Nightshades, Copper as fungicide and Copper poisoning
As a nutritional chemist I strongly believe that we should mainly eat foods that were part of our ancestor's diet. Tomatoes, potatoes and peppers, all part of the nightshade group of vegetables, only grew in America and were introduced to Europe and other parts of the world relatively recently.
E.g. while the potato was brought to Europe by the Spanish conquistadors, the monarchs of Europe discovered the nutritional value of the potato only during the early 18th century, and ordered it planted from then on. Some people might not have been "conditioned" to digest these foods. Interestingly, nightshades are amongst the least tolerated vegetables and we known that they can cause a number of autoimmune diseases or hypersensitivity such as arthritis symptoms, IBS etc. for quite a number of people.
I think we all know that the outbreak of blight wrecked havoc amongst the population of Northern Europe (no, not only in Ireland but vast regions of Europe were affected). But people noticed that blight was not an issue or contained around copper plants and thus, an effective remedy against blight was found. Unfortunately, the heavy use of copper or copper salts and soaps had a devastating effect and caused copper poisoning, brain damage and many times early death of vast numberd of Northern Europeans. Copper is the oldest remedy against blight and other fungal diseases and even today is not stricly regulated by government authorities, which to me is a crime. Commercial organic farmers use copper and needless to say they use a lot. Like lead, copper is a heavy metal that will penetrate your brain barrier and accumulate, and will be toxic above certain limits. Almost anyone of my clients who eats so-called "organic produce" suffers from excessive copper accumulation in their bodies. If you want to use copper be vary careful and only use products such as Soap-Shield, developed in Germany and sold by Garden's Alive in the U.S., quote:
Less copper! In our own field and lab tests, Soap-Shield was effective at much lower concentrations of copper than other copper-based products.
As Carolyn mentioned copper is not all that effective. One of the most effective methods of delaying tomato disease is to start plants early under good conditions. I use metal halide lights with Kawasaki bulbs and cool basement conditions which keeps the plants strong and stocky. It is very important to prevent early fruit set by removing all flowers until plants are well established in your garden (2 to 3 weeks after transplanting). This might sound excessive and you will miss 2 weeks of the first harvest rush but healthier plants make up more than what you loose later.