Inhibition of the growth of Helminthosporium carbonum by L -cysteine

L-Cysteine (1.21 mg/ml) inhibited the linear growth of Helminthosporium carbonum 22% at pH 5.5 and 60% at pH 4. The increased toxicity with increasing acidity was attributed, in part, to the increase in the concentration of the cysteine cation [HS—CH2—CH(NH3+)—COOH]. In the presence of chlorogenic acid (1.77 mg/ml), L-cysteine (1.21 mg/ml) was more toxic than when tested alone. This increased toxicity was due to the acidity of chlorogenic acid, which had little or no fungitoxicity when tested alone. Chlorogenic acid did not increase the toxicity of L-cysteine when the increased acidity was neutralized with base. L-Cysteine at 0.6 mg/ml and 0.3 mg/ml inhibited the growth of H. carbonum, respectively, 32% and 10% at pH 4.5, and 5% and 1% at pH 5.5. Since L-cysteine is toxic only at relatively high concentrations, it appears doubtful whether this amino acid contributes significantly to the fungitoxicity of potato peel extracts.