Trichoderma is a genus of fungi that is present in all soils, where they are the most prevalent culturable fungi. Many species in this genus can be characterized as opportunistic avirulent plant symbionts. Several strains of Trichoderma have been developed as biocontrol agents against fungal diseases of plants. The various mechanisms include antibiosis, parasitism, inducing host-plant resistance and competition. Most biocontrol agents are from the species $T$. harzianum, $T$. viride and $T$. hamatum. The biocontrol agent generally grows in its natural habitat on the root surface and so affects root disease in particular but can also be effective against foliar diseases.