Disease caused by Neofusicoccum parvum in pruning wounds of grapevine shoots and its control by Trichoderma spp. and Xenorhabdus szentirmaii

Exploring the potential of Trichoderma secondary metabolites against Tetranychus urticae (Acari: Tetranychidae)

This study aimed to determine 1) the effects of fungal filtrates containing secondary metabolites from five different isolates of four different Trichoderma species (Trichoderma afroharzianum, T. guizhouense, T. harzianum, and T. virens) grown in different liquid media [malt extract broth (MEB), potato dextrose broth (PDB), yeast peptone glucose (YPG), minimal medium (MM), czapek-dox broth (CDB)] on Tetranychus urticae female, and 2) the effects of Trichoderma filtrates obtained from YPG liquid media on the different biological stages of T. urticae in Petri dish and pot experiments. Results showed that the Trichoderma filtrates produced in the YPG medium exhibited the highest mortality rate of 67.6-83.1 % against T. urticae females at 7 days post-application (dpa) compared to other media. In Petri dish experiments, the mortality rates of Trichoderma filtrates on egg, larva, protonymph and deutonymph stages of T. urticae at 7 dpa were 54.0-57.8 %, 71.5-76.0 %, 72.5-79.8 % and 72.8-80.8 %, respectively. Significant differences were observed between the Trichoderma species and control (P < 0.01) but not among the Trichoderma species (P > 0.05). Trichoderma afroharzianum (83 %) and T. virens (84 %) showed the highest mortality rate on T. urticae adult females at 7 dpa and statistically significant differences were observed among Trichoderma species. Pot experiments revealed that the number of viable T. urticae eggs and mobile stages was significantly lower for T. afroharzianum (110.3 eggs, 105.8 mobile stages) and T. virens (118.5 eggs, 115.3 mobile stages) compared to the control (518.9 eggs, 452.5 mobile stages) at 7 dpa. Significant differences were observed between Trichoderma species and control, but not between T. afroharzianum and T. virens. These findings suggest that Trichoderma secondary metabolites are highly effective against economically important pest such as T. urticae, demonstrating their potential as bio-acaricides. Future research should focus on identifying the specific acaricidal compound(s) within these filtrates.

The bacterial symbionts of Entomopathogenic nematodes and their role in symbiosis and pathogenesis

Entomopathogenic bacteria in the genera Xenorhabdus and Photorhabdus are mutualistically associated with entomopathogenic nematodes (EPN) Steinernema and Heterorhabditis, respectively. Together they form an insecticidal partnership which has been shown to kill a wide range of insect species. The spectrum of dependence in this symbiotic partnership is diverse, ranging from a tight, obligate relationship to a facultative one. A body of evidence suggests that the reproductive fitness of the nematode-bacterium partnership is tightly associated and interdependent. Furthermore, maintenance of their virulence is also critical to the conversion of the insect host as a suitable environment where this partnership can be perpetuated. Disruption of the symbiotic partnership can have detrimental effects on the fitness of both partners. The nematode-bacterial symbiont-insect partnership represents a model system in ecology and evolutionary biology and amenable to investigate beneficial and antagonistic interactions between invertebrates and microbes. Furthermore, the EPN's bacterial symbionts are also viewed as a model system to study the biosynthesis, structure and function of various natural products. Their ability to produce up to 25 different natural product classes is outstanding among the Morganellaceae. These natural products show biological activity, most likely originating from important functions during the life cycle of both the nematodes and their symbionts. Tools and high throughput technologies have been developed to identify ubiquitous and rare molecules and study their function and assess their potential as novel biological activities. We herein summarize the symbiotic relationship between EPN and their bacterial symbionts, focusing on their fitness and their ability to successfully access and utilize an insect host. We also recapitulate the history of natural products research highlighting recent findings and the synthetic biology approaches that are currently implemented to identify non-natural derivatives from Xenorhabdus and Photorhabdus with improved biological activity.

