From experiments carried out on young plants at the cotyledon stage, it has been shown that certain concentrations of metalaxyl did not affect the germination of the conidia, but inhibited their power of contamination. This inhibition occurred if the fungicide was maintained in the suspension of spores even if the period of contact was limited to 4 hours. The action of the systemic fungicide depended on the stage of development of blue mold. Best results were observed during the period of pathogen establishment in the host tissue. On the contrary, after a certain lapse of time, the processes of infestation were not so easily controlled. For example curative treatment applied 72 hours after inoculation needed a concentration 200 times higher than that used at the end of 48 hours in order to obtain a similar control in the development of the disease.