Quoting Penelope Periwinkle:
suze_anderson@spamcop/dot/obvious wrote:

I use Daconil (aka Ortho Garden Disease Control). Once a week spray
as a preventative. Must be applied so as to coat all plant surfaces
(it is not systemic) it 'seals' the plant.

I've been using Daconil this year, too. The spring was wet and cold,
and the summer wet and hot; prefect for growing fungi, but not so
perfect for tomatoes. The problem has been that Daconil needs to be on
the plant for 24 hours, and early this summer, we had rain almost
everyday.

Yep, that's tough. When one needs it most, it won't stay on the
plant. Though it's usually on the dry side here, I experienced this
first hand last year when it just rained and rained late last summer.

One thing I'm considering experimenting with next year is the aspirin
treatment. Sounds like it's more of a systemic way to toughen up a
plant and trigger natural defenses, much like harpin (Messenger) is.

http://www.plantea.com/plant-aspirin.htm

http://www.garden.org/regional/report/arch/gv/1757

You could also do a search on Gardenweb (it's been mentioned a bunch
there), if you're familiar with that venue.