Browsing by Subject "Biologische Kontrolle"

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    Molecular perspectives on the ecologically inconsistent effectiveness of the mycoherbicide Fusarium oxysporum f. sp. strigae against Striga hermonthica
    (2022) Anteyi, Williams Oyifioda; Rasche, Frank
    Cereals are a major staple that is crucial for food security in sub-Saharan Africa (SSA). Sadly, the obligate hemiparasitic witchweed, Striga spp., especially Striga hermonthica (Delile) Benth., is a major biotic constraint to cereal production in SSA, causing enormous crop yield losses estimated at US$10 billion annually. Fusarium oxysporum f. sp. strigae (Fos) is the most renowned fungal biological control agent (BCA) for specifically and significantly tackling S. hermonthica under agricultural systems. Field surveys, however, have revealed the inconsistent effectiveness of Fos isolates against S. hermonthica in differing zones of SSA (i.e., West Africa, East Africa). This daunting phenomenon is a critical challenge that affects Fos reliability and deters its use for S. hermonthica management. The inconsistent effectiveness of Fos against S. hermonthica was presumably ascribed to the interactions that occur between the differing location-specific ecological factors of the pathosystem i.e., abiotic (climate, moisture, or soil physico-chemistry) or biotic (S. hermonthica, Fos isolate, or the plant microbiome). Without doubt, the diversity of a host or pathogen is a primary determinant of the innate susceptibility or virulence of the host or pathogen, respectively. In terms of S. hermonthica diversity, genomic variation of individuals, or regional genetic variation of the sampling zone, were the two major forces suspected. However, the important determiner out of the two forces was unknown. Besides, despite the suppression/death that Fos causes to S. hermonthica, the physiological damage S. hermonthica initiates to an infested cereal crop is mostly irreversible. Hence, in examining strategies for circumventing the main problem of Fos inconsistent effectiveness against S. hermonthica, and the physiological consequences of S. hermonthica on the host cereal crop, the integration of other (non-Fos inoculum) BCA were suggested as possible means for improving the efficiency of S. hermonthica biocontrol. For example, by utilizing a bioherbicide cocktail of Fos and plant growth promoting rhizobacteria (PGPR), or Striga seed germination-inhibiting fungal toxins. Apart from the popular reputation of PGPR in enhancing crop health and growth, certain PGPR strains (especially Bacillus subtilis isolate GB03) have been earlier reported for their highly-promising potential of antagonizing S. hermonthica development. Similarly, certain fungal extracellular metabolites (exometabolites), especially of Fusarium origin, were reported to completely inhibit S. hermonthica seed germination in vitro at very low concentrations (≤ 1 mM). Unfortunately, knowledge of the microbe (Fos)–microbe (PGPR) interaction, their localization and ecological niche, for enabling their expected synergistic impact of simultaneously suppressing S. hermonthica and enhancing the Striga-infected cereal crop biomass, was unknown. Also, it was unknown if highly potent/efficient Striga seed germination-inhibiting fungal exometabolites will consistently suppress S. hermonthica in planta. Thus, in the context of genetic diversity in S. hermonthica, the PhD study focused on gaining (molecular) insights into the inconsistent effectiveness of Fos against S. hermonthica; including the examination of some strategies for improving S. hermonthica biocontrol efficiency, precisely by integrating PGPR, or Striga seed germination-inhibiting Fusarium exometabolites, into a S. hermonthica biocontrol system. The first research examined the molecular genetic basis, underlying the variable susceptibility of S. hermonthica populations sampled from differing zones of SSA (West Africa, East Africa) to contrasting Fos isolates (Foxy-2, FK3). Regardless of sampling zone, the S. hermonthica populations displayed divergent susceptibility patterns to the Fos isolates i.e., a S. hermonthica class was susceptible to both Foxy-2 and FK3, while the other class was susceptible to either Foxy-2 or FK3. This manifestation correlated with nucleotide mutations at certain loci. Thus, genomic variation in S. hermonthica is a superior determinant of the inconsistent effectiveness of Fos isolates, rather than the S. hermonthica sampling zone. The second research examined the impact of coinoculating Fos and a PGPR (B. subtilis isolate GB03) into a S. hermonthica-sorghum parasitic system. Notwithstanding the colocalization of Fos and GB03 in common ecological niches of diseased S. hermonthica shoot (mainly in flavonoid-rich regions), GB03 thwarted Fos suppressive activity against S. hermonthica. Interestingly, a novel, alternative Fos entry route into S. hermonthica (through the trichome) was discovered. The coinoculation of Fos and GB03 presented no added advantage for S. hermonthica control. Finally, the third research screened a set of highly phytotoxic Fusarium exometabolites against S. hermonthica seed germination (in vitro) and incidence (in planta). This was to identify the most potent/efficient Fusarium exometabolite for S. hermonthica biocontrol. Among the tested exometabolites, diacetoxyscirpenol (DAS) was the most potent/efficient to completely suppress S. hermonthica both in vitro and in planta. Fos, however, did not produce DAS, due to underexpression of key genes necessary for Fusarium trichothecene biosynthesis. In conclusion, owing to the obligate outcrossing mating system in S. hermonthica, genomic variation is an inevitable phenomenon. This, therefore, plays a crucial role in the variable susceptibility of S. hermonthica to Fos. The newly discovered Fos (direct) entry route into S. hermonthica (trichome entry), elucidates a novel paradigm to the infection mechanism occurring under the S. hermonthica (host)–Fos (pathogen) interaction, in addition to the previously reported indirect, rhizosphere-transmission. Thus, this novel phyllosphere-transmission, paves the way for further research that exploit this alternative Fos infection route for better S. hermonthica biocontrol. Lastly, considering the potency and broadscale efficacy against diverse S. hermonthica populations, the exometabolite DAS could serve as a new agent for a more efficient S. hermonthica biocontrol. Though, further examination of its specific mode of action against the target weed (S. hermonthica), as opposed to non-target organisms, is required.
