YOu say the potato disease and by that I assume you mean Late Blight, which is P. infestans.
And if that's what your plant has it will be a stinking mass of black tissue in a week or two and I'd get it out of there ASAP.
On the other hand, what's the chance that your plant has one of the more common foliage diseases?
I'd find it a bit odd to have just one plant out of many infected.
Could be the fungal ones Alternaria solani ( Early Blight) or Septoria Leafspot or the bacterial ones Bacterial Speck or Bacterial Spot.
Could you show a picture of the leaf symptoms, or describe them in more detail, or have you looked into the possibility that it's not P. infestans?
The prognosis for the plants and others near it is much better if it's a more common foliage disease than if it's P. infestans.
If it is P. infestans I'd try to protect the other plants ASAP with Daconil, aka Chlorothalonil rather than a copper product. proterction is not very good but the good stuff is not available to the home gardener, at least here in the US. And I mean active against P. infestans.
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Carolyn
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