Fungicide resistance and parasexual recombination in pseudocercosporella herpotrichoides

Hocart, Mark J. (1987) Fungicide resistance and parasexual recombination in pseudocercosporella herpotrichoides. PhD thesis, University of Nottingham.

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A detailed investigation of benzimidazole resistance in field isolates and laboratory mutants of Pseudocercosporella herpotrichoides resulted in the description of four major resistance catagories, based on the sensitivity of isolates to carbendazim: high-level (MIC - >1000 pM), intermediate-level (MIC - 50 - 200 pM), low-level (MIC - 10 - 20 pM) and sensitive (MIC -

0.6 - 2.5 pM). Cross-resistance to other benzimidazole compounds, including thiabendazole, was observed in all cases, although the level of resistance to thiabendazole was not directly related to the level of resistance to carbendazim.

Strains from each resistance catagory were tested for sensitivity to two experimental phenylcarbamate fungicides, MOPC and S32165. Wild-type carbendazim-sensitive isolates were insensitive to both compounds, as were most low-level and intermediate-level carbendazim-resistant mutants. Many high-level carbendazim-resistant strains, including most of the carbendazimresistant field isolates showed increased sensitivity to one or both phenylcarbamates. In addition, a fifth resistance phenotype was identified, showing increased sensitivity to MOPC and S32165 but associated with only slightly reduced benzimidazole sensitivity.

Spontaneous mutants from a high-level carbendazim-resistant strain, selected for reversion to insensitivity to Mope were readily obtained. Analysis of their fungicide response showed some to be back-mutations in which insensitivity to MOPC was associated with the loss of resistance to carbendazim, While others appeared to carry suppressor or modifier mutations producing a range of resistance phenotypes.

A study of sensitivity to a range of ergosterol biosynthesis inhibiting (EBI) fungicides revealed considerable variation between different isolates. In general, BWR-type isolates were much less sensitive to the triazole compounds tested than BWtypes, while the reverse relationship was found for the morpholine fungicide, fenpropimorph. Sensitivity to prochloraz showed much less variation. No correlation was found between benzimidazole resistance and reduced sensitivity to the EBI fungicides.

Two pathotypes of ~ herpotrichoides, BW and BWR, usually distinguished on the basis of cultural morphology, are recognised. The morphological character, however, was found to be unstable, particularly in BWR-types, which readily sectored to give colonies typical of the other pathotype.

Parasexual recombination in P. herpotrichoides was demonstrated using auxotrophic mutants isolated after UVirradiation of conidia. Unstable heterokaryons were obtained following hyphal anastomosis between complementary strains. From these heterokaryons stable prototrophs were recovered both from spores and isolated protop1asts. Genetic evidence and DNA estimations indicated that these stable prototrophs were dip1oids. Recombinant types were isolated from diploids after treatment with 5-fluorouracil. It was also shown that viable heterokaryons could be recovered following PEG-mediated fusion of protop1asts isolated from complementary strains.

Heterokaryon incompatibility between unrelated field isolates was tested using auxotrophic mutants obtained by selection for resistance to chlorate and selenate. Vegetative incompatibility appeared to be widespread, few positive interactions being observed, suggesting that the potential for parasexual gene exchange in the field is low. Protoplast fusion may provide a method whereby this incompatibility can be overcome in the laboratory.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Peberdy, J.F.
Lucas, J.A.
Keywords: Benzimidazole resistance; Carbendazim; Heterokaryon incompatibility; Parasexual gene exchange
Subjects: Q Science > QK Botany
Faculties/Schools: UK Campuses > Faculty of Medicine and Health Sciences > School of Life Sciences
Item ID: 56527
Depositing User: Blore, Mrs Kathryn
Date Deposited: 16 Apr 2019 12:58
Last Modified: 07 May 2020 13:31

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