Factors influencing the effectiveness of non-pathogenic Fusarium solani strain Fs5 in the suppression of root-knot nematode in tomato

Four experiments were carried out under greenhouse conditions to evaluate the effectiveness of Fusarium solani strain Fs5 against the root-knot nematode Meloidogyne javanica. The effect of population densities of M. javanica, various application rates of F. solani, moisture regimes and levels of benzaldehyde, a volatile compound of plant origin affecting the plant-nematode-fungus interaction, were also studied. F. solani parasitized eggs and females of M. javanica and thereby reduced root-knot severity in tomato. Although the fungus was frequently isolated from root tissues, it did not produce phytotoxic symptoms; instead, there was enhanced plant growth. At higher nematode densities, inner root colonization by the fungus increased. The rates of fungal infection on M. javanica eggs and females also increased with increasing nematode densities and fungal inoculum levels. Nematode invasion and subsequent root-knot increased with increasing soil moisture, in both F. solani-treated and untreated plants. However, root-knot development was lower at all moisture regimes when F. solani was applied to the soil. Root colonization by F. solani and parasitism on female nematodes was highest at 50% moisture holding capacity (MHC) whereas egg parasitism by the fungus was greatest at 75% MHC. With increasing concentration of benzaldehyde in soil, nematode penetration and subsequent root-knot infection were progressively reduced. Root colonization by F. solani was greatest in soil treated with benzaldehyde at 2 µg g-1 of soil in the presence of M. javanica. Increasing benzaldehyde concentrations resulted in increased parasitism of M. javanica females by F. solani but in lower parasitism of the eggs. Treatments with F. solani led to better plant growth when they were combined with benzaldehyde at 2 µg g-1 of soil

Phenol-mediated suppression of soil-borne root-infecting fungi in mungbean

Under field conditions, there is a variety of phenolic acids as well as other toxic and non-toxic organic compounds that interact with plant seeds and roots; but in laboratory bioassays, with few exceptions, only single phenolic acids are usually tested. In this study, the effect of various concentrations of two phenolics (caffeic acid and p-hydroxybenzoic acid) on the wilt-inducing fungus Fusarium solani and the damping-off fungus Rhizoctonia solani was tested in pot experiments. The effect of these phenolics on the biocontrol efficacy of Pseudomonas aeruginosa, a plant growth-promoting rhizobacterium, was also evaluated. Caffeic acid and p-hydroxybenzoic acid significantly suppressed F. solani and R. solani infection in mungbean. However, high concentrations of the phenolic acids interfered with plant growth. P. aeruginosa in the rhizosphere declined in the presence of caffeic acid and p-hydroxybenzoic acid

Avicennia marina (mangrove) soil amendment changes the fungal community in the rhizosphere and root tissue of mungbean and contributes to control of root-knot nematodes

The effect of soil amendment with Avicennia marina (mangrove) on mungbean growth and mungbean infestation with Meloidogyne javanica was determined in greenhouse pot experiments. Galling and final nematode population densities were reduced by all soil amendments with mangrove. To better understand whether nematode suppression by A. marina was caused directly by the release of nematicidal factor(s) into the soil, or was due indirectly to changes in the fungal community, the diversity of the rhizosphere populations of culturable fungi was assessed before organic amendment (day 0), after decomposition but before seed sowing (day 15) and at harvest (day 73). Thirteen out of 20 fungal species were isolated from both A. marina-amended and unamended soils, the most frequent genera being Alternaria, Aspergillus, Fusarium, Penicillium, Trichoderma, Mucor, Myrothecium and Rhizoctonia. The other seven were found only in amended soils. At different times in the course of the experiment amended and unamended soils differed significantly in the fungi isolated from the rhizosphere and/or in the concentrations of A. marina. Trichoderma viride was isolated only from surface-sterilized mungbean roots grown in amended soils, whereas Chaetomium sp. was isolated only from unamended soils

Nematicidal and allelopathic responses of Lantana camara root extract

The impact of root leachates of Lantana camara L., a tropical weed, against Meloidogyne javanica, the root-knot nematode, was tested under laboratory and pot conditions. Concentrated and diluted root leachate caused substantial mortality of M. javanica juveniles. Significant suppression of the nematode was achieved when soil was treated with a full-strength concentration of the leachate. Whilst this high concentration retarded plant height and shoot fresh weight, more diluted concentrations actually enhanced plant growth. To establish whether this inhibition of plant growth from the leachate was the result of depleted nitrogen levels in the soil due to the leachate, soil treated with such leachates was given urea as an additional nitrogen source. Urea not only enhanced nematode suppression activity of the root leachates but also increased seedling emergence and growth of mungbean. Application of the L. camara root leachates in combination with Pseudomonas aeruginosa, a plant growth-promoting rhizobacterium, significantly reduced nematode population densities in roots and subsequent root-knot infection, and enhanced plant growth. While a high concentration of root leachate slightly reduced P. aeruginosa colonization in the rhizosphere and inner root tissues, the nematicidal efficacy of the bacterium was unaffected. The root leachate of L. camara was found to contain phenolic compounds, including p-hydroxybenzoic acid, vanillic acid, caffeic acid, ferulic acid and a quercetin glycoside, 7-glucoside. It also contained weak enzymic hydrogen cyanide

Role of Salicylic Acid in Pseudomonas aeruginosa Strain IE-6S+-Mediated Induction of Systemic Resistance against Meloidogyne javanica in Tomato

Root colonization by certain non-pathogenic bacteria can induce systemic resistance to pathogen infections in plants. In a split-root assay with tomato plants, we investigated which determinants of the rhizobacterium Pseudomonas aeruginosa IE-6S+ were important for induction of resistance to the root-knot nematode Meloidogyne javanica. P. aeruginosa IE-6S+ produced 3.9±1.1 µg ml-1 salicylic acid (SA) in a liquid casamino acid medium under laboratory conditions. The bacterial inoculant induced resistance equivalent to the application of 10 mM synthetic SA. However, SA at this concentration did not produce significant mortality of M. javanica juveniles in vitro. Soil iron (2.4 mM FeCl3·6H2O) did not markedly alter the resistance that P. aeruginosa IE-6S+ induced in tomato roots, which suggested that P. aeruginosa IE-6S+ activity was not iron-regulated. However, the resistance reaction was greatly enhanced when IE-6S+ and SA were co-inoculated with 0.5% Tween-20. While IE-6S+ colonized the tomato rhizosphere at 6.38 log cfu per g fresh weight of root during the first 3 days after inoculation, the bacterial populations declined steadily, reaching a mean population density of 4.73 log cfu g-1 fresh weight of root at 21 days. The bacterium was not isolated from the unbacterized half of the split root system

Physiological changes in leaves of mungbean plants infected with <I>Meloidogyne javanica</I>

Sequential changes induced by the root-knot nematode Meloidogyne javanica (Treub) Chitwood in mung bean (Vigna radiata (L.) Wilczek cv. MN95) were studied. Physiological and biochemical changes were recorded 15, 30 and 45 days after nematode inoculation. The changes noted varied with the length of exposure to the nematode. Chlorophyll and carotenoid contents decreased in nematode-infected plants. Total phenols increased in the leaves compared with the controls for up to 30 days after inoculation. Protein content declined significantly at 30 days after exposure to the nematodes. Amylase activity was enhanced in both the leaves and the stems as compared with the controls. The results suggested that plants responded to the nematode by adopting biochemical strategies to withstand the adverse effects of infection