The salacylic acid of aspirin mimics a hormone in tomato plants. If you do a Google search you will see a lot of documentation on this fact. The hormone naturally triggers a defense response in tomatoes. You are tricking your tomatoes into to triggering a defense response by spraying it with aspirin. The theory is to induce a defense response before diseases arrive. You are boosting your tomatoes defenses BEFORE an attack of leaf spot or blight occur. I have done this for years and believe it works.
The Basic Recipe
325 mg of aspirin to 1 gallon of water
Spray your tomatoes 2-3 times monthly
You have to use aspirin because you need the salacylic acid to mimic the hormone.
I've been doing this for several years and notice thicker stronger tomato leaves. My non-scientific observation is that the aspirin triggers a response that makes the leave less susceptible to the spores of diseases. The spores have a harder time getting established. Try it out.
If you would like more information about using aspirin or want to join the great aspirin tomato experiment... check out my blog The Rusted Garden
or go to this direct link on my blog for some tomato fun The Great Tomato Aspirin Experiment of 2012