The subject invention provides a method for enhancing the growth of a crop, a plant, or a seed, simultaneously strengthening a plant stem and/or trunk, increasing the yield of a crop, and/or improving the suppression of a phytopathogenic disease, which method comprises applying a material comprising γ-polyglutamic acid ("γ-PGA," H form) and/or its salt, a γ-polyglutamate hydrogel, a fermentation broth comprising γ-PGA, its salt and/or a γ-polyglutamate hydrogel, or a mixture thereof to the crop, plant, or seed, or to a field for growing the crop, plant or seed.
