Evaluation of Trichoderma isolates for biocontrol of tobacco collar rot in Golestan Province
Biological control is one of the control methods against plant pathogens. Trichoderma species have been used as a biological control agent against soil-borne plant pathogenic fungi. Soil samples were collected from tobacco seedbeds from Golestan Province such as: Gorgan, Ali Abad and Minoodasht. Common media including PDA, WA, CMA and Davet's selective medium were used to isolate fungi from soil samples. The fungi isolates obtained after preparation of the pure culture were identified in genus level by using Bissett key. Among 56 soil samples, 39 isolates belonged to genus Trichoderma species. Because of the importance of genus Trichoderma as biocontrol agent, its species were recognized. Species were identified according to their macromorphological and micromorphological characters including feature and size of conidiophores, phialid and phialospere by using keys written to identify species of this genus (Zafari et al, 2003, Gams 8 Bissett, 1998). Dual culture of antagonistic fungi and pathogen on PDA plate revealed that the antagonists reduced and inhibited the mycelial growth of Sclerotinia sclerotiorum . Mycelial growth inhibition rates of Trichoderma species against S. sclerotiorum were different and ranged between 48/75%-90%. Identified Trichoderma spp included: T. harzianum, T. citrinoviride, T. longibrachiatum, T. koningii, T. virens, T. atroviride, T. hamatum . In this study T. harzianum was the most plentiful respectively (33.33%) in comparison with others. Microscopic studies of Trichoderma species on the fungus showed that antagonistic hyphae (Tr82-1 and Tr36-1) grew over to hyphae of the pathogenic fungus and formed branches that coiled around them. The penetration and growth of isolates (Tr53-1 and Tr35-2) inside the hyphae of S. sclerotiorum was observed. All of Trichoderma species finally lysed the hyphae of pathogenic fungus. Volatile metabolites emitted from all isolates inhibited growth of S. sclerotiorum and the inhibitory effects of Tr53-1 was greater compared with the other isolates and inhibited the pathogen by 81.87%. In the greenhouse study, isolates Tr53-1, Tr82-1, Tr36-1 and Tr35-2 were effective in disease reduction percentage by 81.25, 75, 62.5 and 56.25, respectively. In addition, these isolates caused an increase in height, shoot and dry weight.