| dc.description.abstract | Blackspot is a common pineapple disease in many countries. The fungi Penicillium funiculosum and
Fusarium moniliforme, which occur in the blossom
cavities on the old flower parts and are usually recovered from the rot, have been shown to be wound
pathogens (Edmonstone-Sarnrnons,1958). It is not
known what causes this damage. Growth cracks, insect
or mite injuries, and even pathogenic bacteria have
been suggested. There is, in fact, little evidence
of any kind, but what little there is, may favour a
hypothesis that mites are in some way associated with
the disease.
Mites on pineapple plants can be divided into 3 main
groups, namely the plantfeeders, the fungusfeeders
and the predators. To prove or disprove this above
mentioned hypothesis a study of the population density of the different mites at different plant and
fruit stages was made and the influence of certain
pesticides on the mite population and occurrence of
blackspot was tested in field experiments.
In the population study and the field experiments
mite counts were made at 3-weekly intervals. The
amount of fungi and lesions per floral cavity has
also been noted on a scoring basis. At harvest the
percentage clean fruit for each treatment of the
field experiments was determined.
Populations of Steneotarsonemus ananas, the most
common phytophagous mite on pineapples in Swaziland,
are mainly confined to the inner whorl of young leaf
bases. They reach a population peak on the leaves
and in the floral cavities at the "red bud" stage
followed by a steep decrease in population density
as the fruit matures. As the flower parts senesce
in the blossom cavities, fungal growth starts and
attracts the fungus feeding species.
Thiodan sprays gave very good control of mites and
showed no direct fungicidal action. Plots sprayed
weekly with Thiodan also seemed to reduce blackspot
incidence, but plots sprayed only twice didn't
r educe the incidence at all although i t was still
effective in controlling the mite numbers. In all
t he experiments Captan, a fungicide, was the most
effective chemical to use for the control of blackspot. Where only two applications of Captan at
different fruit stages have been applied, the best
time seems to be at and immediately after the "red
bud" stage. A strong correlation between the fungi
and the lesions i n the floral cavities has been found.
The field experiments showed no definite relationship between the number of mites and the incidence
of blackspot. Invasion of the pathogen fungi rather
may take place through wounds, like growth cracks,
caused by an uneven stretching of the lignified and
suberized epidermis of the floral cavity when the
inner cells become turgid during ripening. Excessive
rainfall may also increase the water content of the
cells. Increased growth caused by an excessive
application of nitrogen fertilizers may also be a
cause of growth cracks. | en_US |
