Isolate and plant host specificity of rhizosphere competence in Trichoderma species

Rhizosphere competence, the ability of a microorganism to colonise and proliferate in the rhizosphere of developing roots, is often studied when mechanisms of individual Trichoderma biocontrol agents are investigated. However, the extent of rhizosphere competence of Trichoderma species and isolates within species has not been widely studied. The rhizosphere competence of 22 Trichoderma isolates from a range of species was assessed using Trichoderma coated sweet corn (Zea mays) seeds grown in non-sterile soil. Results showed that 82 % of the Trichoderma isolates inoculated onto the seeds produced rhizosphere populations significantly greater than the control, indicating rhizosphere competence was widespread within the species that were tested. The least and most rhizosphere-competent isolates belonged to the same species indicating that rhizosphere competence was not species specific. The three least (T. crassum LU555, Trichoderma harzianum LU672, and T. virens LU556) and most (Trichoderma atroviride LU132, T. harzianum LU151, and LU673) rhizosphere-competent isolates were assessed on six plant species (sweet corn, ryegrass, cauliflower, carrot, onion, and white clover). Ryegrass and cauliflower were the most receptive plants to colonisation of the rhizosphere by Trichoderma species, and clover the least. Preferential rhizosphere colonisation was observed between some Trichoderma isolates and the plant species indicating that overall rhizosphere competence was dependent on specific interactions between the Trichoderma isolate and the plant species. However, some isolates were more broadly rhizosphere-competent than others and may have greater potential as plant protection agents. Since only one time point was sampled, future work is required to determine the temporal dynamics of rhizosphere colonisation as well as the spatial colonisation along the length of the root to determine whether different isolates preferentially colonise different regions of the root over different time periods.