Characterization and efficacy of Muscodor cinnamomi in promoting plant growth and controlling Rhizoctonia root rot in tomatoes

Muscodor cinnamomi was selected and investigated for its in vitro ability to produce indole-3-acetic acid (IAA) to solubilize different toxic metal (Ca, Co, Cd, Cu, Pb, Zn)-containing insoluble minerals and tolerance to metals, herbicides and an insecticide. The results indicated that this fungus is able to produce IAA (45.36 ± 2.40 μg ml⁻¹) in liquid media. This phytohormone stimulated coleoptile elongation, and increased seed germination and root elongation of tested plants. The metal tolerance and solubilizing ability depended on the type of insoluble minerals. M. cinnamomi showed the highest growth tolerance on Ca-containing media at 150 mM, followed by Zn-containing media at 100 mM and Cd-containing media at 10 mM. This fungus tolerated the three herbicides (2,4-d-dimethylammonium, glyphosate and paraquat dichloride) and an insecticide (methomyl) at the recommended dosages for field application. Moreover, M. cinnamomi completely controlled Rhizoctonia solani AG-2 root rot in tomato plants, and increased root length, shoot dry weight and root dry weight. This is the first report of in vitro IAA production, solubilization of insoluble metal minerals, and tolerance to herbicides, an insecticide and metals as well as the plant growth promoting ability of M. cinnamomi.     

Interaction between Meloidogyne incognita and Rhizoctonia solani on green beans

The interaction between Meloidogyne incognita (race 2) and Rhizoctonia solani (AG 4) in a root rot disease complex of green beans (Phaseolus vulgaris) was examined in a greenhouse pot experiment. Three week-old seedlings (cv. Contender) were inoculated with the nematode and/or the fungus in different combinations and sequences. Two months after last nematode inoculation, the test was terminated and data were recorded. The synchronized inoculation by both pathogens (N + F) increased the index of Rhizoctonia root rot and the number of root galls; and suppressed plant growth, compared to controls. However, the severity of root rot and suppression of plant growth were greater and more evident when inoculation by the nematode preceded the fungus (N → F) by two weeks. Nematode reproduction (eggs/g root) was adversely affected by the presence of the fungus except by the synchronized inoculation. When inoculation by nematode preceded the fungus, plant growth was severely suppressed and roots were highly damaged and rotted leading to a decrease of root galls and eggs.     

Isolation and characterization of rhizosphere bacteria for the biocontrol of the damping-off disease of tomatoes in Tunisia

Fluorescent Pseudomonas spp., isolated from tomato and pepper plants rhizosphere soil, was evaluated in vitro as a potential antagonist of fungal pathogens. Pseudomonas strains were tested against the causal agents of tomatoes damping-off (Sclerotinia sclerotiorum), root rot (Fusarium solani), and causal agents of stem canker and leaf blight (Alternaria alternata). For this purpose, dual culture antagonism assays were carried out on 25% tryptic soy agar, King B medium and potato dextrose agar to determine the effect of the strains on mycelial growth of the pathogens. In addition, strains were screened for their ability to produce exoenzymes and siderophores. All the strains significantly inhibited Alternaria alternata, particularly in 25% TSA medium. Antagonistic effect on Sclerotinia sclerotiorum and Fusarium solani was greater on King B medium. Protease was produced by 30% of the strains, but no strain produced cellulase or chitinase. Finally, the selected Pseudomonas strain, Psf5, was evaluated on tomato seedling development and as a potential candidate for controlling tomato damping-off caused by Sclerotinia sclerotiorum, under growth chamber conditions. In vivo studies resulted in significant increases in plant stand as well as in root dry weight. Psf5 was able to establish and survive in tomato plants rhizosphere after 40 days following the planting of bacterized seeds.     

UVC-priming mediated modulation of forskolin biosynthesis key genes against Macrophomina root rot of Coleus forskohlii⿿A tissue culture based sustainable approach

Coleus forskohlii is susceptible to root rot disease that reduces yield of root specific metabolite forskolin. In this communication, we first time reported sustainable management of Macrophomina phaseolina root rot in C. forskohlii via Ultraviolet-C (UVC)-hormesis using tissue culture. Compact calli were developed using 2,4-dichlorophenoxy acetic acid (2.0 mg l^⿿1) and tolerance level of UVC exposure was optimized. Calli exhibited 88% viability, 8.4% relative growth in biomass and higher absorbance in 2,3,5-triphenyl tetrazolium chloride analysis subsequent to 10 min of UVC exposure. Pathogenicity of M. phaseolina was reduced in UVC-primed calli-regenerated plants compared with un-primed ones under controlled condition. Plant height, leaf number, shoot and root biomass significantly improved in UVC-primed calli-regenerated plants over un-primed ones when challenged by M. phaseolina in glasshouse. Significant elevation of economically important forskolin via up-regulation of key forskolin biosynthesis genes GGPP synthase, CfTPS2 and CfTPS3 highlighted the potentiality of UVC-priming to boost plant⿿s defence mechanism and protect from drastic economic loss.     

