Manganese is a microelement required for several physiological functions of plants such as photosynthesis, N metabolism (especially reduction of nitrate), and synthesis of aromatic ring compounds as precursors for some amino acids, hormones (auxins), phenols and lignin. As a consequence, manganese plays a major role in both the growth and disease resistance of plants. Manganese can occur in more than one oxidation state, but it is available to plants only in the reduced form (Mn+2), whereas the oxidized form (Mn+4) is essentially insoluble. Manganese availability in soil mainly depends on pH and soil microflora that can either oxidize or reduce manganese. Trichoderma harzianum1295-22 (T-22) is an effective biocontrol agent that also elicits increased growth response (IGR) in several crops, e.g. sweet corn, bean, potato. The manganese solubilization by T. harzianum T-22 could play a role in both biocontrol and IGR. In in vitro experiments, the fungus solubilized Mn contained in MnO2 regardless of the medium, and solubilization occurred regardless whether or not Mn+2 was limiting. Solubilization was not correlated with pH changes that occurred during culture of the fungus. Thus, the fungus appears to be able to solubilize Mn+4 by reducing and/or chelating Mn. This ability appears to be constitutive and does not require a deficiency of Mn+2 to be present. In greenhouse tests, seed treatments with the fungus resulted in growth enhancement of sweet corn, with concomitant colonization of the entire root system. Total quantities of Mn, Fe, Zn, Cu, P, and Mg all were greater in the larger plants grown from treated seeds, but the concentration of all of these elements in ppm was generally lower than in plants grown from nontreated seeds. Thus, the increase in the total concentration of these elements may be due primarily to the larger sink provided by the larger plant and not to be related to the capability of the fungus to enhance micronutrient uptake. However, Mn and other elements were not limiting for the plant growth in these tests.