Die akarofauna op pynappelplante en hulle moontlike verband met die voorkoms van swartvlek
Grobler, Douw Gerbrand
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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.