Cracking (also called sugar crack) is one of the most common physiological problems on sweet peppers in South Africa. Cracks can be small or large on the fruit shoulder, the sides or the blossom end of fruit. Fruit with fine, inconspicuous cracks (Figure 1, A & D) are often marketed, especially during times of short supply. Fruit with deep unattractive cracks are not marketable (Figure 1, B & C).
In contrast to other fruit where cracking is a problem, the fruit of sweet peppers are hollow. This makes sweet peppers more susceptible to crack since the fruit is more prone to shrinking and expansion.
Sugar cracks start off as microscopic cracks in the cuticle (outer wax layer) when the fruit start to turn colour. Initially the cracks are in the cuticle only and they can be observed by means of a microscope. As the condition develops, the cracks become wider, tear through the underlying epidermal layer and later through the deeper lying parenchyma cells. At this stage, cracks can be observed by the naked eye, especially if a magnifying glass is used. The fruit tissue responds by secreting a corky wound callus to seal the wound, thus making cracks very conspicuous and the fruit unmarketable (Figure 2).
Why does the fruit cuticle crack only when fruit reach the mature stage?
The cuticle is a non-living, protective layer covering the surface of the fruit. It protects the fruit wall cells against dehydration and pathogens and it is formed through secretions from epidermal cells. Although the cuticle is non-living, its contents changes as the fruit develop. When small fruit develop and the circumference increases on a daily basis, the cuticle is elastic. Should cracks form in the cuticle at this stage, they are filled by secretions from the epidermal cells. By the time fruit reach maturity and start to colour up, the circumference remains unchanged so less wax is secreted into the cuticle. Researchers also think that the contents of the cuticle may change as it becomes less elastic and more penetrable by water.
What are the causes of cracks on sweet peppers?
The primary cause of cracking is expansion of the fruit wall during the night and shrinkage during the day. The daily shrinkage and expansion weakens the cuticle of mature fruit. During the day water is lost through transpiration. During the night water is taken up by the roots and transported to fruit. This causes a build up of pressure (turgor) in the fruit wall (Figure 2). Because transpiration does not take place at night, the turgor pressure can be so high that the cuticle, and later the epidermis crack.
Developing fruit are less prone to crack partly because the cuticle is supplemented continuously and partly because the wall is thin and contains little sucrose. As fruit develop, the fruit wall becomes thicker and when they are ripe (red or yellow) the sucrose contents can be as high as 20%. The high sucrose content leads to a low water potential and causes water to flow into the fruit during periods of low transpiration – when it is cool or during the night.
Plant characteristics associated with crack
Research in Israel and elsewhere, has shown that cracking can be associated with the following characteristics:
- Genetics – Some varieties are prone to crack.
- Thickness of the fruit walls – The thicker the fruit wall, the more the water it can hold and the higher the pressure that can build up at night. Because breeders select against cracking, not all varieties with thick walls are prone cracking.
Environmental factors associated with cracking
- Reduced transpiration results in less water flowing from fruit to leaves and this result in pressure builing-up in the fruit walls.
High relative humidity is the main cause of low transpiration during the night. Especially during periods when minimum temperatures tend to be low, typically during spring and autumn. Transpiration is also low during cool or cloudy days.
Transpiration is low when the leaf canopy is insufficient. This can happen when a plant is pruned incorrectly and too many leaves are removed, or when plants loose leaves as a result of leaf diseases.
- Water stress during the day cause a net flow of water from fruit to leaves because it becomes difficult for roots to take up water from the soil, yet transpiration takes place for temperature control.
Water supply during hot, dry days is critically important to maintain the water balance in sweet peppers.
- Direct irradiation. The temperature of fruit exposed to direct sunlight, can be 3°C higher than shaded fruit. Researchers speculate that increased temperature can lead to increased water potential that in turn lead to more water flowing out of the fruit to leaves.
Management to reduce cracking
- Plants must at all times receive enough water. This will reduce water flow from fruit to leaves under conditions when the rate of transpiration is high.
- Plants must have a good foliage cover at all times so that fruit are protected against direct sunlight and also to make sure that the surface for transpiration is optimal.
- Reduce the fruit load should leaves be lost as a result of pests and diseases, even if it means that green fruit are harvested.
- Provide for good ventilation, especially in plastic greenhouses. During the day, ventilation removes humid air around leaves resulting in improved transpiration. During the night, ventilation helps to prevent high relative humidity.
- The risk for fruit cracking increases in spring and autumn weather. Minimum temperatures tend to be low, while daily maximum temperatures and transpiration are high.
- Choose varieties less prone to cracking.
- Keep in mind that thick fruit walls are more prone to cracking.
Note: Management to reduce fruit cracking cannot be based on one or two actors. It should be an integrated approach taking all the possible factors into consideration.
Resistance is the ability of a plant variety to restrict the growth and development of a specified pest or pathogen and/or the damage they cause when compared to susceptible plant varieties under similar environmental conditions and pest or pathogen pressure. Resistant varieties may exhibit some disease symptoms or damage under heavy pest or pathogen pressure.
DISCLAIMER: This information is based on our observations and/or information from other sources. As crop performance depends on the interaction between the genetic potential of the seed, its physiological characteristics, and the environment, including management, we give no warranty express or implied, for the performance of crops relative to the information given nor do we accept any liability for any loss, direct or consequential, that may arise from whatsoever cause. Please read the Sakata Seed Southern Africa (Pty) Ltd Conditions of Sale before ordering seed.