Soil Bioassays Research Articles

Distribution of plant pathogenic Aphanomyces species in Sweden, Denmark and Lithuania and its relationship with soil factors

Abstract Three species of Aphanomyces, A. euteiches, A. cochlioides and A. cladogamus are devastating agricultural plant pathogens causing root rot disease on pea, sugar beet and spinach, respectively. Here we investigated the distribution of the three Aphanomyces species in parts of Sweden, Denmark and Lithuania using climate chamber soil bioassays, and tested for correlations with macronutrient levels in the soil. Aphanomyces euteiches was the most widespread pathogen found in 28% of the tested samples in Sweden and Denmark, while A. cochlioides had a more limited distribution, and was found in 22% of the samples. Aphanomyces cladogamus was only detected in Sweden and was present in 16% of the tested samples. Aphanomyces euteiches only infected pea plants, while A. cochlioides and A. cladogamus infected both sugar beet and spinach. There were significant (P < 0.001) differences in disease severity index (DSI) between regions, pathogens and for the interaction between regions and crops. Soil calcium (Ca) content ranged from 82 to 3600 mg/100 g dry soil. No species of Aphanomyces was detected in soils with Ca levels above 250 mg/100 g soil, indicating a pathogen-suppressive effect from calcium. Similar results were observed from 1350 soil samples collected from commercial pea growing fields, as DSI values on pea remained very low (below 20 on a 0–100 scale) in soil samples with confirmed presence of A. euteiches and a Ca value above 210 mg/100 g soil. High Ca levels was a significant (P < 0.05) predictor for low probability of presence of Aphanomyces spp. in soil.

Assessing performance of simplified bioassays for soil-borne pathogens in smallholder systems of western Kenya.

Soil-borne pathogens cause considerable crop losses and food insecurity in smallholder systems of sub-Saharan Africa. Soil and crop testing is critical for estimating pathogen inoculum levels and potential for disease development, understanding pathogen interactions with soil nutrient and water limitations, as well as for developing informed soil health and disease management decisions. However, formal laboratory analyses and diagnostic services for pathogens are often out of reach for smallholder farmers due to the high cost of testing and a lack of local laboratories. To address this challenge, we assessed the performance of a suite of simplified soil bioassays to screen for plant parasitic nematodes (e.g., Meloidogyne, Pratylenchus) and other key soil-borne pathogens (Pythium and Fusarium). We sampled soils from on-farm trials in western Kenya examining the impact of distinct nutrient inputs (organic vs. synthetic) on bean production. Key soil health parameters and common soil-borne pathogens were evaluated using both simple bioassays and formal laboratory methods across eleven farms, each with three nutrient input treatments (66 samples in total). The soil bioassays, which involved counting galls on lettuce roots and lesions on soybean were well correlated with the abundance of gall forming (Meloidogyne) and root lesion nematodes (e.g., Pratylenchus) recovered in standard laboratory-based extractions. Effectiveness of a Fusarium bioassay, involving the counting of lesions on buried bean stems, was verified via sequencing and a pathogenicity test of cultured Fusarium strains. Finally, a Pythium soil bioassay using selective media clearly distinguished pathogen infestation of soils and infected seeds. When examining management impact on nematode communities, soils amended with manure had fewer plant parasites and considerably more bacterivore and fungivore nematodes compared to soils amended with synthetic N and P. Similarly, Pythium presence was 35% lower in soils amended with manure, while the Fusarium assays indicated 23% higher Fusarium infection in plots with amended manure. Our findings suggest that relatively simple bioassays can be used to help farmers assess soil-borne pathogens in a timely manner, with minimal costs, thus enabling them to make informed decisions on soil health and pathogen management.

