Agriculture Reference
In-Depth Information
period of fl owering show greater susceptibility to induction (Fig. 12.9a), as
smaller plants are not as susceptible to natural fl owering and are not as easily
forced to fl ower. The minimum size for forcing is much larger under optimum
growing conditions than when growth is restricted by nutrient, water or low-
temperature stress. Plant weights are used to estimate suitability for forcing,
as the larger the plant weights at forcing the bigger the fruit harvested (Fig.
12.9b). For 'Smooth Cayenne', when the plant fresh weight is between 2 and
4 kg the fi eld will be forced to fl ower, depending on plant growth status and
weather. Ratoon suckers are more easily induced to fl ower than the plant crop.
The Spanish group is also easier to force in conditions impossible for Cayenne.
Pest management
Except for small plantings in subsistence-farming systems, pineapple is
largely a monocultured crop, involving relatively large fi elds under more or
less standardized cultural practices. Monoculture, as well as the perennial
nature of the crop, is conducive to accumulation of diseases and pests, some
confi ned to certain regions, while most have become established worldwide
(Lim, 1985; Rohrbach and Schmitt, 1994) as vegetative germplasm has been
transported around the world since the 1600s. The 'Smooth Cayenne' cultivar
is relatively resistant to most pineapple diseases (Rohrbach and Johnson,
2003). Occasionally, damage associated with specifi c pests or diseases can
cause serious economic losses.
Diseases and nematodes
One of the most important diseases worldwide is mealybug wilt (Table 12.4).
This wilt is caused by a closterovirus and associated feeding by the mealybugs
Dysmicoccus brevipes
and
Dysmicoccus neobrevipes
(Sether and Hu, 2002), both
of which are able to transmit the virus; the disease does not occur if only the
closterovirus or the mealybugs are present. Ants, which protect the mealy
bug colonies and transport individuals from plant to plant, also are essential
to disease development. The dominant species of ant on pineapple in Hawaii
and in South Africa is the big-headed ant (
Pheidole megacephala
). Ant control
is essential to the control of mealybugs and the wilt; in the absence of ants,
parasitoids and predators keep mealybugs under control. Mealybugs feed on
the roots, on the stem above the roots, on leaf sheaths or leaves and in fruitlet
fl oral cavities. Wilt-af ected plants are stunted, becoming yellowish fi rst and
reddish later. These symptoms resemble the ef ects of severe drought.
Some pineapple cultivars possess partial resistance to the mealybug
wilt. However, the only practical control of mealybugs and the associated
ants is by use of insecticide baits to control the ants. Cultivation eliminates
mealybugs, and in a newly planted fi eld they gradually move into the fi eld
from uncultivated grassy or weedy borders, where mealybugs survive on
