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    Union Catalogue of Agricultural Libraries in the Netherlands

    The WUR Library Catalogue contains bibliographic data on books and periodicals held by the libraries of Wageningen University and Research Centre and some 15 associated libraries. Holding data are added to each record.

    Subjects covered include Agrotechnology, Food and Food Production, Plant and Animal Sciences, Soil Science, Geo-information, Landscape and Spatial Planning, Water and Climate, Ecosystem Studies, Economics and Society.

    The joint collections of the participating libraries cover a substantial part of the internationally available scientific literature in these disciplines.

    As far as Dutch scientific literature in these fields is concerned, coverage can be considered near 100%, including much of the so-called "grey literature".

    All titles are entered in their original language. Keywords are added to facilitate subject searching.

    The database is updated every day and now contains over 830.000 records.

Record number 104697
Title Mechanisms of action of the organophosphorus fungicide pyrazophos
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M.A. de Waard
Author(s) Waard, M.A. de
Publisher Wageningen : Veenman
Publication year 1974
Description 98 p ill
Notes Proefschrift Wageningen
Ook verschenen als handelsuitgave
Tutors Dekker, Prof. Dr. Ir. J.
Graduation date 1974-10-18
Dissertation no. 596
Author abstract show abstract

In plant protection organophosphorus compounds are well-known for their insecticidal and acaricidal action. Since 1960, a number of organophosphorus fungicides have also been introduced. In an in vivo screening of these and some related insecticidal compounds against Sphaerotheca fuliginea on cucumber, pyrazophos (O,O-diethyl O-(5-methyl-6-ethoxycarbonylpyrazolo[1,5-a]pyrimidin-2-yl) phosphorothioate) proved to be most active. The chemical displayed both a protective and curative action against the disease; in addition, considerable toxicity to Pyricularia oryzae on barley was also found. Pyrazophos proved to be rather selective because hardly any other fungi sensitive to the chemical were found in a study on the in vitro spectrum of antifungal activity.

The organophosphorus fungicides Hinosan (O-ethyl S,S-diphenyl phosphorodithioate) and Kitazin (O,O-diethyl S-benzyl phosphorothioate) were about as toxic as pyrazophos to P. oryzae, but were much less toxic than pyrazophos to S. fuliginea. In view of these results, investigations on mechanisms of fungitoxic action of orianophosphorus fungicides were mainly focused on pyrazophos.

Both pyrazophos and its phosphate analogue (PO-pyrazophos) were found to inhibit the activity of carboxylesterases of S.fuliginea. However, because no correlation could be established between in vivo inhibition of the activity of these enzymes by pyrazophos and PO-pyrazophos, and their fungitoxicity, this effect probably does not account for the mechanism of action of pyrazophos.

Neither could the toxicity of pyrazophos, in all other experiments studied with P. oryzae, be attributed to an effect on cell membrane permeability, as was shown to be present upon incubation of mycelium of the fungus with Hinosan and Kitazin. In addition, pyrazophos hardly affected nucleic acid and protein synthesis, and only slightly inhibited oxygen uptake.

In short-term experiments, using an incubation time of 2 hours, pyrazophos was 100-1000 x less toxic to fungal growth in mycelial suspensions than to radial growth on agar and growth in liquid media inoculated with conidia. In the latter tests, growth was assayed after I week of incubation. These results can be partly ascribed to the fact that pyrazophos is metabolically converted in the fungus into two fungitoxic breakdown products, PO-pyrazophos and 2-hydroxy-5-methyl-6-ethoxycarbonylpyrazolo(1,5-a)pyrimidine (PP). In short-term experiments the toxicity of PP for mycelial growth in suspensions buffered at pH 4.0 even proved to be considerably higher than that of pyrazophos and PO-pyrazophos. PP might, therefore, be regarded as the actual fungitoxic principle of pyrazophos. This hypothesis is supported by the finding that PP, in contrast to pyrazophos, also displayed an inhibitory activity towards nucleic acid and protein synthesis and towards oxygen uptake of the fungus. The weak effects of pyrazophos on these processes and on mycelial growth in short-term experiments can probably be ascribed to an insufficient conversion of pyrazophos into PP under these conditions.

Regarding the site of fungitoxic action of PP in P. oryzae, two hypotheses could be suggested. First, PP could inhibit specifically oxygen uptake and hence indirectly cellular synthetic processes like nucleic acid and protein synthesis. Secondly, PP might react aspecifically with cellular components and hence, directly affect both oxygen uptake and biosynthetic processes.

Pythium debaryanum and Saccharomyces cerevisiae are practically insensitive to pyrazophos. Upon incubation of these fungi with the fungicide no breakdown products could be detected. Therefore, sensitivity of fungi for pyrazophos seems to be the result of a selective uptake of pyrazophos and/or of its conversion into PO pyrazophos and PP as has been demonstrated for P.oryzae.

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On paper FORUM ; STACKS ; NN08202,596
FORUM ; STACKS ; NN08200,596
Keyword(s) (cab) plant protection / fungicides / pesticides / adverse effects / selectivity / toxicity / nontarget effects / nontarget organisms
Categories Fungicides, Bactericides
Publication type PhD thesis
Language English
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