Screening five local entomopathogenic nematode species for their virulence against pupae of the Eucalyptus snout beetle, Gonipterus sp. n. 2, under laboratory conditions

The Eucalyptus snout beetle, Gonipterus sp. n. 2, is an important pest of eucalypt trees in South Africa and other countries. The pest is partially controlled with a mymarid egg parasitoid, Anaphes nitens. Identifying additional biological control agents that target other developmental stages of Gonipterus sp. n. 2 is necessary to improve control. Entomopathogenic nematodes (EPNs) are used as biological control agents for numerous soil pests and thus have the potential to be used against the pupal stage of Gonipterus sp. n. 2, which occurs in the soil. In this study, five South African EPN species were screened for their virulence on uncased pupae of Gonipterus sp. n. 2. At a concentration of 200 IJs/pupa in 50 μl of sterile water, Steinernema jeffreyense, S. fabii, Heterorhabditis noenieputensis and H. safricana caused pupal mortality of less than 40 %, while S. yirgalemense caused the highest pupal mortality of 100 % two days post-inoculation. Steinernema yirgalemense was selected and applied at different concentrations, namely 0, 12, 25, 50, 100, 200, and 400 IJs/pupa to determine its lethal concentration. Probit analysis indicated that the LC50 and LC90 of S. yirgalemense were 48.29 and 260.63 IJs/pupa, respectively. The study also showed that the pupal cases do not affect the efficacy of S. yirgalemense as it caused more than 90 % mortality of cased pupae of Gonipterus sp. n. 2 in soil bioassays. In addition, pupal age did not affect the efficacy of S. yirgalemense. Based on our findings, S. yirgalemense is a potential candidate for the biological control of pupae of Gonipterus sp. n. 2.

A bioassay of mine-affected soils and ameliorants in semi-arid rehabilitation

Global restoration is a priority given the onset of the United Nations’ decade for restoration. It is challenging to restore arid lands from mining impacts, especially where topsoil deficits exist. Successful rehabilitation is essential for alternative land use after mining. This bioassay aimed to establish whether and why some combinations of iron ore mine-affected soils and ameliorants are more effective in promoting plant growth than others. We examined the responses of one thousand radishes (Raphanus sativus) to five mine-affected soils and four soil ameliorations (soil improvement using fertiliser and organic matter) over two different seasons (early and late rainy seasons). We found that soil characteristics affected plant production regardless of amelioration and that some soil/amelioration combinations were significantly better than others. Topsoil ameliorated by moderate fertiliser application produced the best results. Waste rock could support germination and plant production when ameliorated with certain organic compost and fertiliser combinations. Untreated mine-affected soils had low to no plant production. Plant responses to the different soil/amelioration combinations were affected by season. Our findings suggest that area-specific bioassay investigations and their results could improve management decisions to achieve rehabilitation targets for mine-affected soils.

Phytotoxic Effects of Al on Root Growth Are Confounded in the Presence of Fulvic and Humic Acids

Background and Aims: In acidic soils, aluminum (Al) toxicity remains a critical crop limitation that can be ameliorated by organic amendments through Al complexation with high-molecular-weight carbon compounds, particularly fulvic and humic acids (FA and HA) However, no research discriminates between the direct effects of FA and HA on plant growth and the indirect effect that occurs through ameliorating Al toxicity. This study delineates the direct and indirect effects of FA and HA on plant growth. Methods: Eucalyptus and Hay FA and HA, and Al effects on maize (Zea mays) root growth were investigated using dilute nutrient solution. Five Al concentrations (0–270 µM) were combined with four organic acid (OA) treatments, including Nil-OA, FA40, and HA40 (each at 40 mg C L−1) and a combined treatment FA40HA40 (80 mg C L−1). Results: Eucalyptus FA and HA stimulated root growth by ~20% compared with root growth in the Nil-OA (17.4 cm). In the absence of Al, Hay FA and HA inhibited root growth (by ~20%) compared with the Nil-OA but the addition of Al resulted in stimulation of root growth. In the presence of FA and HA, root growth was not inhibited by nominally toxic monomeric Al (Al3+) concentrations (~20 µM Al). However, when expressed on a relative basis to remove the direct effect of the ligand, the response was consistent with Al toxicity. Conclusions: The effects of FA and HA were either inhibitory or stimulatory depending on the source while both sources of FA and HA mitigated Al toxicity through complexation. The study provides mechanistic data that highlights limitations of soil bioassays where the direct effects of organic ligands on root growth are confounded with the indirect effect of their reduction of Al toxicity. These two independent processes must be considered in evaluating the amelioration of Al by organic amendments.

