Bordeaux mixture-tomato blight

A bit of advise please guys.
Having seen from fellow forum member that the dreaded tomato blight is about again I have got some Bordeaux mixture this morning.
I do not have the blight at this time and the instructions quote:
Spraying should commence when the desease becomes active in late july early August
Now this suggest to me that I should not start using the stuff until I see signs of it. (Treatment)
Question is:Can it be used as a (Preventive) before the main event?
Whats your thoughts on this?
I understand the blight threat is high with certain humidity and temperature values over a four day period.
I`m guessingt that across the country we have reached those parameters
Thankyou
robert

 

we've already started with it down here!... I'd give a pecautionary spray if I was you, by the time you see the symptoms...it's taken hold!

 

consider it done:gnthb:

 

Or check the Blight site which will send you a warning of a full Smith Period in your postcode (dunno if that is good enough as a prophylactic treatment though ...)

http://www.gardenerscorner.co.uk/forum/showthread.php/blight-map-21523.html?p=332263#post332263

 

Thanks Kristen
I just could not remember these details
I am now subscribed:gnthb:
Seems to be clear in my area at the moment
robert