The sugarbeet cultivar Kaumera was found to be highly susceptible to infection by the root-rot pathogens Rhizoctonia solani and Sclerotium rolfsii in the absence of salinity stress. Under this environmental condition, R. solani was more efficient than S. rolfsii in producing cell wall-degrading enzymes in infected hypocotyls. Xylanase and galactanase were most effective. The rate of cell wall degradation by R. solani was nearly 2.5 times that of S. rolfsii when cells walls of healthy hypocotyls were used as sole carbon substrate for the in vitro produced crude enzymes.Under salinity stress the pathogenicity and the performance of cell wall-degrading enzymes of R.solani and S. rolfsii varied profoundly. Pathogenicity studies showed that R. solani appeared to be more tolerant than S. rolfsii of the salinity stresses applied, and relatively more virulent to cv Kaumera. The activities of cell wall enzymes of R. solani decreased and those of S. rolfsii increased with increased salt concentration when cell wall material was used as a sole carbon source. The metabolic products produced under salinity stress by R. solani and S. rolfsii in the cell wall amended culture media shifted the initial pH towards neutrality or slight alkalinity for R. solani and to high acidity for S. rolfsii.When model substrates were used, xylan and galactan were the most responsive substrates for degradation by the cell wall enzymes of the two fungi studied. The rate of degradation was higher for S.rolfsii than for R. solani. The excessive acidity in salt stressed S. rolfsii culture media suggested reduced activities of the enzymes involved in cell wall degradation in vivo. This may explain the decreased virulence potentialities.
The preliminary phytochemical screening of extracts of Rumex vesicarius L. and Ziziphus spina-christi (L.) Willd. showed the presence of compounds that are biologically active against the two root rot pathogens Drechslera biseptata and Fusarium solani in vitro. The relative efficacy of this action, however, differed according to the extracted plant, solvent used, extract concentration, the target fungus and phase of growth. Ethanolic extract ranked first, followed by the remaining aqueous layer fraction. Eight flavonoid subfractions (rutin, quercetin, myricetin, apigenin, quercetin-3-O-galactoside, luteolin, kaempherol and kaempherol-3-O-robinoside) and six flavonoid subfractions (apigenin-7-O-glucoide, quercitrin, quercetin, isovitexin, rutin and quercetin-3-O lucoside-7-O-rhamnoside) were isolated from the remaining aqueous layer fraction of R. vesicarius and Z. spina-christi, respectively. Generally, spore production and germination as well as cellulolytic and pectolytic activity of F. solani were affected by plant extracts more than that of D. biseptata. F. solani failed completely to produce spores when treated with ethanolic extract of Z. spina-christi at the concentration of 20%. However, growth of D. biseptata was more sensitive to plant extracts than that of F. solani. Maximum activity of plant extracts was observed against spore production. It was evident that plant extracts could provide potential source of antifungal compounds.
Pyradur applied to soil at 0.6–2.4 µg∙g−1 active ingredients suppressed infection of three sugarbeet cultivars by Rhizoctonia solani and Sclerotium rolfsii. In the absence of Pyradur, R. solani was more virulent than S. rolfsii against 'Raspoly' and 'TOP', whereas S. rolfsii was more virulent than R. solani against ‘Tribel’. Virulence was directly correlated with the activities of cell wall degrading enzymes produced by mese pathogens in vivo and on cell walls in vitro. Reduced virulence of R. solani and S. rolfsii under Pyradur stress was due to decreased inoculum potential of the two pathogens at the utilized concentrations of herbicide in situ and to reduced production of cell wall degrading enzymes in vitro and in host tissues. In addition, shifts in the pH of cell wall amended media, because of changes in the nature of metabolic products of the pathogens under Pyradur stress, suggest possible repression or stimulation of the activity of the enzymes involved in degradation in vivo, of which cellulase and polygalacturonase are favoured by acid conditions, and galactanase, mannase, and pectate lyase are favoured by alkaline conditions. Keywords: sugarbeet, Rhizoctonia solani, Sclerotium rolfsii, Pyradur, metolachlor, chloridazon, growth activities, pathogenicity, virulence, cell wall enzymes.
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