Woodland wildfire enables fungal colonization of encroaching Douglas‐fir

Abstract Self‐reinforcing differences in fire frequency help closed‐canopy forests, which resist fire, and open woodlands, which naturally burn often, to co‐occur stably at landscape scales. Forest tree seedlings, which could otherwise encroach and overgrow woodlands, are killed by regular fire, yet fire has other effects that may also influence these feedbacks. In particular, many forest trees require symbiotic ectomycorrhizal fungi in order to establish. By restructuring soil fungal communities, fire might affect the availability of symbionts or the potential for symbiont sharing between encroaching trees and woodland vegetation. To investigate this possibility, we performed a soil bioassay experiment using inoculum from burned and unburned oak woodlands and Douglas‐fir forests. We examined how fire, ecosystem type, and neighboring heterospecific seedlings affect fungal root community assembly of Douglas‐firs and oaks. We asked whether heterospecific seedlings facilitated fungal colonization of seedling roots in non‐native soil, and if so, whether fire influenced this interaction. External fungal colonization of oak roots was more influenced by fire and ecosystem type than by the presence of a Douglas‐fir, and oaks increased the likelihood that Douglas‐fir roots would be colonized by fungi in oak woodland soil. Yet, fire increased colonization of Douglas‐fir in oak soil, diminishing the otherwise crucial role played by oak facilitation. Fire also strengthened the positive effect of Douglas‐firs on oak root‐associated fungal diversity in Douglas‐fir forest soil. Prior work shows that fire supports woodland ecosystems by stemming recruitment of encroaching seedlings. Here, we find evidence that it may contrastingly reduce fungal limitation of invasive seedling growth and establishment, otherwise relieved only by facilitation. Future work can investigate how these opposing effects might contribute to the net impact of changes in fire regime on landcover stability. Read the free Plain Language Summary for this article on the Journal blog.

Islands in the shade: scattered ectomycorrhizal trees influence soil inoculum and heterospecific seedling response in a northeastern secondary forest.

The eastern deciduous forest is a mix of arbuscular (AM) and ectomycorrhizal (EM) trees, but land use legacies have increased the abundance of AM trees like Acer spp. (maple). Although these legacies have not changed the abundance of some EM trees like Betula spp. (birch), EM conifers like Tsuga canadensis (hemlock), and Pinus strobus (pine) have declined. We used a soil bioassay to investigate if the microbial community near EM birch (birch soil) contains a greater abundance and diversity of EM fungal propagules compatible with T. canadensis and P. strobus compared to the community associated with the surrounding AM-dominated secondary forest matrix (maple soil). We also tested the effectiveness of inoculation with soil from a nearby EM-dominated old-growth forest as a restoration tool to reintroduce EM fungi into secondary forest soils. Finally, we examined how seedling growth responded to EM fungi associated with each treatment. Seedlings grown with birch soil were colonized by EM fungi mostly absent from the surrounding maple forest. Hemlock seedlings grown with birch soil grew larger than hemlock seedlings grown with maple soil, but pine seedling growth did not differ with soil treatment. The addition of old-growth soil inoculum increased hemlock and pine growth in both soils. Our results found that EM trees are associated with beneficial EM fungi that are mostly absent from the surrounding AM-dominated secondary forest, but inoculation with old-growth soil is effective in promoting the growth of seedlings by reintroducing native EM fungi to the AM-dominated forests.

EFFECT OF DIFFERENT CONCENTRATIONS OF EPNS AND MALATHION ON SUGARCANE WHITE GRUBS, Heteronychus licas (Coleoptera: Scarabaeidae) MORTALITY

