Nico Labuschagne

In vitro antagonism of endophytic Fusarium oxysporum isolates against the burrowing nematode Radopholus similis

Radopholus similis is one of the key pests of banana worldwide. In this study, nine endophytic Fusarium oxysporum isolates were screened for the production of secondary metabolites antagonistic to R. similis in culture. Undiluted and diluted culture filtrates were tested against motile stages and eggs of R. similis. All isolates tested demonstrated in vitro antagonistic activity, causing paralysis of R. similis motile stages. The percentage of paralysed nematodes increased with increase in the length of exposure time to culture filtrates. After 24 h exposure in culture filtrates up to 100% of the treated nematodes were paralysed compared to 26.5% in the control treatments. Nematode mortality rates after 24 h exposure in culture filtrates ranged from 76.4% to 100%. Paralysis was reversible at lower filtrate concentrations. Radopholus similis males were more sensitive to culture filtrates than females. Culture filtrates of all isolates demonstrated inhibitory effects on hatching of R. similis eggs. The results demonstrate the potential for using endophytic F. oxysporum as biological control agents against R. similis and for toxic derivatives from their secondary metabolism to be used as potential nematicides.

Plant Growth Promoting Rhizobacteria as Biocontrol Agents Against Soil-Borne Plant Diseases

Soil-borne diseases are responsible for major crop losses worldwide. Alternatives to the use of synthetic chemicals for disease control are increasingly being sought due to among other reasons, the detrimental effects of these compounds on the environment. In this chapter, biological control of soil-borne plant diseases by means of plant growth promoting rhizobacteria (PGPR) is reviewed with emphasis on cereals. The concepts and definitions of PGPR, biocontrol agents, biopesticides, biofertilizers, and soil inoculants are discussed and overlap between these categories are illustrated. Advantages and disadvantages of the use of PGPR as biocontrol agents are mentioned. Biocontrol of soil-borne diseases of crops is discussed and illustrated by means of specific examples of effective application of growth promoting rhizobacteria for control of soil-borne pathogens on cereals such as wheat and sorghum. The modes of action of PGPR with biocontrol activity is discussed with reference to the production of antibiotics, siderophores, and cell wall degrading enzymes as well as induction of systemic resistance, root colonization efficacy, and rhizosphere competence.

In-vitro inhibition of mycelial growth of several phytopathogenic fungi by soluble potassium silicate

Soluble silicon has been reported to suppress some plant diseases, but in vitro inhibition of phytopathogenic fungi has not been demonstrated. In the current study in-vitro dose-responses towards soluble potassium silicate (20.7% SiO2) were determined for Phytophthora cinnamomi, Sclerotinia sclerotiorum, Pythium F-group, Mucor pusillus, Drechslera sp, Fusarium oxysporum, F. solani, Alternaria solani, Colletotrichum coccodes, Verticillium theobromae, Curvularia lunata and Stemphylium herbarum. Inhibition of mycelial growth was dose-related with 100% inhibition at 80 ml (pH 11.7) and 40 ml (pH 11.5) soluble potassium silicate per litre of agar, for all fungi tested with the exception of Drechslera sp. and F. oxysporum at 40 ml in one experiment. Only Sclerotinia sclerotiorum and Phytophthora cinnamomi were completely inhibited at all soluble potassium silicate concentrations between 5 and 80 ml.1−1 agar, while all the other fungi were only partially inhibited at potassium silicate concentrations of 5, 10 and 20 ml.1−1 agar. Percentage inhibition was positively correlated with dosage. Soluble potassium silicate raised the pH of unameliorated agar from 5.6 to 10.3 and 11.7 at potassium silicate concentrations of 5 and 80 ml.l−1 agar respectively. Subsequent investigations into the effect of pH in the absence of potassium silicate showed that fungal growth was only partially inhibited at pH 10.3 and 11.7. Clearly, potassium silicate had an inhibitory effect on fungal growth in vitro and this was mostly fungicidal rather than attributed to a pH effect.

