You are missing a lot of more fundamental issues by trying to understand the end game production. Its hard to understand what you cannot see, but you have seen the results of contaminated soil and now want to spray the plant. Its not what you need to be doing. Its gonna shock you when you get into soil prep and how much it influences the 90-120 day fruit window.

The active ingredient in Daconil is chlorothalonil (Daconil is just a brand name). Chlorothalonil can be researched online to determine both its potential toxicity to humans and other mammals, to birds and aquatic organisms and to bees and beneficial insects and other pollinators. It can also be researched as to soil half life (how long it will persist) and what impact it can have on soil organisms. Here is just one website that outlines all these issues:Cornell's Extoxnet for Chlorothalonil.

When using any kind of pesticide, it is always a good idea to determine a) what the active ingredient is (the thing that makes the product work or be effective), and b) what the potential toxicological implications are of that ingredient to you, your pets or kids and to any other living organisms that might be affected. Once you are armed with that kind of information, you are then able to make a properly informed choice.

It is too hard to keep up with sometimes. A home garden is not a place where that level of detail can be easily integrated, because the rule must apply to one and all. In other words, to deeply understand one aspect such as a chemical, one should also understand every. single. other. thing. and how that chemical interacts with those myriad other things. Which of course is a virtual impossiblity. Thus the shotgun effect of a product like daconil flies off the shelves of Walmart. If I ever thought of one thing as more important than the others in my garden, it would be the sterility issues before I plant. But that is just my experience to this point in a hot and humid pathogen ridden environment.

There are so many chemicals in our environment and our bodies - why add more? I grow tomatoes in South Florida. It is humid even during the winter, when tomato season peaks. I have disease from the time the plants begin to bloom. I live with it. Of course I mulch, water only the ground, cut off the diseased leaves . But I never spray. I still harvest a bunch of tomatoes. Are you in Arizona? If so, I would think you could keep your plants fairly dry and fungus would not be a huge problem, no?

Actually I live in northwestern Arkansas. Springs and summers are very wet here, creating a great atmosphere for blights. Thanks for the links and information about Daconil, Gardengal. I'll keep reading up on it and make a decision. The disease normally wipes out my tomatoes before I harvest, before I can get a harvest....The plants are always so very healthy as well, which can disappoint pretty quickly.

Thanks for the help everyone.

Fungal disease usually live in the soil and it can live there for years until it is starved or burnt out by the sun so spraying plants with anything won't work. The best solution is to prevent the fungal diseases but diligent gardening. Understanding how they spread is half the battle. Rain or watering hits the soil and splashes up on to a leaf carrying the fungal spores with it and that leaf is infected and water splashing on that leaf can effect other leafs the same way until the whole plant is infected. Never, never water a plant instead water the soil. I build a moat around each plant and put the water in it. Mulch or put down plastic and cut an x in to to plant then fold the x back around the plant. Remove most of the bottom leaves and if a leaf becomes infected remove it. Put all debris in the garbage not the compost and do the same with infected plants and any fallen leaves so they can't reinfect the soil. When you pull the plants if it is still warm weather cover the area with black plastic. It may take years to get rid of fungal diseases but with diligence you should at least get tomatoes. If you still want to spray use baking soda and warm water but don't spray the soil or you will be a fungal enabler