Sugarcane, Saccharum officinarum L. is one of the major commercial crops grown in Eswatini. However, pest and diseases are pivotal to the decrease in yield output in the country. Sugarcane white grubs (SWG), Heteronychus licas Klug (Coleoptera: Scarabaeidae) are amongst the important pests of sugarcane in the country. Entomopathogenic nematodes (EPNs) were found to be effective in controlling both soil borne as well as above ground insect pests. The current study determined the effect of different concentrations of Malathion and EPNs on the mortality of SWG. SWG larvae were dipped in different concentrations (0, 0.625, 1.25, and 2.5 g/L) of malathion and (0, 200, 400, and 600 IJ/insect) EPNs. SWGs were also exposed to the same concentrations in soil bioassays. Results obtained in the bioassays showed that Steinernema yirgalemense at 600 IJs/insect (85.4%) gave the highest mortality which was not significantly different from S. yirgalemense at 400 IJs/insect (72.9%). The Malathion concentrations gave 0, 79.2, 83.3 and 89.6% mortality for 0, 0.625, 1.25 and 2.5g/L of Malathion, respectively. Results obtained in the soil bioassays showed that Steinernema yirgalemense, at 600 IJs/insect (80.5%), provided higher (p < 0.05) mortality of the H. licas larvae which was not significantly different from S. yirgalemense at 400 IJs/insect (70.2%). The Malathion concentrations gave 0, 83.3, 85.4 and 87.5% mortality for 0, 0.625, 1.25 and 2.5g/L of Malathion, respectively. The results indicated that both EPN and Malathion were tested were effective against H. licas larvae. Steinernema yirgalemense at 400 IJs/insect and Malathion at 0.625g/L can effectively H. licas larvae but large-scale field trials are recommended to demonstrate the potential use of the biocontrol agent within integrated pest management programs.

Dry Matter Yield Stability Analysis of Maize Genotypes Grown in Al Toxic and Optimum Controlled Environments

Breeding for Al tolerance is the most sustainable strategy to reduce yield losses caused by Al toxicity in plants. The use of rapid, cheap and reliable testing methods and environments enables breeders to make quick selection decisions. The objectives of this study were to (i) identify high dry matter yielding and stable quality protein maize (QPM) lines grown under Al toxic and optimum conditions and (ii) compare the discriminating power of laboratory- and greenhouse-based testing environments. A total of 75 tropical QPM inbred lines were tested at seedling stage for dry matter yield and stability under optimum and Al toxic growing conditions across six laboratory- and greenhouse-based environments. The nutrient solution method was used for the laboratory trials, while the soil bioassay method was used for the greenhouse trials. A yield loss of 55% due to Al toxicity was observed, confirming the adverse effects of Al toxicity on maize productivity. The ANOVA revealed the presence of genetic variation among the set of genotypes used in this study, which can be exploited through plant breeding. Seventeen stable and high-yielding lines were identified and recommended. Greenhouse-based environments were more discriminating than laboratory environments. Therefore, we concluded that greenhouse environments are more informative than laboratory environments when testing genotypes for Al tolerance.

Soil chemistry and fungal communities are associated with dieback in an Endangered Australian shrub

Background and aimsField surveys across known populations of the Endangered Persoonia hirsuta (Proteaceae) in 2019 suggested the soil environment may be associated with dieback in this species. To explore how characteristics of the soil environment (e.g., pathogens, nutrients, soil microbes) relate to dieback, a soil bioassay (Experiment 1) was conducted using field soils from two dieback effected P. hirsuta populations. Additionally, a nitrogen addition experiment (Experiment 2) was conducted to explore how the addition of soil nitrogen impacts dieback.MethodsThe field soils were baited for pathogens, and soil physiochemical and microbial community characteristics were assessed and related to dieback among plants in the field and nursery-grown plants inoculated with the same field soils. Roots from inoculated plants were harvested to confirm the presence of soil pathogens and root-associated endophytes. Using these isolates, a dual culture antagonism assay was performed to examine competition among these microbes and identify candidate pathogens or pathogen antagonists.ResultsDieback among plants in the field and Experiment 1 was associated with soil physiochemical properties (nitrogen and potassium), and soil microbes were identified as significant indicators of healthy and dieback-affected plants. Plants in Experiment 2 exhibited greater dieback when treated with elevated nitrogen. Additionally, post-harvest culturing identified fungi and other soil pathogens, some of which exhibited antagonistic behavior.ConclusionThis study identified candidate fungi and soil physiochemical properties associated with observed dieback and dieback resistance in an Endangered shrub and provides groundwork for further exploring what drives dieback and how it can be managed to promote the conservation of wild populations.

