Biological control of Pythium damping-off and root-rot diseases of tomato using Trichoderma isolates employed alone or in combination

Tomato (Solanum lycopersicum L.), one of the most widely grown vegetables worldwide, is susceptible to damping-off and root rot caused by Pythium aphanidermatum (Edson) Fitzpatrick. In in vitro assays, five Trichoderma isolates, viz., Trichoderma harzianum (Th), T. asperellum (Ta), T. virens (Tvs1), T. virens (Tvs2) and T. virens (Tvs3) were compared for their ability to suppress P. aphanidermatum. The mycelial growth of the pathogen was inhibited in vitro after placing each Trichoderma species and isolates on the opposite sides of the same Petri dish. Trichoderma isolates were able not only to arrest the spread of the pathogen but also invade the surface of its colony and sporulate over the colony. Additionally, conidia of Trichoderma isolates were able to inhibit the germination of zoospores of P. aphanidermatum in vitro. Control of tomato damping-off and root rot diseases by soil treatment with the inoculum preparations of Trichoderma isolates employed either alone or in combination was attempted. In greenhouse experiment, the combined inoculation of five Trichoderma isolates suppressed damping-off induced by P. aphanidermatum and increased the survival of tomato plants by 74.5%. In field experiment, the possibility of reducing plant death resulting from root rot disease caused by P. aphanidermatum using Trichoderma isolates, employed either alone or in combination, was investigated. The combined inoculation of five Trichoderma isolates was the most effective treatment, decreasing root rot by 57.2% and increasing the survival of tomato plants by 87.5%. The tested Trichoderma isolates stimulated systemic defence responses in tomato plants grown in the field by activating defence enzymes including peroxidase, polyphenoloxidase and chitinase. Additionally, the chlorophyll contents in the leaves of treated tomato plants were markedly increased. Moreover, the combined inoculation of the five isolates yielded the highest records of growth parameters and fruit yield compared with individual inoculation. Therefore, it was concluded that the mixture containing Trichoderma species and isolates may be used to control damping-off and root rot of tomato caused by Pythium aphanidermatum.