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.