Evaluation of Allelopathic Potential of Pluchea indica on Brassica chinensis and Zea mays

Weeds have become one of the leading causes of crop production losses, including the cost of weed management, weed crop competition, and weed intervention with crop management practices. Moreover, herbicide use in agricultural systems is currently being debated due to the potential adverse effects on the environment and human health. The utilization of the allelopathic phenomenon is widely considered an alternative weed management strategy. However, allelopathic study of Pluchea indica plant was limited. P. indica is a flowering plant under family Asteraceae, locally known as Beluntas. Thus, this study was conducted to determine the allelopathic effects of the aqueous leaf extract of P. indica on the seedling growth of tested plant species and to investigate the effect of P. indica leaf debris on the growth of tested plant species. In this study, the allelopathic potential of P. indica leaves were investigated through petri dish bioassay and soil bioassay method. The two tested plant species were chosen for the bioassay: monocotyledonous; maize (Zea mays) and dicotyledonous; mustard (Brassica chinensis). These tested plant species were chosen for the experiment due to their known quick seedling growth behaviours and commonly used in allelopathic studies. The result showed that as the concentration of aqueous leaf extract increased, the inhibition percentage of the radicle length of B. chinensis and Z. mays were increased. However, stimulatory effect was observed on the radicle length of B. chinensis and Z. mays when 1.3%and 2.5% w/w P. indica leaf debris were applied. Thus, aqueous leaf extract and leaf debris of P. indica have potential allelopathic effects on the growth of B. chinensis and Z. mays. Further studies need to be conducted on other bioassay species especially weed species, to elucidate the allelopathic potential of P. indica in weed control.

Are Ni-Cd toxicity models derived from simple bioassay applicable to natural soils? A bioassay-MSMs coupling approach

The application of terrestrial toxicity models to real soil environments is a challenge due to the heterogeneity of soil. We presented here a novel bioassay-geochem-model coupling approach to predict the bioaccumulation and toxicity of a metal mixture in different soils. Equations describing dose-accumulation and dose-toxicity relationships of Cd-Ni mixtures to wheat root elongation were generated using a standard sand culture bioassay. A geochemical multi-surface speciation model (MSM) with default parameters was employed to predicted the dissolved metal contents Cdis of three soils collected in China with different properties. Several toxicity models (TU/CAM + FIAM/BLM/GCSM) were screened to identify the one yielding the most accurate predictions. The results showed that metal bioaccumulation was better predicted by the MSM-Cdis integrated with the dose-accumulation equation (RMSE < 0.58) than by the detection of metal contents in soil porewater (RMSE < 0.74). The MSM predicted metal ion activities combined with the BLM+CAM model successfully predicted Cd-Ni toxicity in all three soils (RMSE = 20.7%), but the deviation was slightly larger than that obtained in measurements of metals in soil porewater (RMSE =17.2%). Our study demonstrates that the MSM coupled with the dose-accumulation and dose-toxicity equations derived from sand bioassays offers a promising approach in risk assessments of mixed metals in actual soils while avoiding tedious soil bioassays, which will greatly improve the practical application value of terrestrial toxic models.

Sugarcane root exudate impact on the potential nitrification rate and N dynamics in the rhizosphere

Soil nitrification is a key process in regulating nitrogen transformation of various inorganic N forms for plant uptake and reducing N loss (N2O and NO3−). Nitrification inhibition by plant-released root exudates is referred to as biological nitrification inhibition (BNI). This study aimed to detect the BNI capacity of sugarcane (Saccharum spp.) root exudates and determine the influence of BNI compounds (BNIs) on the net nitrification rate in rhizosphere soil during sugarcane growth. Three sugarcane varieties were considered in this study, i.e., wild cane, UT13 and KK3. This study included 3 experiments, namely, Experiment I, determination of the potential nitrification rate (PNR) via a soil bioassay and assessment of the effect of dicyandiamide (DCD) on the PNR via a microcosm incubation study; Experiment II, assessment of the effect of sugarcane cultivation on soil N dynamics and the net nitrification rate in rhizosphere soil (in situ); and Experiment III, assessment of the PNR and immobilization in soil via microcosm incubation (ex situ). Sugarcane root exudates were collected under hydroponics, and the BNI capacity was obtained through a soil bioassay. Pot experiments were conducted to determine the influence of sugarcane growth on the net nitrification rate relative to unplanted soil. The results of Experiment I indicated that sugarcane root exudates significantly inhibited the nitrification rate via a soil bioassay (p < 0.05), with a high variation in the BNI capacity according to the sequence of Spone (84.29% inhibition) > UT13 (80.94% inhibition) > KK3 (45.4% inhibition). In addition, Experiment II confirmed a reduced net nitrification rate (p < 0.05) among all three sugarcane varieties. However, net nitrification rate reduction was more notably the result of heterotrophic N immobilization and plant N uptake than the result of BNIs. Evidence of BNI-induced reduction in gross nitrification among all sugarcane varieties was confirmed via soil incubation (Experiment III), and a notable difference in the BNI capacity was found among all sugarcane varieties. Based on the difference in plant mechanisms controlling the net nitrification rate, our study demonstrates that the dominant mechanisms are determined by the soil N availability.

