Physiological Disorders Related to Irrigation and Fertilization

Fruit cracking

Description
Cracks (Fig. 4) may occur in circles around the stem scar (concentric cracking) or may radiate from the stem scar (radial cracking).

Causes
Fruit cracking is a complicated disorder. In areas where rainfall is common during ripening, problems and losses can be very heavy. Cracked fruit is also susceptible to disease and insect contamination. Environmental and cultural factors associated with the disorder were summarized by Peet (1992) as follows: "Fruit cracking occurs when there is a rapid net influx of solutes and especially water into the fruit at the same time ripening or other factors reduce the strength and elasticity of the tomato skin. Increases in fruit temperature raise gas and hydrostatic pressures of the pulp on the skin, resulting in immediate visible cracking in ripe fruit. In green fruit, cracking occurs later in the ripening process when minute cracks created earlier expand to become visible. High light conditions, especially on unshaded fruit have also been associated with higher rates of fruit cracking. High light intensity raises fruit temperatures, especially on exposed fruit. Under high light conditions, fruit soluble solids and fruit growth rates are higher; both of these factors sometimes are associated with increased cracking."

Kamimura et al. (1972) proposed the following explanation for cracking of tomatoes after rain based on their studies of the effects of soil moisture levels on fruit skin. They showed that high soil moisture lowered the tensile strength of tomato fruit skin. Because of this low tensile strength, the fruit enlarged rapidly and developed many minute cracks. These minute cracks later developed into visible cracks. Under low soil moisture, Kamimura et al. found that the tensile strength of the skin was greater. As a result, plants grew more slowly and had fewer minute cracks. Changes in soil moisture during fruit growth also affected skin strength. Skin strength increased if soil moisture content decreased. Conversely, skin strength decreased if soil moisture content increased. In fact, changes from low to high soil moisture lowered skin strength compared to continued growth under any moisture regime. Such changes typically occur when drought is relieved by irrigation or rain. Cracking is particularly likely with continued wet weather or overhead irrigation because water enters the fruit through these minute cracks.

In the greenhouse, excess watering has been shown to increase the incidence of radial cracking, and there are also a few reports in field tomato crops of increased cracking at higher levels of soil moisture (Peet and Willits, 1995).

Anatomical characteristics of crack-susceptible cultivars are: 1) large fruit size, 2) low skin tensile strength and/or low skin extensibility at the turning to pink stage of ripeness, 3) thin skin, 4) thin pericarp 5) shallow cutin penetration, 6) few fruits per plant and 7) fruit not shaded by foliage. (Peet, 1992). Although it has been difficult to breed for cracking resistance per se, (Stevens and Rick, 1986) commercial cultivars bred for firm fruit and tough skin in order to decrease handling and shipping losses in field tomato production in North America are often quite resistant to fruit cracking. This is probably because these qualities are components of resistance to cracking. In addition, much of the crop is harvested at the green-mature stage, when it is less susceptible to cracking. With the increasing production of vine-ripened tomatoes in high-rainfall areas and the increased use of cultivars with the long-shelflife characteristics, cracking may become more of the problem for commercial field production.

Control
Cultural practices which result in uniform and relatively slow fruit growth such as constant, preferably relatively low soil moisture, offer some protection against fruit cracking (Peet, 1992). In field crops, cracking is usually attributed to fluctuations in the water supply. The classic occurrence is when a long period of drought is followed by heavy rain. Cultural practices that reduce diurnal fruit temperature changes also may reduce cracking. In the field, these practices include maintaining vegetative cover. Greenhouse growers should maintain minimal day and night temperature differences and increase temperatures gradually from nighttime to daytime levels. For both field and greenhouse tomato growers, harvesting before the pink stage of ripeness and selection of crack-resistant cultivars probably offer the best protection against cracking.