more severe on the lower trusses. Circular cracking often occurs on ripe tomatoes that are on the vine too long.

Causes of cracking 
1. Alterations in the growth rate.
Plants have periods where they might have very fast growth followed by slow growth and then fast again. These changes can cause fruit nearing maturation to crack. If the cells have "hardened" during the last slow growth then in the next fast growth period they may not be able to stretch enough and the epidermis cracks.

Fast growth.
Some varieties have periods of very fast fruit growth with high temperatures and moisture levels.

3. Fruit temperature fluctuations and leaf removal.
Wide fluctuations in temperature can also induce cracking. This is true especially when plants have been de-leafed too early leaving fruit without protection. The exposed fruit heats up dramatically in the sun. At night it cools relatively quickly and the differential is bigger than it would have been had the leaves covered the fruit. The expansion and contraction of the epidermis and its cells can result in cracking.

4. Succulent plants. Succulent plants that are high in nitrogen and low in potassium are more susceptible to cracking. 

5. Rain and irrigation.
Rain and excess irrigation will often cause cracking and if the fruit lacks leaf cover then the effect is even more dramatic. Tomato crops that do not receive water at regular intervals but rather receive it periodically at large intervals are likely to have cracking. This problem is related to the Conductivity Factor (CF) of the soil solution.

Control of Cracking 
A good fertilizer programme that will avoid overly succulent plants. 

Proper pruning and leaf removal. Excess removal will result in fruit being exposed too much to the sun. 

3. A good preventative spray programme to control foliar diseases and reduce the loss of leaves. More leaves give the fruit better protection. 

4. Proper water management: do not over irrigate; water at fixed intervals and increase or decrease quantities as needed. Use tensiometers.
5. Constantly monitor the CF levels of the soil solution, using soil extractors, CF meters and equipment to measure nutrient levels in the soil solution. 

The CF levels should remain fairly constant in the different stages of the crop. Big fluctuations in the CF levels mean that the plant will probably absorb too much water when the level suddenly decreases. 

When it rains then the rainwater entering the soil will dissolve the nutrient elements and take them down deeper into the soil. This causes the CF levels in the root zone to fall to a lower level and water moves more easily and quickly into the plant. The epidermis of the fruit cannot stretch enough to absorb all the extra water and the result can be cracking. To control the CF in rain, it is recommended to give potassium chloride (5 kg/ha more or less depending on the situation) to the crop while it is raining or when this is not possible, immediately after the rain. This will help restore the CF without adding extra nitrogen.

NOTE: Some growers also believe that Greenback varieties are more tolerant to cracking, russeting and wind marking, compared to the uniform green, non-greenback types. Greenback types at this stage seem to have a tougher, more resilant outer skin, which may combat a wider range of environmental conditions

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