Combined application of entomopathogenic nematodes and fungi against fruit flies, Bactrocera zonata and B. dorsalis (Diptera: Tephritidae): laboratory cups to field study.

The peach fruit fly, Bactrocera zonata, and the Oriental fruit fly, B.dorsalis (Diptera: Tephritidae), are economically important fruit fly species in various regions of the world. We evaluated the effects of separate and combined applications of the entomopathogenic fungi (EPF) Beauveria bassiana (WG-18) and Metarhizium anisopliae (WG-02), and the entomopathogenic nematodes (EPNs) Heterorhabditis bacteriophora (VS strain) and Steinernema carpocapsae (ALL strain) against larvae, pupae and pharate adults, of B.zonata and B.dorsalis under laboratory, glasshouse and field cage conditions. Combined applications of EPF and EPNs produced greater mortality than individual treatments under all conditions. Against both species, the combination of B. bassiana and H.bacteriophora consistently exerted strong effects that were similar to the combined application of B.bassiana and S.carpocapsae whereas M.anisopliae applied with S.carpocapsae was least effective in all combinations. In a laboratory bioassay, synergistic interactions were detected between B.bassiana and H.bacteriophora applied against larvae and pharate adults of both fly species, between B.bassiana and S.carpocapsae against larvae of both species and pharate adults of B.zonata, and between M.anisopliae and H.bacteriophora against B.zonata larvae. Other combined treatments resulted in additive effects, especially against fly pupae. In a potted soil bioassay, there were only additive interactions in all combinations against different stages of both flies. The 3rd instar of both flies was more susceptible than pharate adult and pupal stages. Additive interactions between EPNs and EPF were detected in the glasshouse against 3rd instars and pupae, and under field conditions against 3rd instars of both fly species. These results indicate how particular combinations of entomopathogenic fungi and nematodes could be deployed in integrated pest management of tephritid fruit flies in orchard agro-ecosystems. © 2022 Society of Chemical Industry.

Enantiomer-Selective Characterization of the Adsorption, Dissipation, and Phytotoxicity of the Plant Monoterpene Pulegone in Soils.

Plant monoterpenes have received attention for their ecological functions and as potential surrogates for synthetic herbicides, but very little is known about the processes that govern their behavior in the soil environment, and even less about the possible enantioselectivity in the functions and environmental behavior of chiral monoterpenes. We characterized the adsorption and dissipation of the two enantiomers of the chiral monoterpene pulegone in different soils, and their phytotoxicity to different plant species through Petri dish and soil bioassays. R- and S-pulegone displayed a low-to-moderate non-enantioselective adsorption on the soils that involved weak interaction mechanisms. Soil incubation experiments indicated that, once in the soil, R- and S-pulegone are expected to suffer rapid volatilization and scarcely enantioselective, biodegradation losses. In Petri dishes, the phytotoxicity of pulegone and its enantioselectivity to Lactuca sativa, Hordeum vulgare, and Eruca sativa was species-dependent. Lactuca sativa was the most sensitive species and showed higher susceptibility to S- than to R-pulegone. Biodegradation and volatilization losses greatly reduced the phytotoxic activity of S-pulegone applied to soil, but the addition of a highly-adsorptive organoclay stabilized the monoterpene and increased its phytotoxic effect. Stabilization by adsorption may represent an important mechanism by which the bioactivity of plant monoterpenes in soils can be increased.