Biocontrol of Rhizoctonia solani damping-off and promotion of tomato plant growth by endophytic actinomycetes isolated from native plants of Algerian Sahara

Thirty-four endophytic actinomycetes were isolated from the roots of native plants of the Algerian Sahara. Morphological and chemical studies showed that twenty-nine isolates belonged to the Streptomyces genus and five were non-Streptomyces. All isolates were screened for their in vitro antifungal activity against Rhizoctonia solani. The six that had the greatest pathogen inhibitory capacities were subsequently tested for their in vivo biocontrol potential on R. solani damping-off in sterilized and non-sterilized soils, and for their plant-growth promoting activities on tomato seedlings. In both soils, coating tomato seeds with antagonistic isolates significantly reduced (P < 0.05) the severity of damping-off of tomato seedlings. Among the isolates tested, the strains CA-2 and AA-2 exhibited the same disease incidence reduction as thioperoxydicarbonic diamide, tetramethylthiram (TMTD) and no significant differences (P < 0.05) were observed. Furthermore, they resulted in a significant increase in the seedling fresh weight, the seedling length and the root length of the seed-treated seedlings compared to the control. The taxonomic position based on 16S rDNA sequence analysis and phylogenetic studies indicated that the strains CA-2 and AA-2 were related to Streptomyces mutabilis NBRC 12800^T (100% of similarity) and Streptomyces cyaneofuscatus JCM 4364^T (100% of similarity), respectively.     

Impact of soil salinity on the plant-growth – promoting and biological control abilities of root associated bacteria

The effectiveness of plant growth – promoting bacteria is variable under different biotic and abiotic conditions. Abiotic factors may negatively affect the beneficial properties and efficiency of the introduced PGPR inoculants. The aim of this study was to evaluate the effect of plant growth – promoting rhizobacteria on plant growth and on the control of foot and root rot of tomatoes caused by Fusarium solani under different soil salinity conditions. Among the five tested strains, only Pseudomonas chlororaphis TSAU13, and Pseudomonas extremorientalis TSAU20 were able to stimulate plant growth and act as biological controls of foot and root rot disease of tomato. The soil salinity did not negatively affect the beneficial impacts of these strains, as they were able to colonize and survive on the roots of tomato plants under both saline and non-saline soil conditions. The improved plant height and fruit yield of tomato was also observed for plants inoculated with P. extremorientalis TSAU20. Our results indicated that, saline condition is not crucial factor in obtaining good performance with respect to the plant growth stimulating and biocontrol abilities of PGPR strains. The bacterial inoculant also enhanced antioxidant enzymes activities thereby preventing ROS induced oxidative damage in plants, and the proline concentrations in plant tissue that play an important role in plant stress tolerance.     

Interactive effects of α-NAA and UV-B radiation on the endogenous hormone contents and growth of Trichosanthes kirilowii Maxim seedlings

The impact of UV-B radiation on endogenous hormones in plants has recently drawn attention from researchers. The mechanism for reduced stem elongation by UV-B might be due to changes in the phytohormone levels, especially IAA, which plays a role in stem elongation. In this study, effects of UV-B radiation on Trichosanthes kirilowii Maxim (T. kirilowii) seedlings in greenhouse-grown plants were investigated. The results indicated that: (1) In comparison to controls, exposure to 0.029 Jm^?2 s^?1. UV-B radiation led to accumulation of endogenous abscisic acid (ABA) and zeatinriboside (ZR) in the plant contents, and decreased contents of endogenous indole-3-acetic acid (IAA) and gibberellic acid (GA_1/3). Exposure to UV-B radiation reduced the height and leaf area of plants. As a result, total biomass (plant dry weight) was lower. (2) In comparison to controls, addition of 2 mg l^?1 α-naphthaleneacetic acid (α-NAA) slightly increased the contents of IAA, GA_1/3 and ZR, and decreased the content of ABA in leaves. This addition of α-NAA significantly increased plant height and leaf area, but only slightly increased total biomass. (3) Addition of α-NAA to UV-B-exposed plants: increased the content of endogenous IAA, GA_1/3 and ZR; decreased accumulation of endogenous ABA; and increased plant height and leaf area in comparison to plants that only were exposed to UV-B. Moreover, total biomass increased slightly. This suggests that addition of α-NAA may compensate to a certain extent for the lack of IAA resulting from UV-B radiation; it also increases the content of GA_1/3 and ZR, decreases the accumulation of ABA, and promotes the growth of plants.     

