Growth promotion of apple plants is the net effect of binucleate Rhizoctonia sp. as rhizosphere-colonizing fungus

Abstract

The functional relationship between binucleate Rhizoctonia sp. (BnRhizoctonia) and apple was investigated. Apple rootstock plantlets (Malling 9) were grown in BnRhizoctonia inoculated and non-inoculated soil after preliminary treatment with gamma-rays. A growth assay was performed in phytotron under two soil-water content regimes: an optimal one for plant growth and another inducing periodic plant water stress. At the end of the growth test, plant growth as fresh and dry weight of biomass produced over the trial period and fungal root colonization with culture-based methodology were estimated. Relative quantification of Rhizoctonia and soil fungi DNA using specific primers was performed. Overall, BnRhizoctonia inoculation gave a mean plant growth increase which accounted for 50% as compared to non-inoculated treatments. Although root colonization frequency did not differ between inoculated and uninoculated plants, the inoculated were highly colonised by BnRhizoctonia, whilst the others were colonized by a series of ascomycetes such as Fusarium spp., Alternaria sp. which quickly recolonized soil after preliminary soil treatment with gamma-rays acting soil microbial restoration. Water stress overall reduced plant growth; however, inoculation with BnRhizoctonia reduced this negative effect by about 30%. Findings of this study indicate that BnRhizoctonia acts as growth promoter; but this beneficial effect depends on its degree of root colonization and is affected by environmental interaction.