Jamieson, Jennifer Agnes
Thiabendazole residues and its effects on the storage quality of potatoes.
PhD thesis, University of Glasgow.
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The objective of this thesis was to gain more information about the post-harvest fungicide thiabendazole (TBZ), in terms of chemical residues in potatoes and in processed products and on factors affecting the storage quality of the crop. So that TBZ residues could be determined, a sensitive and accurate method for the extraction, clean-up and quantification of TBZ from potatoes, was derived. The recovery of the method was calculated at 93.8% 2.1%, and levels in the range of 0.004g TBZ could readily be detected. This method was then used to determine the distribution and penetration of TBZ in stored potatoes. The residue was found to associate with the skin of the potato, as its polarity hindered any further penetration of the chemical into the tuber. TBZ residue analysis was also carried out in processed potato products, including boiled potatoes, baked potatoes and crisps. As little work had been carried out in this area, these findings should provide the basis for future research, so that a complete picture can be built up with respect to residues in processed products. In general, cooking did not seem to affect either the chemical nature or the absolute residue value of TBZ in the product. The exception to this was the microwaved baked potatoes. In this sample, the residue had decreased after cooking. However, no decomposition products were detected and so the decrease in residue remains unaccounted for. Factors considered which affected the storage quality of the crop included i) the treatment of seed with TBZ to try to reduce the amount of chemical applied to the ware crop, while still maintaining the desired level of disease control, ii) the effect that TBZ and its formulations had on the wound healing capacity of potatoes and iii) the metabolism of TBZ by fungi and bacteria. The results of the seed treatment study were very disappointing. It was shown that a seed treatment of TBZ did not affect emergence or yield to any significant extent. The effects on disease control could not be studied satisfactorily though, because a particularly healthy crop was grown and no disease was in evidence (including the controls). The effect that TBZ had on wound healing was measured by following the development of resistance to water loss in cut potato discs. TBZ was found to promote the development of resistance to water loss in the early stages of wound healing, compared with the control, but after 21 days (the conclusion of each experiment) resistance to water loss was the same in TBZ treated discs, as in the controls. The initial increase in the development of resistance to water loss in TBZ treated discs may be sufficient to inhibit fungal and bacterial pathogens from entering the tubers. The formulation components of the TBZ also influenced the wound healing capacity of discs. Storite, a neutral suspension of TBZ, promoted wound healing to the same extent as TBZ in methanol. However, the acidic formulation of TBZ, Storite Clear, was found to inhibit wound healing and encourage bacterial rotting. The combined formulation of TBZ and 2-aminobutane, Storite Plus, had mixed effects and therefore no satisfactory conclusions could be made with respect to its effects on the wound healing process. TBZ was shown to have no significant, beneficial effects on curing in whole tubers. Therefore, although the chemical promotes wound healing, it does not necessarily follow that it will have the same beneficial effects on curing. It was important to study the metabolism of TBZ for three reasons:- i) to discover if loss of activity via metabolism was likely to be a problem, ii) to see if any metabolite formed was toxic and iii) to try to ascertain whether the development of resistance to TBZ was likely to become a problem. A number of fungal and bacterial cultures were set up, and resistant strains of the micro-organisms, if they developed, isolated. Of the micro-organisms subjected to TBZ, only soil bacteria, Erwinia carotovora, Phoma exigua and Phytopthora infestans developed resistance. From these isolates, P. exigua was the only micro-organism which metabolised TBZ. Very small amounts of TBZ (less than 0.08% of the TBZ initially administered) were metabolised to 5-hydroxy TBZ. These metabolism studies were carried out under optimum growing conditions for the bacteria and fungi, therefore the likelihood of P. exigua developing resistance to TBZ in a store is fairly low.
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