Ridomil (DL-methyl N-[2,6-dimethylphenyl]-N-[2-methoxyacetyl]alaninate), a new systemic fungicide effective against plant pathogenic species of the Peronosporales, effectively controlled P. infestans on tomato [Lycopersicon esculentum] plants. A single soil drench containing 0.25 mg a.i. [active ingredient] applied to a 7-leaf plant in a 1-l pot was sufficient to protect plants against blight. Protection of potted plants treated by soil drench was observed within 1 h from time of fungicide application at 25.degree. C. The fungicide was slightly toxic to sporangia and zoospores in vitro. Penetration and initial establishment of P. infestans in leaves and fruits was not inhibited by the fungicide. Instead, minute, restricted, sterile lesions were produced, even if the compound was applied 2-3 days after inoculation. To achieve the most efficient control of blight, plants must be treated with the fungicide either before, or within the first 2 days after, inoculation.