Sensitivity of Fusarium oxysporum f. sp. melongenae, the causal agent of Fusarium wilt of eggplant to some ammonium, potassium, and sodium compounds in vitro and in vivo bioassays

This study evaluated the efficacy of various salts against Fusarium oxysporum f. sp. melongenae (Fomg) in vitro and in vivo bioassays. Ammonium bicarbonate/carbonate, potassium benzoate/sorbate, sodium benzoate/carbonate/metabisulphite at 2.0% entirely halted mycelial growth and spore germination of the fungus. Considering the EC50, minimum inhibitory concentration, and minimum fungicidal concentration values of the salts, sodium metabisulphite was more toxic to the fungus. In soil bioassays, at 0.4% and 0.6%, sodium metabisulphite and potassium sorbate reduced the mycelial growth of the fungus by 98.89%. However, in the soil/seed germination bioassays, potassium sorbate and sodium carbonate at 0.05–0.1% concentrations had a much lower inhibitory effect on root elongation compared to the others. The two salts may be possible to reduce the number of traditional fungicides required by integrating them with disease management strategies. However, the possibility of including inorganic salts in programs to treat fungal diseases requires further research in the greenhouse conditions.

Plant growth-promoting bacteria belonging to the genera &lt;em&gt;Pseudomonas&lt;/em&gt; and &lt;em&gt;Bacillus&lt;/em&gt; improve the growth of sorghum seedings in a low-nutrient soil

Background: Deficiency in sorghum growth in ecosystems of low-nutrient soils has been scarcely studied. This soil deficiency can be overcome by the addition of plant growth-promoting bacteria which increase sorghum growth. Questions and/or Hypotheses: indole acetic acid (IAA) producing and phosphate solubilizing bacteria can promote sorghum growth under nutritional stress. Studied species: Sorghum bicolor (L.) Moench. Study site and dates: Mexico City, 2018. Methods: Of the twelve bacterial strains utilized, three produce IAA (group BI), two strains produce IAA and siderophores (BIS group), four strains produce IAA and solubilize phosphate (BIP group), and three strains produce IAA, solubilize phosphate, and produce siderophores (BIPS group). Hydroponic bioassays and low-nutrient soil bioassay were used. Results: In hydroponic bioassays, for BI and BIS groups, five strains significantly increased the growth parameters with respect to the control, and for the BIP and BIPS groups, two strains promoted stem development and shoot dry weight. In a low-nutrient soil bioassay, Pseudomonas sp. BI-1 (from BI group) was the one that presented the highest percentages 32, 48, 140 and 79 % in stem diameter, height and dry weight of the shoot and dry weight of the root, respectively, followed by the P. mohnii BIPS-10 strain (from BIPS group) that exhibited similar results. Conclusions: IAA producing Pseudomonas strains improve the sorghum growth in a low-nutrient soil and suggest thatPseudomonas sp. BI-1 and P. mohnii BIPS-10 could be used as potential bioinoculants for sorghum.

Burial Environment Drives Seed Mortality of Kochia (Bassia scoparia), Wild Oat (Avena fatua), and Volunteer Canola (Brassica napus) Irrespective of Crop Species.

Models of weed population demography are critical to understanding the long-term viability of management strategies. The driving factors of weed seedbank persistence are often underrepresented in demographic models due to the cumbersome nature of seedbank research. Simplification of weed seedbank dynamics may induce substantial error in model simulations. A soil bioassay was conducted to determine whether growth of different crop species, including wheat (Triticum aestivum L.), canola (Brassica napus L.), and field pea (Pisum sativum L.), differentially impact seed mortality of kochia [Bassia scoparia (L.) A.J. Scott], wild oat (Avena fatua L.), and volunteer canola in seven burial environments in western Canada. Weed seed survival after the 7 week burial period varied widely among burial environments (from 8% to 88% when averaged among weed and crop species), whereas growth of the different crop species had negligible impact on seedbank persistence. Among environments, wild oat seed survived the greatest (79%), followed by kochia (20%), and volunteer canola (6%). Weed seed survival was associated with soil physical properties (texture) and seed microsite characteristics (temperature), but not crop species or soil chemical properties. Overall, these data support the need for greater integration of soil and environmental parameters into models of weed population demography.

Virulence of Entomopathogenic Nematodes to Pupae of Frankliniella fusca (Thysanoptera: Thripidae).

