Biological Activity of Four Trichoderma Species Confers Protection against Rhizoctonia solani, the Causal Agent of Cucumber Damping-Off and Root Rot Diseases

Abstract

The cucumber (Cucumis sativa L.) is often subjected to several fungal diseases. Rhizoctonia solani-induced cucumber damping-off and root rot are the most common diseases reported from the commercial greenhouses of the eastern area of Saudi Arabia. The objective of the current study is to explore the antagonistic activity of four Trichoderma species against R. solani in vitro and in vivo. Ten R. solani isolates (eight belonging to AG-4 and two belonging to AG-A and AG2-1) were studied. AG4 isolates were pathogenic to cucumber plants, while AG-A and AG2-1 isolates were non-pathogenic. Seven isolates of Trichoderma spp., named T. hamatum KSATR8, T. harzianum (KSATR9 and KSATR10), T. asperellum (KSATR11, KSATC, and KSAT1E), and T. longibrachiatum KSATS were isolated, and the identities of both R. solani and Trichoderma isolates were confirmed based on the phylogenetic analysis of the DNA sequence of the ITS region. The dual culture findings indicated that T. asperellum KSATC and KSAT1E exhibited the most significant inhibitory activities against R. solani, with values of 79.33 and 70.89%, respectively. Scanning electron microscope (SEM) images showed a considerable degradation in the cell wall and collapsing of R. solani hyphae by all Trichoderma species. Under greenhouse conditions, the application of T. asperellum KSATC and KSAT1E at concentrations of 2 × 108 conidia/mL revealed a reduction in root rot and damping-off incidence percentages with values that did not reveal a significant (p < 0.05) difference from those of Rizolex-T fungicide. Nevertheless, the efficacy of the fungicide attained 86.67%, being higher than that of T. asperellum KSATC, which reached 80%. Trichoderma asperellum KSATC and KSAT1E were the greatest in increasing peroxidase, catalase, and chitinase enzymes activities in cucumber plants. Conversely, a significant (p < 0.05) elevation in polyphenol oxidase enzyme (0.762 and 0.97 U/g FW) and total phenol content (0.55 and 0.62 mg/g FW) was recorded in cucumber plants treated with T. harzianum KSATR9 and KSATR10, respectively. The statistical analysis results displayed no considerable variations among cucumber plants regarding total chlorophyll content as a response to treatments with Trichoderma species and fungicides. Therefore, we endorse using T. asperellum KSATC and KSAT1E as an alternative to fungicides to manage root rot and damping-off in cucumbers.