Effect of endophytic Fusarium oxysporum on root penetration and reproduction of Radopholus similis in tissue culture-derived banana ( Musa spp.) plants

In Uganda, banana (Musa spp.) production is constrained by the burrowing nematode Radopholus similis. Non-pathogenic, endophytic Fusarium oxysporum has been isolated from healthy banana plants, and several strains have shown potential as biological control agents against R. similis. Plant infection by R. similis can be characterised in three steps: host searching, root penetration and reproduction. In this study, we investigated the effects of three endophytic F. oxysporum strains (Eny1.31i, Eny7.11o and V5w2) on root penetration and reproduction of R. similis in tissue culture-derived banana plants. The number of R. similis that penetrated roots of endophyte-inoculated and untreated plants was not influenced by the endophytes in either laboratory or screenhouse experiments. However, R. similis reproduction was reduced by endophytes. Strain V5w2 gave the greatest suppression of R. similis reproduction. The results of this study imply that antagonism of endophytic F. oxysporum against R. similis in banana plants is post-infectional and mediated through disruption of nematode reproduction.

Effect of fertiliser application on yam nematode (Scutellonema bradys) multiplication and consequent damage to yam (Dioscorea spp.) under field and storage conditions in Benin

Abstract Studies were carried out at the International Institute of Tropical Agriculture (IITA-Cotonou Station, Benin) and at Save (central Benin), on the influence of chemical fertiliser application on yam nematode (Scutellonema bradys) densities and damage in the field and in storage to three yam (Dioscorea spp.) cultivars namely two D. rotundata cvs, TDr 131 and Ala, and one D. cayenensis cv, Kokoro. Treatments with nematodes (1000 per plant) and NPK, DAP (Diamonic Phosphate) and KCl at rates of 300, 155, and 60 kg/ha, respectively, were compared to un-inoculated and no-fertiliser control in field plots in terms of the damage to the crop and the multiplication of the nematode. At harvest, yield and the number of tubers per plant were not significantly affected by fertiliser application or nematode inoculation. However, the fertilisers suppressed nematode multiplication in tubers, especially DAP. In storage, tuber weight loss was recorded in all treatments. Weight loss was more pronounced in tubers from plots receiving fertiliser application than from those receiving none and when infected with S. bradys than when healthy, and mostly in tubers from infested plots which had received fertiliser. Nematode multiplication rates were greater in tubers during the first 3 months of storage, declining in the fourth and fifth months.

The effect of chloride on four different citrus rootstocks

Seedlings of Troyer citrange [Poncirus Mollata L. Raf. × Citrus sinensis L. Osbeck] (Tro), Citrus volkameriana (Vol), Carrizo citrange [Poncirus trifoliata L. Raf. × Citrus sinensis L. Osbeck], (Car) and Rough lemon [Citrus jambhiri Lush.] (Rie) were tested for tolerance to chloride in irrigation water, ranging from 0 to 1147 mg L−1 chloride, as NaCI. Based on the occurrence of toxicity symptoms on the leaves, Vol was the most tolerant to CI, followed by Rle. Decrease in dry mass of shoots and roots with increased chloride levels again showed Vol to be the most tolerant rootstock followed by Rle. Tro and Car were the most sensitive to chloride. The shoot: root ratio increased with increasing chloride levels indicating that more shoots were produced in relation to roots at increasing chloride levels. The percentage chloride in the leaves showed a constant increase with increasing CI levels to the point where leaf drop occurred.

Allelopathic interaction between Chenopodium album L. and certain crop species.

