Posted: 11/08/2012 at 09:03
Leggi, if the stems are quickly turning black it sounds like it could be Late Blight. If so, there's no saving the plant.
In general, spraying once any fungal disease symptoms are already present is a waste of time and money. It's too late. Spraying has to be preventive, meaning you spray before the spores arrive, starting a couple of weeks after planting out, then weekly or so afterwards, respraying if it rains.
Spraying doesn't guarantee you won't get fungal disease but it gives you a solid defence.
Dove - good luck!