Selection of Trichoderma isolates for the biological control of Sclerotium rolfsii and Rhizoctonia spp. in sugar beet

Sclerotium rolfsii and Rhizoctonia solani var. betae cause serious damage to sugar beet crops. At present, only agronomic techniques are used to control these diseases. The aim of this study was to evaluate the activity of different Trichoderma isolates against the above pathogens. Trichoderma spp. are known to be effective biocontrol agents against several soil-borne plant pathogens, acting through different mechanisms, such as competition, antibiosis, mycoparasitism, induction of defence responses and growth promotion. Moreover, the ability to colonise the infection court is an important pre-requisite for effectiveness in the open field. More than 120 Trichoderma strains were tested for direct and indirect in vitro antibiosis. Some of these were also tested for sugar beet rhizosphere competence. The findings showed a wide variability among the different isolates tested. Some stimulated mycelial growth of the pathogens, but most of them inhibited it; less than 5 isolates reduced the pathogen growth in vitro by 80%. These results, combined with rhizosphere competence indications, made it possible to select some Trichoderma strains, which are under evaluation in an open field trial on sugar beet.