Laboratory and glasshouse studies were conducted to assess the allelopathic effect of Chenopodium album L. on the germination and early growth of certain crop species. Test species were grown in soils containing 1% (m/m) freeze-dried shoot material of the weed. Aqueous extracts of soils containing weed residues and organic solvent extracts of shoots of the weed were tested for effects on the germination of certain crop species. Radish (Raphanus sativus L.) was the only crop not affected by the weed residue in the soil. Growth inhibition was 68% for cucumber (Cucumis sativus L.), 85% for onion (Allium cepa L.), 47% for tomato (Lycopersicon esculentum L.) and 51% for sunflower (Helianthus annuus L.). Except for radish and sunflower, stand loss contributed to reductions in growth. Germination of crop species was not affected by aqueous extracts of soil portions (100g) from soil either grown with the weed (residues later removed by sieving) or containing shoot material of the weed. In fact, the growth of radi...

Determination of scoparone in citrus roots by micellar electrokinetic capillary chromatography

A new MEKC method has been developed to determine the amount of the phytoalexin, scoparone, in Citrus roots. The separation and analysis was achieved with a running buffer of 100 mM SDS, 25 mM phosphate and 12% (v/v) methanol pH 6.2. Separation was performed at 12 kV with 25 degrees C and UV detection at 200 nm. A near complete recovery of scoparone was obtained with the extraction procedure. The MEKC method was compared with a fluorescence TLC method. The detection limit for scoparone with the MEKC method (2 mu g/mL) was better than the TLC method (10 mu g/mL).

The Chemistry of Plant–Microbe Interactions in the Rhizosphere and the Potential for Metabolomics to Reveal Signaling Related to Defense Priming and Induced Systemic Resistance

Plant roots communicate with microbes in a sophisticated manner through chemical communication within the rhizosphere, thereby leading to biofilm formation of beneficial microbes and, in the case of plant growth-promoting rhizomicrobes/-bacteria (PGPR), resulting in priming of defense, or induced resistance in the plant host. The knowledge of plant–plant and plant–microbe interactions have been greatly extended over recent years; however, the chemical communication leading to priming is far from being well understood. Furthermore, linkage between below- and above-ground plant physiological processes adds to the complexity. In metabolomics studies, the main aim is to profile and annotate all exo- and endo-metabolites in a biological system that drive and participate in physiological processes. Recent advances in this field has enabled researchers to analyze 100s of compounds in one sample over a short time period. Here, from a metabolomics viewpoint, we review the interactions within the rhizosphere and subsequent above-ground ‘signalomics’, and emphasize the contributions that mass spectrometric-based metabolomic approaches can bring to the study of plant-beneficial – and priming events.

Seed treatment with selected plant growth-promoting rhizobacteria increases maize yield in the field

Maize, Zea mays is the most important grain crop in South Africa and is a staple food in many African countries. The beneficial effects of plant growth-promoting rhizobacteria on crop growth and yield have been well documented, but obtaining reproducible results under field conditions is often difficult. In the current study, five selected rhizobacterial strains that showed plant growth-promoting activities in pilot studies were evaluated for potential enhancement of maize yield under field conditions. The five strains together with a commercial standard were assessed as seed treatments of maize over three seasons in four different soil types. The strains were identified on the basis of 16S rRNA sequencing as Lysinibacillus sphaericus (T19), Paenibacillus alvei (T29), Bacillus safensis (S7) Bacillus pumilus (A26) and Brevundimonas vesicularis (A40). The best yield increases in maize were obtained during the 2011/2012 and 2012/2013 seasons in the Shortlands ecotope with the rhizobacterial strains T19, T29 and S7, resulting in yield increases ranging from 24% to 34%. Strain T19 rendered the most consistent yield increases during the three successive field trials amounting to 33% and 24% in Shortlands ecotope and 12% in Clovalley ecotope, respectively. During 2013/2014 a consortium of three strains, viz. T19, S7 and A26 gave a 32% yield increase in Clovalley ecotope. All the rhizobacterial strains solubilised phosphate in vitro except T19. Strain T29 showed the best nitrogen-fixing activity in vitro, proliferating on a nitrogen-free substrate and also producing ammonia. All the strains tested positive for indole acetic acid production. The current study demonstrates the ability of rhizobacterial strains T19, T29, S7 and A26 applied as seed treatments to significantly enhance maize yield in the field, making development and commercialisation of these strains a viable option.