Tobacco thrips, Frankliniella fusca (Hinds) is an economically significant pest. Entomopathogenic nematodes (EPNs) have shown promise as biocontrol agents against certain thrips species, but they have not been explored for suppression of F. fusca. We investigated the potential of EPNs to manage F. fusca by conducting three different bioassays: 1) a small cup dose-response bioassay (25, 50, and 100 IJs cm-2) with four EPN species, 2) a broad virulence bioassay with eight EPN species at 100 IJs cm-2, and 3) a potted soil bioassay testing with four EPN species (100 IJs cm-2). In the dose-response bioassay, all treatments showed relatively lower adult emergence when compared with the control group, but the minimum adult emergence (30%) was observed at 7 d post-treatment when Heterorhabditis bacteriophora (FL1-1) was applied at the highest rate (100 IJs cm-2). In the broad virulence study, all EPN treatments caused significant reductions in F. fusca adult emergence (18.3-75.0%) in comparison with the control. H. bacteriophora (Fl1-1) was more virulent than other nematode treatments but statistically not different from Steinernema feltiae and Steinernema riobrave, while Steinernema rarum was the least virulent. In the potted soil bioassay, the lowest emergence (10.6%) was observed in H. bacteriophora (Fl1-1) treatment, followed by S. feltiae (SN), S. riobrave (355), and Heterorhabditis indica (HOM1) treatments. These results indicate that EPNs have the ability to suppress the soil dwelling stage of F. fusca and should be explored further under greenhouse and field conditions for biocontrol potential within an integrated pest management (IPM) context.

Entomopathogenic nematodes as biological control agent against Bactrocera zonata and Bactrocera dorsalis (Diptera: Tephritidae)

We evaluated ten different species of entomopathogenic nematodes (EPNs) Heterorhabditis bacteriophora, H. megidis, H. georgiana, H. floridensis, H. indica, Steinernema carpocapsae, S. riobrave, S. feltiae, S. rarum and S. glaseri against different developmental stages i.e. larvae, pupae and adults of fruit flies species Bactrocera zonata and Bactrocera dorsalis. In addition to this broad virulence screening in the laboratory, we also conducted the first field tests measuring the potential for EPNs to control B. zonata and B. dorsalis. In a Petri plate assays targeting adult insects, all EPN species caused different levels of mortality with maximums of 81.18% and 73.64% caused by H. bacteriophora 72 h after exposure in B. zonata and B. dorsalis, respectively. All the EPN species caused infection with the lowest survival levels observed in H. bacteriophora treatments against B. zonata and B. dorsalis. In a long-term exposure bioassay, both larvae and pupae were susceptible to all the EPN species with maximum mortalities caused by H. bacteriophora at 93.06% and 85.93% and 66.71% and 58.50% against larvae and pupae of B. zonata and B. dorsalis, respectively. In a potted soil bioassay, both developmental stages were found susceptible to the different EPN species, but again H. bacteriophora caused the highest levels of mortality; 95.74% and 86.88% in larvae and 71.27% and 67.65% in pupae of B. zonata and B. dorsalis, respectively. Clear dose response relationships were found in four EPNs (H. bacteriophora, S. carpocapsae, S. riobrave, S. feltiae) against larvae, pupae and adult B. zonata and B. dorsalis; the dose–response assays also confirmed superior virulence in H. bacteriophora relative to other EPN species. In a fruit exposure bioassay under laboratory condition, all EPN species were lethal towards larvae even inside the fruit and also in soil with maximum mortalities caused by H. bacteriophora in both fly species. Under greenhouse conditions, the lowest adult emergences (ranging from 19% to 52%) were observed in H. bacteriophora treatments when EPNs were exposed as larvae and pupae of B. zonata and B. dorsalis. Under field conditions, the same EPN species were tested and all the species significantly reduced adult fly emergence, but the lowest adult emergence was observed when using H. bacteriophora (25.06% and 31.53% in B. zonata and B. dorsalis, respectively). In the field, EPNs performed better in artificially infested fruit with mortalities of 47.18% and 42.03% as compared to natural infested fruit with mortalities of 36.88% and 30.86% in B. zonata and B. dorsalis, respectively. Our results indicate that EPNs could be part of an integrated pest management program for control of soil inhabiting stages of fruit fly complexes in orchard systems of Pakistan.