Production of antifungal compounds by Trichoderma spp. to control Phytophthora nicotianae, causal agent of gummosis on citrus
Orange crops in Brazil are affected by various diseases such as gummosis caused by Phytophthora nicotianae, which is controlled by chemical fungicides, by preventive nature through cultural practices and utilization of tolerant rootstock. However, the high costs and environmental problems caused by the intensive use these fungicides have led to the search for alternative methods of control. The objective this study was to evaluate the in vitro antagonistic activity of Trichoderma spp. isolates against P. nicotianae by the production of antimicrobial compounds and an alfalfa seedling bioassay, thus, 12 out of the 50 isolates of Trichoderma spp. evaluated were selected based on in vitro screening to
compose bioassay treatments. The paired culture showed that all isolates tested inhibited the mycelial growth of the pathogen. In the production of antimicrobial compounds, 41 isolates produced volatile compounds using dextrose as carbon source; however, further assays showed that the use of sucrose or maltose as carbon source increases the production of these compounds. Seven isolates of Trichoderma spp. produced thermostable compounds and 14 isolates produced cell-free antimicrobial compounds of the antagonist. In the alfalfa seedling bioassay, four isolates of Trichoderma spp. inhibited the formation of sporangia and mycelia growth. The in vitro evaluation concerning to the antimicrobial compound production by the Trichoderma spp. isolates and the alfalfa seedling bioassay were able to select biocontrol agents to control P. nicotianae. The four most promising isolates were TB12, TB14, TB28, and TB30 and the mechanisms of action that may be involved in the biocontrol are production of volatile compounds and cell-free filtrates by the antagonists.