A L-lysine derivative used in the preparation of Polylysine, a synthetic, basic poly-α-amino acid. Polylysine inhibited fungal penetration and polyphenol formation in susceptible sweet potatoes infected with C. fimbriata and caused some inhibition of fungal growth, a lessening of the penetration by the fungus, and more callus formation in resistant sweet potatoes. The compound inhibited the infection of tomato cuttings by F. lycopersici but was also toxic to the plants. Polyglutamic acid had no effect on the fungi in vitro.