Maria Agnese Sabatini

Effects of Collembola on plant-pathogenic fungus interactions in simple experimental systems

Abstract Take-all and brown foot rot, caused respectively by Gaeumannomyces graminis var. tritici and Fusarium culmorum, are two important components of the foot and root fungal disease complex of winter cereals world-wide. These fungi persist in soil and in crop debris in the same layer of agricultural soil as Collembola, a well represented taxon of soil animals. Previous in vitro tests showed that these fungi grown on agarised medium were readily consumed by springtails. In a simplified experimental system with wheat plants and the pathogenic fungi grown on millet and wheat kernels, the severity of disease was significantly reduced by collembolan feeding activity.

Effect of Fusarium, Paecilomyces and Trichoderma formulations against aphid Schizaphis graminum

Fungal strains belonging to the genera FusariumPaecilomyces and Trichoderma were tested in vitro in order to study their effects against Schizaphis graminum one of the major pests of cereal crops around the world. Biological assays were performed using a solid formulation that was obtained from fungal cultures grown on rice and then finely ground (≤0.2 mm). The occurrence of toxic secondary metabolites (fumonisin B1 and beauvericin) produced by these fungi was also investigated. In each experiment, three groups of aphids: 15-hour old larvae, 5-day old nymphs with wing buds and wingless morphs were treated with a suspension of a fungal formulation. Some strains belonging to the genera Fusarium and Trichoderma significantly controlled the specimens of the three groups of S. graminum. The F. proliferatum strain ITEM 1407, producing a high level of fumonisin B1 in the culture (1250 μg/g), and F. larvarum strain ITEM 2139 had high insecticidal activity (>60%) within 10 minutes after application. As F. larvarum ITEM 2139 did not produce metabolites toxic to mammals, it might be a good candidate as a biocontrol agent of S. graminum in the field.

Soil-borne plant pathogenic fungi in relation to some collembolan species under laboratory conditions

Interactions between springtails (Onychiurus armatus, O. tuberculatus, Folsomia candida) and plant pathogenic fungi (Gaeumannomyces graminis var. tritici, Fusarium culmorum, Bipolaris sorokiniana and Rhizoctonia cerealis) living in the same soil layer were investigated under laboratory conditions. The fungi are responsible for the foot and root disease complex of winter cereals and can cause serious reduction in yield. Since these species coexist in agricultural soils, feeding preference tests were performed in such a way that the springtails were allowed to interact simultaneously with all the tested fungi. O. armatus, O. tuberculatus and F. candida fed on the mycelia of G. graminis var. tritici, F. culmorum and R. cerealis: in the first hours of the trials individuals of all springtail species distributed themselves among the colonies, though moving from one to the other. Subsequently F. culmorum mycelium became the preferred food of all species tested; however, the other two fungi continued to be consumed. Mycelia of G. graminis var. tritici, F. culmorum and R. cerealis were shown to be an adequate food source for reproduction of Folsomia candida. Bipolaris sorokiniana mycelium had both repellent and lethal effects on F. candida and O. armatus, whereas this fungus was lethal, although not repellent, for O. tuberculatus. Long-term experiments on F. candida indicate that conidia of B. sorokiniana were eaten and proved to be a sufficient diet for reproduction.

Citrantifidiene and citrantifidiol: bioactive metabolites produced by Trichoderma citrinoviride with potential antifeedant activity toward aphids.

Two novel metabolites with potential antifeedant activity were isolated from cultures of the fungus Trichoderma citrinoviride strain ITEM 4484 grown in solid-state fermentation on sterile rice kernels. The producing strain was identified at species level by sequence analysis of the internal transcribed spacer regions ITS-1 and ITS-2 of the nuclear rDNA and a fragment of the translation elongation factor gene TEF-1alpha. Fractionation by column chromatography and TLC of the culture organic extract, followed by feeding preference tests on the aphid pest Schizaphis graminum (Rondani), allowed the purification of 5.8 and 8.9 mg/kg of culture of two bioactive metabolites, which were named citrantifidiene and citrantifidiol ( 1 and 2). Citrantifidiene and citrantifidiol, whose structures were determined by spectroscopic methods (NMR and MS) are a symmetrical disubstituted hexa-1,3-dienyl ester of acetic acid and a tetrasubstituted cyclohexane-1,3-diol, respectively. The pure metabolites influenced the feeding preference of S. graminum restraining individuals from feeding on wheat leaves dipped in 5% aqueous methanol solution containing 0.57 mg/mL of citrantifidiene or 0.91 mg/mL of citrantifidiol.