The use of natural plant volatile compounds for the control of the potato postharvest diseases,black dot,silver scurf and soft rot

Many naturally occurring plant volatile compounds are known for their anti-fungal properties. In this study, acetaldehyde and 2E-hexenal were chosen as prototype volatiles in order to investigate the use of volatile compounds for control of blemish pathogens in fresh-pack potato packaging. Pure cultures of the three main potato blemish pathogens, Pectobacterium atrosepticum (bacterial soft rot), Colletotrichum coccodes (black dot), and Helminthosporium solani (silver scurf), were used in the study. Pathogen cultures were exposed to the pure volatiles that were injected into the atmosphere of sealed jars for 4–8 days at 23 °C. Results showed that 2E-hexenal was the most effective of the two volatiles with 5 μL/L providing complete inhibition of growth for all three pathogens in vitro. Cytological studies showed that a concentration of 2.5 μL/L of 2E-hexenal was capable of inhibiting germination in both fungal pathogens. These results suggest that the primary mode of action of 2E-hexenal was inhibiting germination for fungi and suppressing bacterial growth. The quantities required to achieve pathogen inhibition are extremely low. This study suggests that these volatiles may be used to effectively manage potato postharvest blemish diseases in storage.     

Effect of volatile substances of Streptomyces platensis F-1 on control of plant fungal diseases

Headspace volatile substances (VS) produced by Streptomyces platensis F-1 were preliminarily identified using GC–MS. The effects of VS released by S. platensis F-1 on the control of leaf blight/seedling blight of rice caused by Rhizoctonia solani, leaf blight of oilseed rape caused by Sclerotinia sclerotiorum and fruit rot of strawberry caused by Botrytis cinerea, as well as on the growth of these three pathogenic fungi, were investigated. Results showed that sixteen volatile compounds were tentatively identified in 1-week-old cultures of S. platensis F-1 grown on autoclaved wheat seeds. They could be chemically grouped into alcohols, esters, acids, alkanes, ketones and alkenes. The most abundant composition in volatiles of S. platensis F-1 is geosmin, an earthy-muddy–smelling compound. Two antifungal compounds, phenylethyl alcohol and (+)-epi-bicyclesesquiphellandrene, were detected in the volatile profile of S. platensis F-1. Consistent fumigation of healthy tissues of rice, oilseed rape and strawberry to VS of S. platensis effectively reduced the incidence and/or the severity of leaf blight/seedling blight of rice (R. solani), leaf blight of oilseed rape (S. sclerotiorum) and fruit rot of strawberry (B. cinerea). A significant (P < 0.05) suppression of the mycelial growth of R. solani, S. sclerotiorum and B. cinerea by the VS of S. platensis was observed. The potential of using VS of S. platensis F-1 as a biofumigant to control plant fungal diseases is discussed.     

Microencapsulation of Bacillus subtilis B99-2 and its biocontrol efficiency against Rhizoctonia solani in tomato

Biological control is a promising approach to protecting plants from disease. Bacillus subtilis has been widely used in agriculture for promoting plant growth and biocontrol. However, their short shelf life limits the application of biological pesticides. The objectives of this study were to develop a microencapsulation procedure of B. subtilis B99-2 using maltodextrin and gum arabic as wall materials to determine the optimum conditions of spray-drying in microencapsulation, evaluate storage stability of microcapsules, and assess their biocontrol efficiency against Rhizoctonia solani in tomato under field conditions. We microencapsulated the Bacillus thallus by spray-drying with various concentrations of the wall material. Maltodextrin was found to be an efficient wall material, especially at concentrations higher than 80%, while gum arabic did not affect the bacterial survival rate. The mean survival rate of B. subtilis was more than 90%, when spray drying was performed at 145 °C, with a feed flow rate of 550 mL h⁻¹, and a spray pressure of 0.15 MPa. B. subtilis microcapsule survival rate was 87.53% after 540 d of storage, which was a longer shelf life than that of wettable powders. Moreover, its biocontrol efficacy reached 79.91% when a dosage of 300 g hm⁻² was used, the microcapsule showed higher control efficacy than Thiram wettable powder against R. solani in tomato under field conditions. All these characteristics indicated that B. subtilis microcapsules have the potential to become a successful biocontrol product.