Fatty acid methyl ester (FAME) profiling for species-specific characterization and detection of fungal pathogens that cause tree and grapevine trunk diseases

Fungal trunk diseases are of major concern for tree fruit, nut, and grape growers throughout the world. These diseases include Eutypa dieback of grape, caused by Eutypa lata, band canker of almond, caused by Neofusicoccum mediterraneum and Neofusicoccum parvum, and twig and branch dieback of walnut, caused by N. mediterraneum, Botryosphaeria dieback of grape, caused by Diplodia mutila, Diplodia seriata, N. mediterraneum, and N. parvum, and esca of grape, caused by Phaeomoniella chlamydospora and Phaeoacremonium minimum. Given the common occurrence of mixed infections, and the similar wood symptoms at the macroscopic level, species-specific detection tools are needed. Fatty acid methyl ester (FAME) profiling can be an effective and inexpensive diagnostic tool. FAME analyses were conducted on pure cultures of multiple isolates per species to characterize profiles and assess whether this technique could result in consistent identification. FAME profiles were dominated by oleic acid (18:1 ω9c) and palmitic acid (16:0), with less abundant FAMEs in different ratios for each species and isolates within species. Canonical discriminant analyses revealed which minor FAMEs were most variable, with a total of 20 different FAMEs that can explain 69.01% of profile variance in the first two canonicals. Using these analyses, samples were self-tested and correctly sorted 97.18% of the time. Within species, canonical discriminant analyses were able to separate isolates further, often by original geographic location or by host plant species. These results further suggest that potential novel species, subspecies, or races may be present among the isolates analyzed, demonstrating the capacity of FAME profiling to have a role in discovering cryptic species and accurately identifying fungal pathogens in conjunction with other molecular techniques and genomic analyses.

Natural products from Xenorhabdus and Photorhabdus show promise as biolarvicides against Aedes albopictus

BACKGROUND

In the perpetual struggle to manage mosquito populations, there has been increasing demand for the development of biopesticides to supplant/complement current products. The insecticidal potential of Xenorhabdus and Photorhabdus has long been recognized and is of interest for the control of important mosquitoes like Aedes albopictus which vectors over 20 different arboviruses of global public health concern.

RESULTS

The larvicidal effects of cell-free supernatants, cell growth cultures and cell mass of an extensive list of Xenorhabdus and Photorhabdus spp. was investigated. They were quite effective against Ae. albopictus causing larval mortality ranging between 52-100%. Three Photorhabdus spp. and 13 Xenorhabdus spp. release larvicidal compounds in cell-free supernatants. Cell growth culture of all tested species exhibited larvicidal activity, except for Xenorhabdus sp. TS4. Twenty-one Xenorhabdus and Photorhabdus bacterial cells (pellet) exhibited oral toxicity (59-91%) against exposed larvae. The effect of bacterial supernatants on the mosquito eggs were also assessed. Bacterial supernatants inhibited the hatching of mosquito eggs; when unhatched eggs were transferred to clean water, they all hatched. Using the easyPACId approach, the larvicidal compounds in bacterial supernatant were identified as fabclavine from X. szentirmaii and xencoumacin from X. nematophila (causing 98 and 70% mortality, respectively, after 48 h). Xenorhabdus cabanillasii and X. hominickii fabclavines were as effective as commercial Bacillus thuringiensis subsp. israelensis and spinosad products within 5 days post-application (dpa).

CONCLUSION

Fabclavine and xenocoumacin can be developed into novel biolarvicides, can be used as a model to synthesize other compounds or/and can be combined with other commercial biolarvicides. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Biocontrol of almond canker diseases caused by Botryosphaeriaceae fungi

BACKGROUND

Botryosphaeria dieback is a canker disease caused by fungal species of the Botryosphaeriaceae family that threatens almond productivity. The most common control measure to prevent canker development is the application of fungicides which are being phased out by European Union regulations. In the present study, two sets of bacterial strains were evaluated for their antifungal activity against pathogenic Botryosphaeriaceae species through in vitro and in vivo antagonism assays.

RESULTS

The rhizospheric bacteria Pseudomonas aeruginosa AC17 and Bacillus velezensis ACH16, as well as the endophytic bacteria Bacillus mobilis Sol 1-2, respectively inhibited 87, 95, and 63% of the mycelial growth of Neofusicoccum parvum, Botryosphaeria dothidea, Diplodia seriata, and Macrophomina phaseolina. Additionally, they significantly reduced the length of lesions caused by N. parvum and B. dothidea in artificially inoculated detached almond twigs. All these bacterial strains produce hydrolytic enzymes that are able to degrade the fungal cell wall. P. aeruginosa AC17 also produces toxic volatile compounds, such as hydrogen cyanide. This strain was the most effective in controlling Botryosphaeria dieback in planta under controlled conditions at a level similar to the biocontrol agent Trichoderma atroviride and standard chemical fungicide treatments.