Effects of beauvericin on Schizaphis graminum (Aphididae)

The effects of beauvericin, a toxic fungal metabolite common contaminant of maize and wheat, on aphid fitness were studied in three consecutive generations of females. Aphids were reared on wheat leaves inserted into a sandy substratum wetted with a solution of beauvericin. Ingestion of this solution through leaves did not significantly decrease the lifespan of females of all generations as compared to controls. However, the mean number of offspring from the third generation of treated females was significantly smaller than those in controls. Furthermore, treated second and third generation females produced a greater number of abortive embryos. Histological analysis revealed abundant DAPI and Feulgen positive material in the cytoplasm of some bacteriocytes of treated third generation females. This material was attributed to the endosymbionts of bacteriocytes. Tests by contact were also carried out and revealed a significantly lower survival of treated first instar aphids as compared to controls 18h after the start of the trial.

Bisorbicillinoids produced by the fungus Trichoderma citrinoviride affect feeding preference of the aphid Schizaphis graminum.

We report the effects of some bisorbicillinoids isolated from biomass of the fungus Trichoderma citrinoviride on settling and feeding preference of the aphid Schizaphis graminum. Purification of the fungal metabolites was carried out by a combination of column chromatography and thin-layer chromatography using direct and reverse phases. Chemical identification was performed by spectroscopic methods including nuclear magnetic resonance and mass spectrometry. The identified bisorbicillinoids appeared to be bislongiquinolide, its 16,17-dihydro derivative, trichodimerol, and dihydrotrichodimerol. A feeding preference test with alate morphs of S. graminum was used to identify the active fractions. Among the four bisorbicillinoids, dihydrotrichodimerol and bislongiquinolide influenced aphid feeding preference, restraining specimens from settling on leaves treated with metabolites. Taste neurons sensitive to these compounds, particularly to bislongiquinolide, were located on tarsi of the S. graminum alate morphs.

Effects of a sulfonylurea herbicide on soil microarthropods.

Abstract The short-term effects on soil microarthropods of the herbicide triasulfuron, belonging to the chemical class of sulfonylureas, were evaluated in two fields which had never been treated with sulfonylureas, and were cultivated with winter wheat. In particular, the effects of single applications at rates corresponding to two- (rate 2) and sixfold (rate 6) the recommended agricultural rate (7.5 g active ingredient ha–1) were analysed and compared with controls. The changes in the populations of the main groups of microarthropods were evaluated. Rate 2 had very low effects, whereas rate 6 produced a significant decrease in the number of microarthropods, Acarina and Collembola in the surface soil layer (0–7.5 cm). The Collembola were analysed at the species level. Statistical analysis revealed significant differences only for a few species, and only after treatment with the highest rate of triasulfuron. Finally, the results of the field tests were compared to those of laboratory tests carried out previously, which examined the effects of the same herbicide on a collembolan species.

Microarthropod communities in soil treated with atrazine

Abstract The effect of atrazine on soil microarthropod populations was studied in an experimental field cultivated with maize. Three doses of herbicide (2, 4, 6 Kg/ha) were tested in three different periods and on four layers. Controls were employed. Collembola and Oribatid mites were studied at species-level. The number of microarthropod specimens in the treated plots was not significantly different from that in the control plots.

Effects of the fungus Lecanicillium lecanii on survival and reproduction of the aphid Schizaphis graminum.

This study investigated the effects of a strain of the fungus Lecanicillium lecanii (Zimm.) Zare and Gams (Hypocreales: Ascomycota) on the aphid Schizaphis graminum (Rondani) (Hemiptera: Aphididae). The fungus was administered to fourth instar nymphs and to alate and apterous adult morphs as a ground rice-kernel formulation. This study showed that L. lecanii formulation affected the survival of the aphids and interacted differently with the studied morphs, the Lethal Time values being lower for alate compared to apterous morphs and nymphs. The treatment also caused a significant reduction in the fecundity of the three treated aphid groups. Histological analysis revealed that the hyphae invaded the host hemocoel of a limited number of alate and apterous morphs: the fungus only entered through the spiracles. Scanning Electron Microscope observations revealed that L. lecanii adhered to the body surface of both adults and nymphs without differentiation. In conclusion, the present study suggests that this strain might be a good candidate for a programme of biological control of S. graminum and other aphid species.

Detection of fungal metabolites of various Trichoderma species by the aphid Schizaphis graminum

The feeding preferences of alate and apterous morphs of the aphid Schizaphis graminum (Rondani) (Homoptera: Aphididae) were evaluated using leaves treated with powdered rice cultures of four fungal isolates belonging to different species of the genus Trichoderma (Deuteromycotina: Hyphomycetes). All of the fungal isolates restrained alate morphs of S. graminum from visiting treated leaves, but only Trichoderma citrinoviride Bisset also influenced the preference of apterous morphs. Trials carried out with supernatants obtained by centrifuging aqueous suspensions of the fungal cultures showed that the feeding preference of aphids was maintained in the absence of fungal spores and mycelia, supporting the hypothesis that at least part of the fungal metabolites responsible for this effect were water‐soluble compounds. Electrophysiological studies showed that the structures involved in the perception of the fungal metabolites are located on the aphid tarsomeres.