This article contains a detailed description of a reliable method for the inoculation of potato tubers with Phytophthora infestans. This technique was developed as a general basic method for greenhouse and field trials to study the development of the primary inoculum source in potato crops. It was applied to increase the reproducibility of infections from tubers to sprouts. Afterwards the pathogen was detected in light sprouts and stems at different stadiums of plant growth by the polymerase chain reaction‐based test (PCR). From this it follows that Phytophthora infestans is able to grow directly up into the stem from the tuber as mycelium. In the course of plant growth the concentration of mycelium fell below the detection limit in some of the assayed stems on account of dilution effects, although the symptoms occurred in the following period. Apparently the pathogen is also able to cause symptoms in the plant at very low concentrations.

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