Trichoderma harzianum is an effective biocontrol agent against several fungal soil-borne plant pathogens. They have an inhibitory effect on the growth of mycelium of Phytium. They suppress the growth of fungi in order to allow the plant to grow favorably. However, possible adverse effects of T. harzianum on plant growth promoting microorganisms, such as arbuscular mycorrhiza (AM) fungi, might be a drawback in the use of this biocontrol agent in plant protection. So, the correct answer is 'Biological control of soil-borne plant pathogens'