http://www.post-gazette.com/pg/09228/991203-455.stm

I hyper-linked it into my name (above) for quick linking . . .

Farmer Troy

I’d just take the science and research for what it says, prophylactic is the
only way. It is problematic for us organic growers because the copper
organic spray is one of the most harsh we can use, perhaps worse than the
chemical fungicides. If we don’t have a problem, why use it at all. If we
wait for the problem (late blight) it’s already too late to use the spray.
Catch 22. So far my experience this hear says that the preventative spray
has helped. The other field where I waited, is almost totally infected.
–Don

There are products that are labeled for organic production such as copper-based fungicides, biological fungicides such as Serenade and even potassium bicarbonate.

If you are not willing to spray even these materials, when we have outbreaks of disease such as late blight on tomatoes and potatoes or downy mildew on cucurbit crops, you are at risk for losing the entire crop.

[Other strategies include] selecting resistant varieties when available; practicing crop rotation, even in a small area; fertilizing based on soil test results; and protecting crops from drought stress and insect damage as the most important cultural controls. Late blight can only overwinter in our climate on potatoes that were infected and overlooked during harvest. It does not overwinter in your garden if you only grow tomatoes.

Best regards,
Sandy