CONCLUSION

Pseudomonas aeruginosa AC17 is the best candidate to be considered as a potential biocontrol agent against Botryosphaeriaceae fungi affecting almond. © 2023 Society of Chemical Industry.

Biosynthesis of selenium nanoparticles using cell-free extract of Xenorhabdus cabanillasii GU480990 and their potential mosquito larvicidal properties against yellow fever mosquito Aedes aegypti

Nanomaterials are successful due to their numerous applications in various domains such as cancer treatment, environmental applications, drug and gene delivery. Selenium is a metalloid element with broad biological activities and low toxicity especially at the nanoscale. Several studies have shown that nanoparticles synthesized from microbial and plant extracts are effective against important pests and pathogens. This study describes the bio fabrication of selenium nanoparticles using cell free extract of Xenorhabdus cabanillasii (XC-SeNPs) and assessed their mosquito larvicidal properties. Crystallographic structure and size of XC-SeNPs were determined with UV-a spectrophotometer, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), Energy-dispersive X-ray spectroscopy (EDAX), Zeta potential and Transmission electron microscopy (TEM). The significant surface plasmon resonance at 275 nm indicated the synthesis of XC-SeNPs from the pure cell-free extract of X. cabanillasii. The XRD result exhibits the crystalline nature of XC-SeNPs. The Zeta potential analysis confirmed that the surface charge of XC-SeNPs was -24.17 mV. TEM analysis revealed that synthesized XC-SeNPs were monodispersed, spherically shaped, and sized about 80-200 nm range. In addition, the larvicidal potentials of the bio-fabricated XC-SeNPs were assessed against the 4th-instar Ae. aegypti. XC-SeNPs displayed a dose-dependent larvicidal effect; the larval mortality was 13.3 % at the minimum evaluated concentration and increased to 72 % at higher dose treatments. The LC50 and LC90 concentration of XC-SeNPs against mosquito larvae were 79.4 and 722.4 ppm, respectively.

Antifungal Effect of Metabolites from Bacterial Symbionts of Entomopathogenic Nematodes on Fusarium Head Blight of Wheat

Fungal diseases such as Fusarium head blight (FHB) are significant biotic stressors, negatively affecting wheat production and quality. This study explored the antifungal activity of the metabolites produced by the bacterial symbionts of entomopathogenic nematodes (EPNs) against FHB-causing Fusarium sp. Fusarium graminearum. To achieve this, the symbiotic bacteria of nine EPN isolates from the EPN collection at the Agricultural Research Council-Small Grains (ARC-SG) were isolated from the cadavers of Galleria mellonella (Lepidoptera: Pyralidae) larvae after infection with EPNs. Broth cultures (crude) and their supernatants (filtered and autoclaved) of each bacterial isolate were used as bacterial metabolite treatments to test their inhibitory effect on the mycelial growth and spore germination of F. graminearum. Mycelial growth inhibition rates varied among both bacterial isolates and treatments. Crude metabolite treatments proved to be more effective than filtered and autoclaved metabolite treatments, with an overall inhibition rate of 75.25% compared to 23.93% and 13.32%, respectively. From the crude metabolite treatments, the Xenorhabdus khoisanae SGI 197 bacterial isolate from Steinernema beitlechemi SGI 197 had the highest mean inhibition rate of 96.25%, followed by Photorhabdus luminescens SGI 170 bacteria isolated from Heterorhabditis bacteriophora SGI 170 with a 95.79% mean inhibition rate. The filtered metabolite treatments of all bacterial isolates were tested for their inhibitory activity against Fusarium graminearum spore germination. Mean spore germination inhibition rates from Xenorhabdus spp. bacterial isolates were higher (83.91 to 96.29%) than those from Photorhabdus spp. (6.05 to 14.74%). The results obtained from this study suggest that EPN symbiotic bacterial metabolites have potential use as biological control agents of FHB. Although field efficacy against FHB was not studied, the significant inhibition of mycelial growth and spore germination suggest that the application of these metabolites at the flowering stage may provide protection to plants against infection with or spread of F. graminearum. These metabolites have the potential to be employed as part of integrated pest management (IPM) to inhibit/delay conidia germination until the anthesis (flowering stage) of wheat seedlings has passed.