Plant-mediated effects of Trichoderma spp. and Beauveria bassiana isolates on insect and pathogen resistance

Abstract

Brassica oleracea var. capitata is grown worldwide under temperate to tropical climate conditions. However, cabbage is attacked by a wide range of insect pests and plant diseases. The phloem-feeding green peach aphid, Myzus persicae is a major brassica pest while Sclerotinia sclerotiorum and Leptsophaeria maculans are two important plant fungi causing white mold and blackleg or phoma stem canker respectively in cabbage and other brassica vegetables. Control of these pests and diseases is largely dependent on chemical pesticides. Due to the many negative effects associated with chemical pesticides and their decreasing availability, biological control options involving endophytic fungi are currrently being explored for pest and disease management. Some entomopathogenic fungi have been reported to be endophytes, living asymptomatically in plant tissues and protecting the host from insect pests and plant diseases. In this study, six fungal isolates (Trichoderma atroviride LU132, T. hamatum LU593, T. virens LU556; and Beauveria bassiana BG11, FRh2 and J18) were tested in a glasshouse for their ability to affect insect performance and disease severity on cabbage. Seven day old cabbage seedlings were inoculated with the biocontrol fungi as root drench and challenged with the insect pests and plant diseases 14-days after inoculation. The results showed that plants inoculated with T. hamatum LU593 and T. virens LU556 delayed the time taken for aphids to produce the first offspring compared to the control treatment. Total aphid reproduction was significantly reduced when fed on fungal endophyte treated plants compared to control treatment except B. bassiana J18 which did not show any significant effect. Aphids fed on plants inoculated with T. hamatum LU593, and B. bassiana isolates FRh2 and BG 11, had reduced longevity compared to aphids fed on uninoculated control plants. For the effect of the fungal endophytes on Sclerotinia sclerotiorum, the strongest effects were observed on plants inoculated with B. bassiana isolates J18 and BG 11 and T. hamatum LU593, with less leaf lesion area (mm²) compared to the uninoculated control treatment. No fungal endophytes showed any significant effect against Leptosphaeria maculans infection. All fungal endophyte inoculated plants except B. bassiana J18 promoted plant root growth, but there was no significant effect in the number of leaves and shoot growth across all treatment when compared to the untreated controls, indicating that the fungal endophytes likely established and colonised the root rhizospheres. Fungi recovered from endophytic colonisation showed colonies characteristics of Trichoderma species and B. bassiana. High recovery rates from surface sterilized root tissue segments plated were observed in T. atroviride LU132 (67%), B. bassiana BG11 (58%) and B. bassiana FRh2 (57%), whereas the other fungal endophytes showed less than 50% colonisation effect. Colonies characteristic of B. bassiana isolates FRh2 (17%) and BG11 (8%) were recovered from surface sterilized leaf tissues plated. The present study demonstrates that root drench inoculation of cabbage seedlings with Trichoderma spp. and B. bassiana can contribute to crop protection by enhancing the resistance of cabbage towards aphids and foliar diseases.