Physiological Disorders Related to Irrigation and Fertilization

Russeting/Cuticle cracking

Description
'Russeting' is a disorder of the tomato skin in which minute, hairline cracks, invisible to the naked eye, cover up to 25% of the fruit surface (Bakker, 1988). It is also called rain check, crazing, swell cracking, shrink cracking, hair cracking, and cuticle blotch (Emmons and Scott, 1997). The fruit has a rough feel, and when examined closely the surface appears crazed rather than smooth (Fig. 5). It is sometimes described as a poor skin 'finish' and significantly reduces the shelf life of harvested fruit (Hayman, 1987). Cracks appear as fruit approach maturation, six to seven weeks after fruit set and incidence of the disorder was highest early and late in the production season (Bakker, 1988).

Causes
Russeting is not well understood in either the field or greenhouse. It is more common in soilless culture than fruit cracking (Bakker,1988), but conditions conducive to fruit cracking also appear to be conducive to russeting. Presumably in both cases, the expansion of the epidermis cannot keep pace with the rate of fruit enlargement. Cultivars with greatest susceptibility to russeting had an epidermis with an outer cell wall up to 80% thicker than that of fruits without cracks (Den Outer and van Veenendaal, 1987). This may represent a tougher, less extensible epidermis, which is more likely to develop cracks. Fruits with high growth rates late in the maturation period were most affected by russeting (Bakker, 1988). A low fruit load relative to leaf area tended to increase russeting, presumably by increasing the fruit growth rate. These conditions would be present at the beginning of harvest because relatively few fruit are present and late because the plants have been topped. Emmons and Scott (1997) did not find an increase in russeting in field-grown tomatoes in Florida by pruning either leaf or fruit, but did find the amount of russeting correlated with the amount of rain during the entire 2-week period before harvest.

Control
To control russeting in soilless systems, the following practices are suggested: selecting a resistant cultivar; avoiding big fluctuations between day and night temperatures and relative humidities; avoiding large changes in electrical conductivity (EC) of the nutrient solution and maintaining a minimum EC of 3.0 in rockwool slabs; and minimal deleafing (Hayman, 1987). Procedures that stabilize the leaf area to fruit ratio and reduce fruit growth rate are sometimes also suggested. For example, not topping plants or permitting suckers to grow at the top of the canopy should reduce fruit growth rates at the end of the season and may also reduce fruit temperatures. In the field, Emmons and Scott (1997) suggest: staking plants; avoiding direct exposure of the fruit to the sun by protecting foliage from disease and damage during harvest; using resistant cultivars; and harvesting before rains, if possible.