Production Practices and Quality Assessment of Food Crops. Vol. 1

28 R. M. Madakadze and J. Kwaramba
4.3.8. Button
This refers to unusually small heads due to premature generative stage. This happens
under poor environmental conditions which arrest vegetative growth. Cabbage heads
also become small due to boron deficiency and will also be yellow (Yamaguchi,
1983).
4.3.9. Leaf abscission
Cabbages are very sensitive to ethylene (Cantwell and Suslow, 2001). Very low levels
of the ethylene will lead to leaf fall and yellowing. In the field, if cabbages are
exposed to other plants or fruits nearby that produce ethylene the disorder will
appear. In storage poor ventilation and mixing with ethylene producing plants and
fruits will result in leaf abscission.
4.3.10. Black speck, pepper spot, petiole spot, gomasho
Very small to moderate size discolourations in the form of lesions appear on the
midrib and leaf veins. In the field black speck is as a result of low temperatures
coupled with harvesting over-mature cabbage heads whereas in storage and in transit
this is due to exposure to low temperatures suddenly followed by warmer temperatures
(Cantwell and Suslow, 2001).
4.4. Sweet potatoes
The sweet potato (Ipomoea batatas) is a member of the Convolvulaceae family
and is a warm season tender perennial root crop grown as an annual for its storage
roots. Other common names include Louisiana yam and Spanish potato. The crop
is grown throughout most of the tropics. Sweet potato is the major staple crop in
Papua New Guinea (Hartemink et al., 2000). It is also an important secondary
food crop for many Eastern and Southern African countries whose staple diets are
based on cereals, particularly maize (Gakonyo, 1993). Sweet potato is an important
food security crop when maize is in short supply or in years of drought (Mutuura
et al., 1992). Sweet potato is grown in tropical, subtropical and warmer temperate
regions during the frost-free periods (Yamaguchi, 1993). They are widely grown
from 40° N to 40° S and above 2500 m at the equator (Hahn and Hozyo, 1984).
It is the most widely adapted of the agriculturally important root crops native to
the humid tropics. Sweet potatoes require a minimum frost-free period of 120–150
days and a minimum average daily temperature of 24 °C (Yamaguchi, 1983). It
also requires approximately 2 cm of soil moisture per week, uniformly distributed
during the growing season. Sweet potatoes are adapted to a wide range of soil textural
classes but sandy loams at a pH of 5.3–7.0 are considered ideal (Norman et al.,
1995). Heavy clay soils often give low yields of poor quality (Kay, 1973) and
irregular tuber shape (Martin, Leonard and Stamp, 1976). However, in spite of
their high clay content (� 60%) oxisols and histosols are particularly suited to sweet
potatoes (Feliciano and Lugo-Lopez, 1977; Chew, 1970). The quality of the sweet
potato root is affected by several environmental conditions during the growing