Crown and root rot of sunflower (S. minor and S. sclerotiorum) is one of the most important diseases of sunflower. Chemical control of this disease is very difficult, because the causal agent is a soil-born pathogen, therefore biological control is the best method. In this investigation interaction between 9 isolates of Trichoderma (6 isolates of T. harzianum, 2 isolates of T. viride and I isolate of T. virens) and S. minor and S. sclerotiorum were studied. Microscopic studies showed that hyphal contact, coiling and penetration of Trichoderma isolates within hyphae of the pathogen was not observed, but the lysis and deformation of hyphal tip of pathogen was observed. In dual culture, the. mycelial growth of S. minor and S. sclerotiorum were suppressed upon contact with the competitor's hyphae of all isolates, then parasite's growth continued over the host mycelium and then some of the isolates inhibited the formation of sclerotia of S. minor and S. sclerotiorum. Study on volatile metabolites showed that T. harzianum (ll) among all isolates was the most effective one, which inhibited mycelial growth of the pathogen by 90.42 % and 77.77%, respectively. Study on culture filtrates that were studied in three concentrations (10, 20 and 30%) showed that T. harzianum (l6) was more effective and inhibited mycelial growth of the pathogen by 86.47% and 78.88%, respectively at 30% concentration. Study on colonization ability of isolates on 14-day-old culture of pathogen showed that all isolates were grown over the sclerotia of pathogen, but some of them only were able to sporulate and auto lysing the sclerotia of S. minor and S. sclerotiorum.
