Endophytic establishment of Beauveria bassiana in wheat (Triticum aestivum) and its impact on Diuraphis noxia

Abstract

The entomopathogenic fungus (EPF) Beauveria bassiana (Balsamo-Crivelli) Vuillemin (Hypocreales: Cordycipitaceae) is globally known to infect a wide range of insect hosts, including aphids. The current study investigated possibilities for endophytic establishment of the EPF B. bassiana in wheat (Triticum aestivum L) (Poaceae) and explore its biocontrol potential against the Russian wheat aphid (RWA), Diuraphis noxia Kurdjumov (Hemiptera: Aphididae). Twelve indigenous B. bassiana isolates were pre-screened against Galleria mellonella larvae and the virulent strain (PPRI 7598) was selected based on >90% of insect mortality. PPRI 7598 was compared with PPRI 7861 (commercial strain) for pathogenicity against D. noxia. PPRI 7598 strain was further passaged through D. noxia and wheat cultivar Tugela (designating “IN” and “PL” isolates, respectively), from which wheat seedlings were inoculated with 1 x 108 conidia ml-1 using three inoculation techniques. Endophytic colonisation of five cultivars was evaluated post inoculation. Furthermore, the endophytic effect was evaluated on the aphid population growth with intrinsic rate of increase, plant response towards RWA herbivory, plant damage rate post infestation with RWA and impact on the aphid masses (g/plant). The B. bassiana strains used in this study were therefore confirmed with PCR assay using B. bassiana species-specific primers. The two B. bassiana strains were proved to be pathogenic to D. noxia, however PPRI 7598 caused significantly higher mortality of 55.64% than PPRI 7861 (43.1%). PPRI 7598”IN” isolate also recorded the highest RWA mortality as compared to 50% by PPRI 7598PL. Also, PPRI 7598IN showed significantly higher (29.74) mean level of endophytic colonisation of seedlings than the PL isolate (26.13%). The highest levels of colonisation were 44.15% and 38.59%, recorded in roots with PPRI 7598IN and PPRI 7598PL, respectively. Overall, endophytic B. bassiana improved plant growth by 71% average over the controls. The significant effect of the endophyte on RWA population reduced the aphid net reproductive rate by 14 nymphs female-1, although the intrinsic rate of population increase was not significant. The endophyte presence significantly decreased the aphid mass by 13% over the controls. However, the damage rating varied significantly across the three cultivars, although there was no significant differences between the treatment levels within each cultivar. The PCR assay successfully amplified 7 of 9 indigenous B. bassiana strains including PPRI 7598 and PPRI 7861 at 300bp. Through this study, endophytic colonisation of five South African wheat cultivars by B. bassiana PPRI 7598 was confirmed, presenting new possibilities as IPM component including improved plant growth and potential insect and/or disease suppression