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    Molecular systematics of selected Diadegma species (Hymenoptera: Ichneumonidae: Campoplegine) important in biological control
    (2006) Wagener, Barbara; Zebitz, Claus P. W.
    The genus Diadegma (Hymenoptera: Ichneumonidae: Campopleginae) represents a large group of parasitoids with 201 species worldwide. Adult Diadegma females parasitise larvae of various lepidopteran species and some species, in particular Diadegma insulare (Cresson) and D. semiclausum (Hellén), have gained economic importance as biological control agents of Plutella xylostella (Linnaeus). A low parasitism rate of <15 % of the parasitoid complex (Diadegma sp., Oomyces sokolowskii (Kurdjumov) and Diaplazon laetatorius (Fabricius)) in unsprayed cabbage and kale fields infested with P. xylostella in eastern and southern Africa was the starting point for the development of a biological control project for P. xylostella which was implemented by the International Centre of Insect Physiology and Ecology (ICIPE), Kenya. One of the objectives of the biocontrol project was to examine the taxonomic status of Diadegma species associated with P. xylostella in eastern and southern Africa and the exotic parasitoid D. semiclausum imported to Kenya from Taiwan (Asian Vegetable Research and Development Centre, AVRDC) by cross breeding experiments and molecular methods. Thus, two different molecular regions, a fragment of the mitochondrial cytochrome c oxidase subunit (COI) and the second internal transcribed spacer (ITS2) of ribosomal DNA were amplified utilising polymerase chain reaction (PCR) and digested afterwards with several restriction enzymes (PCR-Restriction Fragment Length Polymorphism-RFLP). In the due course of the study examinations of several Diadegma species attacking P. xylostella were undertaken with the PCR-RFLP method developed previously for the African Diadegma. This molecular method could solve some taxonomic difficulties of the genus Diadegma. Sequence analyses were used to investigate the phylogenetic relationship of nine Diadegma species (D. blackburni (Cameron), D. insulare, D. leontiniae (Brèthes), D. chrysostictos (Gmelin), D. armillata (Gravenhorst), D. fenestrale (Holmgren), D. mollipla (Holmgren), D. semiclausum, D. rapi (Cameron)) and the phylogenetic relationship of the genus Diadegma within the superfamily Ichneumonoidea. Cross breeding experiments were carried out between two populations of D. mollipla from eastern and southern Africa. No significant differences in the total number of progeny per female and the number of male offspring were obtained, whereas the female progeny showed significant differences. Hybrid females resulting from both reciprocal crosses were reproductively compatible with males of both parental lines, which indicated that no genetic incompatibility was apparent between the two D. mollipla populations. In contrast, crosses between D. mollipla and D. semiclausum resulted only in the occurrence of male offspring, which is typical for unfertilised progeny in Diadegma. The laboratory cultures of D. mollipla and D. semiclausum were highly male biased. Inbreeding, where homozygosity is much higher, is leading to a higher diploid male production. Diploid males can easily be detected by isoenzyme variations as a genetic marker. Heterozygote females/males of D. semiclausum and D. mollipla were identified by phosphoglucomutase (PGM) electrophoretic banding patterns. Crosses between a mother (heterozygote, diploid) and her son (homozygote, haploid) resulted in one diploid male in D. mollipla and none in D. semiclausum. Information about diploid males in D. semiclausum detected with PGM has already been published and different methodologies might be the reason why in D. semiclausum no diploid male was detected. Therefore the present analyses with PGM as molecular marker should be seen as a preliminary study.